Post on 15-Mar-2020
The Efficacy of LED Light Polymerization Units in Private
Dental Offices in Toronto, Canada
by
Dave Darko Kojic
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Faculty of Dentistry
University of Toronto
© Copyright by Dave Darko Kojic (2018)
ii
The Efficacy of LED Light Polymerization Units in Private Dental Offices in
Toronto, Canada
Dave Darko Kojic
Doctor of Philosophy
Faculty of Dentistry
University of Toronto
2018
Abstract
Light-emitting-diode (LED) light-polymerization units (LPUs) are currently routinely
used for resin-based composites (RBC) photopolymerization. This study aimed to determine
the effectiveness of LED-LPUs used in participating private dental practices in Toronto,
Canada. One-hundred dental practices were recruited, and 113 LED-LPUs were examined.
Part 1 evaluated radiant exposure (RE) of participants’ LED-LPUs. Majority of LPUs (n=87,
77%) passed the study-required criterion (delivering equivalent to 16 J/cm2 in 20s). A strong,
negative correlation between LPUs' age, light-probe damage, and RE was found. Part 2
evaluated operator's technique to deliver 8 J/cm2 of RE in 10s to simulated restorations
before and post-instructional. Before instructions, 73.6% of participants delivered required-
energy to anterior restoration, and 32.2% of participants delivered required-energy to
posterior restoration. Following instructions, 94.3% of participants delivered required-energy
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to anterior restoration, and 73.6% of participants delivered required-energy to posterior
restoration. Part 3 examined relative-hardness (RH) of two RBCs polymerized at 1mm and
4mm, tip-to-target distance. Both RBCs polymerized at 1mm had average RH (ratio of mean
lower/upper surface hardness) greater than 0.80, but not at 4mm tip-to-target distance. Part 4
evaluated the diametral tensile strength (DTS) of two RBCs polymerized with participating
LED-LPUs. No significant difference between mean DTS values of two RBCs was observed.
Mean DTS values were highly correlated among the LED-LPUs tested. In conclusion, 23%
of LED-LPUs tested were deemed as incompetent for sufficient RBC polymerization. A high
percentage of dental practitioners failed to deliver 8 J/cm2 in 10s exposure cycle, even after
receiving specific instructions (5.7% to anterior restoration and 26.4% to posterior
restoration). The outcome was due to a combination of LPUs performance and operator's
technique. This finding is alarming as it relates well to early failure of Class-2 RBC
restorations due to insufficient polymerization.
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Dedication
This thesis is dedicated to my beloved Father and Mother, who are the source of love, inspiration,
and wholehearted support since the beginning of my life journey, and without whom, none of this would
be possible.
Also, this thesis is devoted to my sons, Petar and Lazar for their endeavor of bringing so much
love into my life, unshakable faith in the righteousness of my life's decisions and for making my whole
existence relevant.
Finally, this thesis is devoted to my sister and my brother-in-law who have supported and
encouraged me in this venture since the early beginning.
v
Acknowledgements
First and foremost, my genuine gratitude goes to my supervisor, Dr. Omar El-
Mowafy, who has supported me throughout my academic journey with his patience and
knowledge, while permitting me to work in my way. His expertise, understanding, time and
helpfulness, added considerably to my extraordinary pleasant graduate experience. I attribute
the level of my doctorate degree to his encouragement and effort, especially with his support
of my international exposures at the IADR conferences.
Special thanks to my co-supervisor, Dr. Wafa El-Badrawy, for her expertise,
understanding, patience, professionalism, and mentorship during my educational venture at
the Faculty. Her insight and guidance contributed significantly to this project.
Also, I wish to thank my other thesis advisory committee members, Dr. Richard Price
and Dr. Gildo Santos for the time they provided throughout this project. Dr. Price's valuable
and thorough comments, expertise, incredible support and suggestions helped significantly in
improving the quality of this project. Dr. Santo's ideas and dedication have been invaluable
for the completion of this project, for which I am incredibly grateful.
Additionally, I would like to thank my sons: Petar and Lazar for their support and
great patience at all times. My parents and my sister's family have provided me their help
unconditionally, as always, for which my mere appearance of thanks furthermore does not
satisfy. Without their love, encouragement, on-going support and sacrifices throughout this
process, I would not be able to complete this graduate program.
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Furthermore, I am very grateful to my research assistants: Ms. Ana Burilo and Ms.
Ivana Orlovic whose contribution to this project was significant for the duration of this
project. I would like to express my sincere gratitude to Heather Sanguins from the Health
Sciences Writing Centre University of Toronto for her enormous patience and the
tremendous help with my thesis writing.
In conclusion, I realize that this research would not have been possible without the
generous research material donations from Shashikant Singhal (Ivoclar-Vivadent, Amherst
NY) and Sridhar Janyavula (Dentsply, York PA), as well as technical support and
CheckMARC loan from Colin Deacon (BlueLight Analytics, Halifax NS). Their support is
duly acknowledged. Furthermore, I am very appreciative to the University of Toronto for the
financial assistance during the entire course of my graduate training and the funding of this
project.
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Table of Contents:
Dedication ………………………………………………………………………… iv
Acknowledgements ………………………………………………………………. v
Table of Contents ………………………………………………………………… vii
List of Tables ……………………………………………………………………. x
List of Figures …………………………………………………………………… xi
Chapter 1 Introduction and Literature Review……….………………….……. 1
1.1 Introduction ………………………………………………….………………... 1
1.2 History of Resin Based Composites (RBC) …………………………………… 4
1.3 Classification of RBC …………………………………………………………. 11
1.4 Classification of Bonding Agents……………………………………………… 18
1.5 Evaluation of Photopolymerization Efficiency ………………………………. 23
1.5.1 Degree of conversion………………………………………………………… 25
1.5.2 Degree of crosslinking………………………………………………………. 27
1.5.3 Mechanical properties of RBCs .……………………………………………. 30
1.5.4 Shrinkage and shrinkage stress………………………………………………. 33
1.5.5 Depth of polymerization……………………………………………………... 36
1.5.6 Biocompatibility……………………………………………………………... 37
1.6 Evolution of LPUs……………………………………………………………. 40
viii
1.7 Intrinsic Factors Affecting Photopolymerization Efficiency…………………. 50
1.7.1 Photoinitiator systems………………………………………………………. 52
1.7.2 Viscosity, monomers, and fillers……………………………………………. 58
1.7.3 Optical properties……………………………………………………………. 62
1.8 Extrinsic Factors Affecting Photopolymerization Efficiency………………… 67
1.8.1 Light polymerization units (LPUs) and emission spectrum ……………….. 67
1.8.2 Radiant exposure, irradiance, and irradiation time ………………………… 70
1.8.3 Light exposure ………………………………………………………………. 73
1.8.4 The effect of temperature on photopolymerization …………………………. 74
1.8.5 Light guide tip positioning…………………………………………………… 75
1.9 Review of studies on RBCs Longevity and Effectiveness of LPUs………….....77
1.9.1 Longevity of RBCs direct restorative materials……………………………….77
1.9.2 Effectiveness of LPUs in dental practices………………………………….… 83
Chapter 2 Objective and Aims………………………….……………………….. 91
Chapter 3 Manuscript 1 ……………………………………………………….... 93
Assessment of LED Light-Polymerization Units Used in Toronto Private
Practices…………………………………………………………………………….. 94
Chapter 4 Manuscript 2………………………………………………………….. 125
Assessment of photopolymerization application effectiveness among private
dental practices in Toronto, Canada ….…….…………………………............. 126
ix
Chapter 5 Manuscript 3…………………………………………………………. 159
Relative Hardness of Resin Based Composites polymerized by LED Light-
Polymerization Units in Toronto Private Practices………………………………. 160
Chapter 6 Manuscript 4………………………………………………………….. 190
Assessment of Biaxial Tensile Strength of Resin Based Composites Polymerized by
LED Light-Polymerization Units in Private Dental Practices in Toronto, Canada
………………………………………………………………………...…...……...191
Chapter 7 Findings, Analysis, Discussion, Limitations and Future
Directions, and Conclusion …………………………………………………… 212
7.1 Research Findings ………………………………………………………….. 213
7.2 Analysis ……………………………………………………………….…….. 218
7.3 Discussion ………………………………………………………………..…. 220
7.4 Limitations and Future Directions ….………………………..………………221
7.5 Conclusions …………………………………………………………..…….223
Appendices ……………………………………………………………………….225
Appendix I ………………………………………………………………………..226
Studies on survival of RBC restorations …………………………………………227
Appendix II ………………………………………………………………………229
Studies of Effectiveness of LPUs in dental practices ……………………………230
References……………………………………………………………….………232
x
List of Tables
(Table 3.1) Listed LED LPUs that failed to reach a minimum of 16 J/cm2 …… 117
(Table 4.1) Radiant exposure delivered by 113 dental practitioners ….………….147
(Table 5.1) Descriptive statistics for RH levels for two RBCs…..….….……….. 179
(Table 6.1) Average BTS values of two RBC brands, polymerized by
monowave (type 0) and multiwave (type 1) LPUs ……………………………... 207
xi
List of Figures
(Figure 1.6.1) Ultralume 5® (Ultradent Products, South Jordan, UT) …….…..….... 48
(Figure 3.1) The CheckMARC® device ….…………………………………..…... 114
(Figure 3.2) Schematic representation of calculated damage levels observed on
light probe surfaces of 113 LPUs tested ….………………………………………. 114
(Figure 3.3) Calculated 20s radiant exposure of 113 LPUs tested …………….. 116
(Figure 3.4) Box and Whisker plots: LED LPUs which met/did not meet the
study-required criterion ……………………………………………………….….. 117
(Figure 3.5) Q-Q plot: multiwave LPUs incident irradiance distribution …………119
(Figure 3.6) Q-Q plot: monowave LPUs incident irradiance distribution…………119
(Figure 4.1) The MARC® Patient Simulator system …………..………………….146
(Figure 4.2) The MARC® PS radiant exposure of each professional group..….….146
(Figure 4.3) 10s RE delivered to the MARC®PS (Anterior before instructions).... 148
(Figure 4.4) 10s RE delivered to the MARC®PS (Anterior post-instructional)…. 148
(Figure 4.5) 10s RE delivered to the MARC®PS (Posterior before instructions)…149
(Figure 4.6) 10s RE delivered to the MARC®PS (Posterior post-instructional)…. 149
(Figure 4.7) Q-Q plot: Anterior restoration before instructions.………………… 150
(Figure 4.8) Q-Q plot: Posterior restoration before instructions…………………. 150
(Figure 4.9) Q-Q plot: Anterior restoration post instructional.….…………..…… 151
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(Figure 4.10) Q-Q plot: Posterior restoration post instructional ………………….151
(Figure 4.11) 10s RE of monowave vs. multiwave LPUs….…………..…………152
(Figure 5.1) Plastic mold used for the specimens’ production .…………….…. 178
(Figure 5.2) Specimen ready for Knoop microhardness testing ………….…… 178
(Figure 5.3) Percentage of LPUs that reached the .80 level of RH …………. …. 179
(Figure 5.4) Average RH by LPU type ..………………….…………………….180
(Figure 5.5) Average RH at 1mm/4mm Tetric EvoCeram ………………………180
(Figure 5.6) Average RH at 1mm/4mm TPH Spectra …………………………...181
(Figure 5.7) Q-Q plot: RH of Tetric EvoCeram at 1 mm distance ………….….181
(Figure 5.8) Q-Q plot: RH of Tetric EvoCeram at 4 mm distance ………….….182
(Figure 5.9) Q-Q plot: RH of TPH Spectra at 1 mm distance ….…………….….182
(Figure 5.10) Q-Q plot: RH of TPH Spectra at 4 mm distance …………………183
(Figure 6.1) The BTS Specimen ……..…………………………………………. 206
(Figure 6.2) The BTS testing in a universal testing machine …………………..206
(Figure 6.3) BTS values polymerized by multiwave /monowave LPUs ….……. 207
(Figure 6.4) Q-Q plot: BTS values of TPH Spectra …………….…..…………...208
(Figure 6.5) Q-Q plot: BTS values of Tetric EvoCeram …………..……………208
1
Chapter 1
Introduction and Literature Review
Introduction
Direct esthetic restorative materials have experienced significant changes since the
1980s. Up to this time, amalgam was the material of choice for direct posterior restorations.
However, over the past decade, extensive research into, and implementation of, innovative
technologies have significantly improved the dependability and predictability of amalgam-
alternative dental materials, especially direct resin-based composite (RBC) materials. At the
same time, restorative dentistry has experienced a dynamic shift to adhesive dentistry. During
the past decade, the use of RBC for direct restorations has increased exponentially, due to the
high esthetic demands and increasing public awareness of mercury in dental amalgam, in
addition to environmental concerns about the industrial use of mercury 1, 2. In 2013, the
Minamata Convention on Mercury committed to the reduction and ultimate dismissal of the
production and use of mercury-containing products globally. The use of RBC continues to
increase worldwide. In some countries, however, RBCs have entirely replaced amalgam,
such as Norway where amalgam elimination started in 2008 3. According to the United States
insurance data claims for the 2010 year, RBC has become the first-choice restorative material
accounting for more than 65% of all private practice placements 4. The RBC has three
2
distinct components, each with its role in influencing material properties: the organic resin
matrix, the inorganic fillers, and the filler-resin interface (coupling agent). The resin matrix
phase is composed of polymerizable monomers that convert to the highly crosslinked
polymers upon photopolymerization, which catalyzes the development of active centers,
which induce polymerization. The filler phase has several roles, including improving the
mechanical properties, reducing thermal expansion, and decreasing polymerization shrinkage
by reducing the resin content. Finally, silane at the filler-resin interface creates a bond
between the filler particles and the resin interface that is important for their bonding
capabilities 5.
Polymerization of RBC materials is initiated by the appropriate spectra and sufficient
quantities of light in the blue region of spectrum from the LPU that activates photoinitiators
with subsequent release of free radicals that initiate the polymerization reaction. During this
process, the terminal aliphatic C=C bonds are severed and converted to the primary C-C
covalent bonds. During the polymerization reaction, the monomer is converted into a
polymer, nonetheless, full conversion is never achieved. Although the majority of monomers
are transformed, however, a certain number of monomers remains unreacted or trapped in the
polymeric matrix, and is the cause of the free monomer elution 6. The percentage of
unconverted aliphatic C=C bonds in the polymeric matrix represents the degree of conversion
(DC), the most important feature that affects the clinical performance of RBC materials 7, 8.
The literature shows that the development mechanism of free radicals in the monomer differs
according to the photoinitiator system used 9. Therefore, for adequate RBC
photopolymerization, light-activated resins must not only receive sufficient radiant exposure
3
(J/cm2) but also must be given this energy within the appropriate wavelength range to match
the spectral range of the photoinitiator system of the resin 10.
The first law of photochemistry, the Grotthuss-Draper law, states that photon has to
be absorbed by a chemical material for a photochemical reaction to take place. Therefore, the
presence of light alone is not enough to trigger a photochemical reaction; the light must
exhibit the correct wavelength match to be absorbed by the reactant used 11. The second law
of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a
chemical system, only one molecule can be activated for the photochemical reaction to occur.
However, this fundamental principle applies to the photoinitiators with a low free radical
production efficiency, such as the camphorquinone system (CQ) 11. The ability to accurately
determine the number of photons, leading to a reaction, enables the efficiency or quantum
yield of the photochemical reaction 11. Thus, the use of LED-LPUs that can generate
adequate radiant power (mW) and spectral irradiance (mW/cm2/nm), which produces
sufficient polymerization, is a crucial factor for the clinical success of RBC direct restorative
materials 12.
The factors affecting the photopolymerization efficiency of RBCs are intrinsic,
related to the RBC composition, and extrinsic, related to the LPUs. The intrinsic factors are
co-monomer composition and filler-content ratio, photoinitiator type, and concentration.
While the extrinsic factors are the light spectrum, irradiation protocols, temperature, and light
guide tip positioning. It is imperative for clinicians to comprehend and optimize the RBC
photopolymerization process to maximize material properties. According to the results of a
recent study (Kopperud et al., 2017) the majority of Norwegian dental practitioners did not
demonstrate sufficient knowledge of this matter 13. Furthermore, the majority of the study
4
participants (78.3%) were unaware of the irradiance value of their LPUs. It was reported that
78.3% of dentists failed to perform the regular maintenance of their LPUs. The research
findings provide the foundation for many clinical practice guidelines developed to improve
dental care procedures 14. Although contemporary dental practice exhibits a substantial
increase in RBC restoration placements, the literature demonstrated that the longevity of
RBCs has fallen short in comparison to that of amalgams 15. Consequently, the practitioners'
selection of LPUs and RBC materials, as well as the latest guidelines to be followed, are
important detrimental factors. The literature demonstrated a substantial effect of LPUs
performance and clinician's polymerizations technique on the longevity of RBC restorations,
and this project attempted to characterize and quantify findings from a group of private
dental practices in the Greater Toronto Area and how these results correlate with published
studies.
1.2 History of Resin-Based Composites (RBC)
The introduction of silicate cement as a direct restorative material in 1871 was the
beginning of esthetic restorative dentistry 16. Silicates were noted for their anti-cariogenic
effects and remained the material of choice for restoring anterior teeth until the early 1950s
17. However, silicates had some undesirable physical properties, especially color instability
and quite a high rate of dissolution in saliva, which led to their discontinuation in the early
1960s. Acrylic resin (unfilled resin), another type of direct restorative material, was
5
developed in 1936, but was not used as a direct esthetic material until the late 1940s 17. It was
composed of Poly methyl methacrylate (PMMA) and has also been used in dentistry in
another form as a denture base material replacing vulcanite. Although unfilled resins offered
benefits over silicates, bonding was not as yet discovered 18. In the mid-1950s, Buonocore
was the first to introduce an orthophosphoric acid etching technique, which enabled the
formation of a stable bond between enamel and acrylic resin 18. Knock and Glenn patented
the first particulate-filled composite in 1951 20. However, their product lacked bonding
between the resin matrix and the filler particles.
Composite dental materials were developed in responses to the shortcomings of
silicate cement and unfilled resins. The breakthrough came in 1962 when Rafael Bowen
patented a new resin, Bisphenol glycidyl di-methacrylate (Bis-GMA). This material
resembled an epoxy resin except the epoxy groups were replaced by methacrylate groups 21.
Bis-GMA formulations could polymerize rapidly under oral conditions, and they had less
polymerization shrinkage than methyl methacrylate. Although Dr. Buonocore continued his
acid etching research with various resins, however, he achieved the best bonding results at
that time using the Bis-GMA resin 22, 23. In addition to Buonocore findings, Regenos 24 and
others further developed the acid-etching concept using Bis-GMA resin to repair fractured
incisors without drilling the dentin 25, 26. The birth of adhesive dentistry was recognised as a
consequence of Bowen's 1963 publication. Bowen successfully demonstrated that chemical
treatment of silica particles' surfaces with vinyl silane, within the Bis–GMA resin matrix
could produce a new direct restorative material with consistent mechanical properties and
clinical effectiveness. Bis–GMA has become the basis for all RBC materials used in
dentistry. Bowen worked at the U.S. National Bureau of Standards. Thus, he was in a
6
position to conduct testing in the areas of polymerization setting times, shrinkage, solubility,
degradation, water sorption, color stability, mechanical testing, and toxicity 27, 28, 29, 30, 31.
The early two-paste chemically-polymerized RBCs required manual mixing. The mixing
process was a technique-sensitive one. During the mixing, the mixture usually trapped air
bubbles that had unfavorable effects on mechanical properties after RBC’s polymerization.
Furthermore, these materials demonstrated excessive shrinkage, leading to sensitivity, and
recurrent decay. Due to their relatively large size filler particles, RBCs from that era
exhibited high wear rates 32. Over the course of 4 decades, the Bis-GMA chemistry has
improved considerably, yielding clinically acceptable RBCs. The particle size of the fillers
has been considerably reduced, and the resin chemistry further developed, allowing RBCs to
exhibit superior physical and mechanical properties, which extended their clinical longevity.
As time progressed, photopolymerized RBC materials replaced the early two-paste self-
polymerized ones in the 1970s. This development was regarded as a revolutionary step in
dentistry which allowed quick chairside polymerization.
The photopolymerization technology was adapted from the chemical industry and
modified for use in dentistry 33, 34, 35. The first LPUs were designed to emit ultraviolet light
(UV) 36. A 40s polymerization exposure was advocated, but 60s exposure provided enhanced
results 37. The common issues associated with UV polymerization of RBCs were: a limited
light penetration throughout the material (limited depth) and health hazards related to UV
light exposure on the operators' eyes 38, 39, 40. However, the rise of UV photopolymerization
was short lived when in 1976 Bassoiuny, at the University of Manchester, School of
Dentistry, placed the first visible light-polymerized RBC restoration. Again, the technology
7
was adopted from the Imperial Chemical Industries of England and modified for dental use
41, 42, 43.
The new visible light polymerization technology initially used a photoinitiation
system based on camphorquinone with a tertiary amine co-initiator, a system that was
enhanced over the years and became a template for new RBCs 44, 45. The light source used to
photopolymerize the newly developed RBCs was quartz–tungsten–halogen (QTH),
developed by the Johnson and Johnson Company. The new venture launched the FotoFil
restorative system and QTH light source unit 46. This technology enabled
photopolymerization of a 2 mm thick increment of RBC 47, 48. Since that time, efforts have
been directed to modify the filler particles of RBC to improve their mechanical properties.
The macrofill RBC had filler particles of about 10 µm that exhibited excessive wear 49. In the
1980s, microfilled RBCs were developed for the first time 50. Due to microscopic filler
particles (0.02 µm to 0.04 µm), microfilled RBCs demonstrated excellent polishing
characteristics. Furthermore, they were translucent, however, due to their inferior mechanical
properties, bulk fractures were common 51.
The concept of combining high strength and esthetic properties was accomplished
with the launch of the first hybrid RBCs in the late 1980s 49, 52. These materials had improved
strength and better wear resistance, however, the esthetic properties were inferior to those of
the microfilled RBCs 53. The next development in the search for an ideal direct restorative
material led to the creation of microhybrid RBCs in the mid-1990s. These materials had filler
particles that were smaller than those of hybrid formulation, with better multi-modal
distribution 54. The properties of these RBCs exhibited the strength of the hybrid RBCs with
significantly improved esthetic attributes 54, 55. Further improvements in the filler technology
8
have allowed nanofillers to formulate nanohybrid RBCs, which became available in the early
2000s. Nanohybrid RBCs contain nanometer-sized filler particles (0.005–0.01µm), along
with conventional fillers. They became the first genuinely universal RBCs with the handling
and esthetic properties of micro filled RBCs and the strength and wear resistance of
traditional hybrid RBCs 56.
The first generation of flowable composites, introduced in 1996, belonged to the
hybrid category of RBCs. A notable reduction in the percentage of fillers and a particular
modification of the resin matrix resulted in the low viscosity of the flowable RBCs 52.
Flowable RBCs were used as luting materials for indirect restorations for a long time.
However, they were reintroduced for direct restorative RBC materials in the late 2010s.
Following the success of nanotechnology and the incorporation of pre-polymerized
resin fillers, a new epoch for nanofilled RBCs began 57. These materials have been produced
with nanofiller and formulated with a nanomer® (Nanocor, Arlington Heights, IL) and
nanocluster filler particles 57. Nanomers® are discrete nano-agglomerated particles with a size
of 20 to 75 nm, while the nanoclusters represent loosely bound agglomerates of nano-sized
particles 58. The combination of nanomer® and nanocluster formations reduces the interstitial
spacing of the filler particles, which creates the ability to increase filler load with excellent
polishing capability 58.
Since the mid-2000s, there has been considerable research for an alternative resin
with less polymerization shrinkage and a higher degree of conversion 58. A newly discovered
resin based on dimer chemistry, which exhibited higher than 70% conversion, was launched
in the late 2000s 59, 60.
9
Following the Dimer based RBCs’ commercialization, a novel organically modified
ceramic (ORMOCER) was launched. Materials based on ORMOCER technology are
inorganic-organic hybrid polymers that form a siloxane network selectively modified through
the incorporation of organic groups 61. A distinctive feature of these RBCs is a blend of
polysiloxane groups with photo-polymerizable methacrylate groups covalently bonded to
silica. Because of its structural relation to silicate ceramic or glass, such materials are also
known as organo-ceramics. These materials have exhibited up to 50 % reduction in
polymerization shrinkage when compared to traditional RBCs 62, 63. The DC was reported to
be in a range from 95% to 99%, which is virtually identical to that of indirect restorative
RBCs 63. The new material exhibited a higher level of resistance to stain and hydrolysis.
Thus, it was able to maintain a high degree of luster for a considerable long time 64, 65. The
latest innovation that has happened with ORMOCER materials was the fusion of nanohybrid
and ORMOCER technology, which exhibited further improvements regarding initial
shrinkage, shrinkage stress, esthetics, and biocompatibility of the nano ORMOCER materials
66.
However, in the late 2000s, a different resin synthesized from oxiranes and siloxane
molecules, which became known as a Silorane RBC material, was developed 67. This new
RBC material was introduced as an alternative to conventional RBC because of its
hydrophobicity and lower polymerization shrinkage 68, 69, 70. When compared to
methacrylate-based resins, the cyclo-aliphatic oxirane functional groups exhibit less
shrinkage 71. Oxiranes are cyclic ethers that polymerize via a cationic ring-opening
mechanism, while methacrylate polymerization is achieved via a free-radical polymerization
process 70, 71. The distinctive feature of Silorane RBCs arises from the ability of the
10
cyclosiloxane backbone to impart hydrophobicity, while the cyclo-aliphatic oxirane sites
have high reactivity and less shrinkage than methacrylate RBCs 72, 73. Silorane RBCs
demonstrated improvements with delayed gel and vitrification points and reduced shrinkage
strain 74, 75. However, these materials tend to produce more heat during the cationic ring-
opening polymerization 76. Furthermore, Silorane RBCs exhibit the high initial flexural
modulus, which may explain its high polymerization stress values, despite the low volumetric
shrinkage (both post-gel and total) 77, 78.
Recently, a new class of RBC materials has been introduced to the dental profession
under the name of bulk-fill restorative materials, with the aim of simplifying and expediting
the restoration process. It was available due to the use of new proprietary resins, individual
modulators, and different filler particles 79. To reduce some of the disadvantages of
incremental layering techniques and to save some chairside time, the innovative bulk-fill
RBCs can be placed in a 4 mm single increment 80. The manufacturers’ claim that these
materials can be adequately photopolymerized in bulk has been confirmed by in vitro
infrared spectroscopy studies 81, 82, 83. More recently, flowable bulk-fill composites have been
developed 84. They have been used along with conventional RBCs in a two-layer
arrangement for esthetic considerations in posterior teeth. It was proven their use could lower
the polymerization stress, enhance the flow and handling, and improve adaptation to the
cavity walls, which resulted in substantial stress reduction at the cavity walls 84, 85, 86. The
bulk-fill flowable RBCs can be used to restore posterior teeth using 4 mm maximum
increments because this procedure does not reduce the mechanical resistance of the restored
teeth while rendering the entire procedure less technique sensitive and more efficient 87.
11
The most recent development in contemporary dentistry was the introduction of
flowable RBCs containing adhesive monomers. This new technology is based on enhanced
traditional methacrylate resin that incorporates acidic-monomer chemistry from bonding
agents, making them capable of interacting mechanically and chemically with tooth surfaces
in an adhesive manner 89, 90. Despite their limited use for direct restorations, they are
considered to be futuristic materials in the continued search for universal, self-adhesive RBC
direct restorative materials.
1.3 Classification of RBC
RBCs emerged during the past 50 years and with different formulations that have
been successfully used as restorative material luting agents, sealants, provisional materials,
and much more. The RBC material consists of three components: resin matrix (organic
content), fillers (mineral content), and a coupling agent. The features of the filler particle and
filler particle loading, are the most critical factors that influence the mechanical properties of
restorative RBCs 91, 92. The classification introduced by Lutz and Philips in 1983 divided the
RBCs into macrofills; microfills; and hybrids, which are mostly a blend of ground glass and
the microfill particles 93.
• Type I: Macrofilled RBCs
Type I, macrofilled RBCs, which were also known as conventional or traditional
RBCs, provide some historical perspective. The fillers were composed of amorphous silica or
quartz with relatively large particles from 1μm to 50 μm. Due to the relatively large filler
particles, however, they exhibited inferior wear resistance as a consequence of differential
12
wear, which led to rapid loss of resin compared with the filler. In addition, plucking of the
large filler particles from the restoration surface resulted in surface roughness and was more
receptive to stain and plaque accumulation. Because of these limitations, the use of these
materials by dental practitioners has rapidly declined.
• Type II: Microfilled RBCs
For type II, microfilled RBCs, traditional microfillers were made from fumed silica
prepared by a pyrogenic process, with an average particle size of 40 nm. The filler load in
these materials was low, however, it was increased by adding highly filled, pre-polymerized
resin fillers (PPRF) 94. Due to the increased surface area of these relatively smaller filler
particles, overall fillers percentage was 35 to 67 by weight 94. The microfilled RBCs
exhibited superior esthetic properties and for that reason, they were recommended for
restoring the anterior teeth and cervical abfraction lesions. However, they were contra-
indicated in heavy-stress-bearing restorations because of an elevated risk of bulk-fractures
and marginal chippings 94. These materials were truly nanocomposites, with an average
particle size of 40 nm. Although nanotechnology was not revealed in the 1980s, earliest
microfilled RBCs would be recognized as nanofilled RBCs today.
• Type III: Hybrid RBCs
Hybrid RBCs were developed to retain the advantages of both traditional macrofilled
and microfilled RBCs by combining the fillers of different particle sizes and multi-modal
particle distributions. Thus, the hybrid RBCs were typically filled with a blend of 40 nm and
1µm to 5 µm particles. These materials exhibited much better wear resistance compared to
13
traditional macro filled RBCs, but surface properties remained less than ideal because of the
natural wear pattern due to the presence of larger filler particles 95. In current terms, nearly all
RBCs are hybrids, with particle distribution of two or more size ranges. In addition, the
presence of amorphous silica has been used to improve handling properties of RBC
restorative materials.
A classification introduced by Willems and Lambrechts in 1992, which is
based on a filler volume fraction and filler size, categorized RBCs as: densified, microfine,
miscellaneous, traditional, and fiber-reinforced 96. Based on Young's modulus value for
dentin, a computed value of 60% filler load was established that further classified density
filled RBCs as either midway-filled (<60 volume percentage) and compact-filled (>60
volume percentage). Furthermore, each category has been further divided into ultrafine (<3
µm) and fine (>3 µm) RBCs, based on the mean filler particle size. The average particle size
of hybrids, microhybrids, and nanohybrids, is typically less than one μm, but more than 0.2
μm. The larger particle sizes can extend to well over one μm. The nanohybrids have some
particles in the nanofiller size range less than 100 nm, but they also contain particles in the
range (0.2 to 1 μm). When the surface of the RBCs is subjected to wear, the resin between
and around the particles is lost, leading to the creation of bumps and craters with subsequent
loss of polish at the surface of restorations 97, 98, 99. Innovations in the sintering process
enabled manufacturers to produce loosely agglomerated nanoparticles, i.e., nanoclusters. The
nanoclusters exhibited a significant structural difference in comparison with densified
particles 99. However, nanoclusters behaved similarly to the densified particles found in other
RBCs that enable a high filler loading. The new nanohybrid RBCs were formulated using
both engineered nanoparticle and nanocluster fillers. The nanocluster filler particles consist
14
of loosely bound aggregates of engineered nanofiller particles. The addition of engineered
nanoparticles to formulations containing nanoclusters, the reduction of the filler particles
interstitial space, results in higher filler loadings 99. The filled matrix (resin plus engineered
nanoparticles) is harder and more wear resistant than the resin alone. The increased filler
loading resulted in better mechanical properties and wear resistance 97-99. Therefore,
nanohybrids are considered to be the first genuinely universal RBC materials, with the
handling properties and polishability of microfilled RBC materials and the strength and wear
resistance of traditional hybrid RBCs 97.
Since their introduction, flowable RBCs have been universally accepted among the
clinicians, due to their remarkable versatility in a wide variety of clinical applications. From
a materials science standpoint, the flowable RBCs represent a subclass of microfilled RBCs.
The filler loading was reduced to 30% to 55% by volume, compared to 57% to 72% by
volume as in traditional RBCs 99. The amount of fluidity or flowability varied considerably
among the brands. Because of their physical properties, these materials are used in low-stress
areas, such as the small Class I occlusal restorations, Class V restorations, and for pit and
fissure sealants or conservative restorations 100. The rationale for their application in the
Class V carious lesions and abrasion/abfraction lesions is because these materials exhibit a
low modulus of elasticity and low viscosity. The use of flowables for an initial increment in
the proximal box portion of a Class II restoration is somewhat controversial 101. Despite the
many controversies associated with the use of flowables, these materials are often used as
liners under hybrid materials for an initial increment in the proximal boxes of the Class II
restorations 101, 102.
15
For many years, an incremental layering technique was followed when placing
posterior composite restorations. This procedure was deemed to offer the optimum
polymerization of the restorations. In the late 1990s, the packable RBCs were developed to
enhance incremental material placement 103. These products had high viscosity and contained
a much higher filler content. Manufacturers claimed that their handling is amalgam-like and
that their stiffness aided in forming good proximal contacts. However, the high viscosity of
these materials made the cavity adaptation quite challenging 103, 104. Furthermore, these
materials exhibited a lower degree of monomer conversion and reduced polymerization
depth. Even if the polymerization was acceptable, the clinical outcomes of high shrinkage
and polymerization stress became more prominent with thicker layers of RBCs 105. These
materials did not have high clinical success, and currently, they are no longer considered to
be the mainstream of the RBCs.
The concept of bulk-fill materials gained momentum as a result of further RBC
development. Through the use of innovative proprietary resins, alternative photoinitiator
systems, different fillers, bulk-fill materials were designed to photopolymerize up to 5 mm
thick increments 106. They exhibited low volumetric polymerization shrinkage and low-stress
build-up upon polymerization, when compared with incrementally placed conventional RBCs
106, 107, 108. Adequate polymerization was achieved by efficiently adding novel photoinitiators,
with higher quantum yield and polymerization modulators, which were responsible for
decreased shrinkage stress 109. Furthermore, increased translucency and the incorporation of
mixed oxide fillers with a refractive index matching that of the resin matrix or glass fibers
enhanced the light penetration, which resulted in improved photopolymerization 110, 111.
16
A further development in bulk-fill flowables has occurred by combining an
enhanced material formulation with a sonic delivery system 112. The sonic energy generated
by a particular handpiece causes a temporary reduction in RBCs viscosity, thus considerably
improving the ability of the material to adapt to the internal surfaces of the cavity 88. By
ending the sonic energy from the handpiece, the RBC gradually returns to its original
viscosity before it is photopolymerized and finished using traditional techniques 88.
According to the manufacturers’ claims, the material exhibits polymerization shrinkage of
approximately 1.6% and can be adequately photo-polymerized to a depth of 6 mm in a single
increment 88.
The critical breakthrough in the field of restorative materials occurred with the launch
of advanced giomer restoratives. This class of restorative materials has the distinguishing
feature of a stable glass ionomer core in a protective resin matrix 113. The different filler,
Surface Pre-Reacted Glass (S-PRG), gives the material a composite-like feature in addition
to a significant release of fluoride ions 114, 115. Studies have shown that these materials, in
both low and high viscosity consistencies, were able to achieve adequate polymerization, had
excellent mechanical properties and demonstrated low polymerization shrinkage when
compared with conventional RBCs 116, 117.
Recently, increased use of bioactive restorative materials has been noticed as these
products promise untapped benefits for long-lasting restorations. The idea of bioactivity of
dental materials was proposed by Hench 118 in the late 1960s, with the following definition:
“A bioactive material is one that elicits a specific biological response at the interface of the
material which results in the formation of a bond between the tissues and the material.” The
concept of bioactive restorative dental material with the capacity of adhesion to tooth
17
structure and the fluoride-releasing ability has been widely used in dentistry for decades.
However, the current concept of bioactivity is defined as the ability of the restorative
material to develop a surface deposition of mineral apatite in the proximity of the restoration
margin in the presence of an inorganic ions solution 119. In 2014, the Activa™ Bioactive
(Pulpdent, Watertown, MA) was launched, and that event changed a paradigm in the arena of
bioactive types of cement and restoratives. Activa represents the first bioactive dental
material with an ionic resin matrix (Embrace resin), a shock-absorbing resin component, and
bioactive fillers that mimic the physical and chemical properties of natural teeth 120. The
ionic resin contains phosphate acid groups. Through an ionization process that is dependent
upon water, hydrogen ions break off from these groups and are replaced by calcium in tooth
structure 120. This ionic interaction binds the resin to the minerals in the tooth, forming a
robust resin-hydroxyapatite complex and a positive seal against microleakage, hence, the
need for a separate bonding system is eliminated 120. The Activa exhibits higher resistance to
fracture and chipping than any other dental restorative material due to a patented rubberized-
resin component 120, 121. In the oral environment, this product responds to pH cycles and plays
an active role in release and recharge of substantial amounts of calcium, phosphate, and
fluoride 120. The presence of these beneficial ions that supersaturate dental pellicle and saliva,
enables hydroxyapatite or fluorapatite precipitation at favorable pH, mimicking the natural
tooth behavior 120. Due to its patented resin formulation, Activa contains no Bisphenol A
(BPA), Bis-GMA, or BPA derivatives 120. At the present moment, this class of bioactive
materials is poised to offer substantial benefits to high-caries-risk patients and those with
proven allergies to BPA derivatives in conventional RBC products, at a reasonable cost.
Although results of in vitro and short-term clinical studies presented favorable results, there
18
is an urgent need for quality long-term clinical studies to confirm the advantages of bioactive
restoratives over currently used restorative materials.
1.4 Classification of Bonding Agents
Since Dr. Michael Bounocore's work has revolutionized the world of adhesive
dentistry, significant developments have taken place, which has contributed to an improved
understanding of factors affecting the durability of adhesive resin restorations 22, 23. The
complexity of dentin-resin interfaces demands in-depth knowledge of all mechanical,
physical, and biochemical aspects that collectively play a pivotal role in the adhesion of
RBCs to the tooth structures. The ionic and hydrophilic nature of modern dental adhesives
yields porous, unstable hybrid layers that are susceptible to water sorption, hydrolytic and
bacterial degradation, and the resin by-products leakage 122, 123. The hydrolytic activity of
host-derived proteases also contributes to the degradation of resin-dentin bonds. Preservation
of the collagen matrix is critical for the improvement of resin-dentin bond durability 124.
Based on experimental results and clinical verification, enamel bonding is very predictable
and reliable 125. However, dentin-bonding procedures are not that predictable. This situation
is not surprising since dentin contains 17 % collagen by volume encapsulated into the
hydroxyapatite structure. Thus, the micromechanical retention can be achieved by utilizing
the pores of dentinal tubules 126. These tubules contain fluid, which may prevent adequate
bonding. Numerous factors influence the bonding mechanism: the number and direction of
19
tubules, the presence of cementum, the age of the tooth, the type of dentin, and the pulp
approximation 127.
Nakabyashi proposed the removal of the mineral phase of the surface layers of dentin
by acid etching, which exposes the dentinal collagen matrix as a bonding substrate 128. The
subsequent penetration and interaction of the hydrophilic monomer within the porous tissue
substrate creates a dentin-resin hybrid layer that represents a significant milestone in dentin–
adhesive bonding 129. The hydrophilic nature of this matrix prompted early researchers to use
monomers with both hydrophilic and hydrophobic groups for improved adhesion. The
rationale was that hydrophilic functionality would help promote penetration of the monomer
into the collagen matrix, leading to the formation of a collagen-resin hybrid layer, whereas
the hydrophobic groups would enhance bonding to the hydrophobic resin matrix of the RBC
restoration 129.
The presence of a smear layer was another hurdle in the dentin bonding process. The
smear layer represents debris that is left untouched on the dentin surface after cavity
preparation. Etching the dentin surface removes the smear layer and provides a predictable
substrate for bonding 130. The enamel conditioning by the acid enhances the removal of the
smear layer and facilitates the creation of a honeycomb structure receptive to bonding 131.
The flow of adhesive into this porous substructure improves the creation of the resin
tags at the enamel-restoration interface for improved micromechanical bonding. In contrast,
the etchant on the dentin surface removes the mineral phase and smear layer, which exposes
the collagen fibril network 132. This network appears to be a very complicated structure,
wherein the naturally wet condition keeps it from collapsing, thereby allowing adhesive
permeation into the substrate for adequate bonding. Research findings indicated that a
20
slightly moist environment at the collagen fibril network enhances the dentin bonding, which
was referred to as wet bonding 131, 132, 133.
Bonding systems have evolved through several generations with improvements in
their chemistry, reduction in the number of steps, simplified application techniques, and
enhanced clinical effectiveness. New bonding agents were developed to combine one or
more steps of etching, priming, and bonding, to reduce standard errors in an already
technique-sensitive process. Thus, current bonding agents represent three categories:
• Three-step systems, whereby the etching (and rinsing), priming and bonding steps are
carried out separately, are also known as total-etch systems.
• Two-step systems that include two different steps:
A. Etching and rinsing step followed by application of a self-bonding primer.
B. Application of a self-etching primer followed by a bonding step.
• Single-step systems, which are composed of self-priming, self-bonding adhesives
where the etching, priming, and bonding are merged into a single step.
In the late 1980s, a new generation of bonding systems was introduced, with the
premise of mildly acid etching of the dentin to partially remove and modify the smear layer
129. These systems utilized a phosphate primer to modify and to soften the smear layer. Due
to limited penetration of the primer, bonding to smear layer did not reach a clinically
accepted level 130.
In the early 1990s, complete removal of the smear layer was accomplished with a
new generation of bonding agents. These systems use the total-etch technique, which
21
involved the simultaneous etching of enamel and dentin with phosphoric acid. The system
facilitated the establishment of resin tags into tubules and adhesive lateral branches for an
enhanced bonding process, while the resin tags sealed the tubule orifices firmly 134. The total-
etch systems exhibited excellent bonding results, which have become a gold-standard for
dental bonding.
The next generation of bonding agents was introduced in the mid-1990s. The
bonding systems from this generation met demands of the profession to simplify the clinical
procedures and to prevent the collapse of the collagen after the acid-etching of the dentin
surface. It consisted of two distinct types of adhesive systems. A single bottle system
combined the primer and adhesive into one solution to be applied after etching the enamel
and dentin. This system exhibited high bond strength to enamel and dentin by
micromechanical interlocking with the formation of the hybrid layer, resin tags and adhesive
lateral branches 133, 134, 135. The next generation of bonding employed another type of bonding
agent, known as a self-etching primer, which was the combination of etching and priming
steps in one solution (phenyl-P and HEMA). However, in comparison to the one-bottle
system from the same generation of bonding agents, the bond strength of this method seemed
to be less predictable 135. Moreover, the self-etching primer system resulted in clinically-
relevant higher microleakage than what is performed with the one-bottle system 136. The
bonding agents of the next two generations eliminated the need for etching with phosphoric
acid by incorporating an acidic primer.
In the mid-1990s, two ingredients: an acidic primer and wetting agent and an unfilled
bonding agent were combined. Self-etch adhesives, which comprised an aqueous mixture of
acidic functional monomers that were phosphoric acid esters, did not require a separate acid-
22
etch step and subsequent rinsing. This generation of bonding agents had two unique types of
specific two-bottle systems. Type I consisted of self-etching primer and adhesive. This
system required the application of self-etching primer first, which is followed by an adhesive.
Type II consisted of two bottles or unit doses containing acidic primer and adhesive. In this
case, a drop of each liquid was mixed and applied to the tooth. Since the etching and bonding
procedures were almost instantaneous, there was minimal potential for postoperative
sensitivity. However, these bonding systems showed lower enamel bond strength, which
prompted some manufacturers to advocate separate enamel acid conditioning before the
application of self-etching adhesive 136.
The next generation of bonding systems was introduced in the mid-2000s. This
generation of bonding agents, simplified bonding procedures, increased bond strength, and
significantly reduced microleakage. Using a single-step technique lessens the clinical
application time, but also notably reduces technique sensitivity. Adhesion is consequently
obtained micro-mechanically through shallow hybridization and by the supplementary
chemical interaction of particular carboxyl/phosphate groups of functional monomers with
remaining hydroxyapatite 137.
The latest advances in dental bonding systems were launched in 2012 in the form of
universal bonding systems. These single-bottle, multi-mode bonding systems are designed to
bond to tooth structures in either the etch-and-rinse or the self-etch mode, depending on the
clinician's preference. According to the adhesion-decalcification concept proposed for self-
etching adhesives, aggressive demineralization of hard tissues by strong acids will result in
the dissolution of apatite crystallites. Demineralization reduces the opportunity to establish a
chemical bond between functional adhesive resin monomers and hydroxyapatite and also
23
reduces the potential for creating nano-layers of calcium precipitates with phosphate resin
monomers with extended spacer groups 138, 139. Research has shown that universal adhesives
with the acidic resin monomer 10-MDP (10-Methacryloyloxydecyl dihydrogen phosphate),
which has been acknowledged as a gold-standard functional monomer, responsible for
chemical bonding to hydroxyapatite, demonstrated improved bond strength, increased
resistance to hydrolytic degradation, and reduced nano leakage 140, 141.
The ideal bonding system should be biocompatible, have an excellent bond strength
to dentin, be resistant to degradation and be simple to use. Unfortunately, even though
extensive research has been achieved in the past 50 years of adhesive dentistry, the ideal
bonding system has not been realized as yet. However, it is crucial to understand the nature
of bonding to see the clinical implications of bond failure, which ultimately lead to failure of
RBC restorations. In a typical case with weak bonding to dentin, the low bond strength is not
sufficient to resist the forces of RBC polymerization shrinkage (about 24 MPa in Class I
cavities). Thus, one or more of the bonded walls could exhibit some level of separation and
gap formation, which can create bacterial leakage through the permeable dentin, resulting in
the pulp irritation 142.
1.5 Evaluation of Photopolymerization Efficiency
Light-polymerization units (LPU), introduced into dental practice in the 1970s,
paved the way to simplified restoration procedure. A large number of studies has been
conducted on several types of light sources, including quartz–tungsten–halogen (QTH) and
24
light-emitting-diodes (LED) 305-317. In today's contemporary dental office, LED LPUs are the
primary source of photopolymerization. In the majority of LED LPU light-spectrum ranges
from 400 nm to 515 nm, with a high peak around 468 nm, referred to as monowave LED
LPU, the light spectrum which is the most efficiently absorbed by a combination of
camphorquinone (CQ) and a tertiary amine (Type II system) 97, 143. On the other hand, LED
LPUs that are capable of generating more than one peak, are referred as multiwave
(Polywave®) LED LPUs, capable of activating the alternative photoinitiators, in addition to
the CQ systems. The Polywave® technology is registered trademark of Ivoclar-Vivadent. The
light spectrum of multiwave LED LPUs has a high peak around 470 nm and secondary peak
around 400 nm depending on the manufacturer's specification for the coverage of alternative
photoinitiators (Type I systems) 97, 143. For example, phosphine oxide derivatives diphenyl (2,
4, 6-trimethylbenzoyl) phosphine oxide (TPO) and phenylbis (2, 4, 6-trimethylbenzoyl)
phosphine oxide (BAPO), like any other Type II photoinitiator system, do not require a
tertiary amine for generating free radicals 143. The TPO high peak absorption is around 400
nm and features much higher absorption efficiency and quantum yield 143, 144. It has been
reported that TPO is very efficient visible light photo-sensitizer, comparable to CQ for the
initiation of RBC photopolymerization 145. However, there are other commercially available
photoinitiators such as phenylpropanedione (PPD) and Ivocerin®, incorporated into various
formulations of commercially available RBC materials. The physical properties of
polymerized RBCs are affected by the generation of primary radicals during the initial stage
of polymerization 143. Thus, free radicals created in this process, attack the carbon double
bonds of the monomers, and rapidly convert the resin to a cross-linked three-dimensional
polymer network 143. Ideally, during the polymerization process, all monomers should be
25
converted into polymers. However, in clinical settings as well as under laboratory conditions,
not all monomers are polymerized. Also, the monomer molecules exhibit loosely bonded
configuration by weak Van der Waals force. Following the polymerization, the monomers
are tightly linked by covalent bonds into a polymer with a smaller distance between the
molecules, which is the principal factor for polymerization shrinkage 97. Inadequately photo-
polymerized RBCs exhibit inferior mechanical and physical properties, which ultimately lead
to compromised longevity of the restoration. Research has shown that degree of conversion
(DC) is a co-determinant of mechanical properties of restorative RBC materials 146.
Furthermore, the literature has also shown a significant correlation between DC and
hardness, modulus of elasticity and flexural strength of RBCs 147. A large number of factors
can influence the effects of photopolymerization. The filler particle size, geometry, and filler
load, polymeric matrix, refractory indices, radiant exposure, and a distance from the light
probe to the RBC surface. In addition, the polymerization protocol can also influence the DC
of RBCs and thereby their mechanical properties 148, 149. The recent photopolymerization
studies support the assumption that LPUs irradiance and exposure time independently affect
the mechanical and physical properties of RBCs 150, 152, 153. Adequate polymerization of
RBCs is needed for desired physical and mechanical properties, biocompatibility, and
longevity 154, 155.
1.5.1 Degree of conversion
During the RBC photopolymerization reaction, the radiant exposure (RE) and
irradiance are principal factors that influence the degree of conversion (DC) and ultimate
clinical success of the restorations 156-160. The exitant irradiance (mW/cm2) represents the
26
flux incident over the total surface area of the light probe, while RE (J/cm2) is the product of
the exposure time and the incidence irradiance at the RBC surface 280. During the free-
radicals photopolymerization, double bonds in the resin of the RBCs are converted into
single covalent bonds, enabling converting monomers into the linear or cross-linked
polymers 196. Thus, the DC represents the ratio of converted single bonds to a total number of
remaining double bonds. Aforementioned is an important determining factor for
polymerization kinetics, which is highly correlated to mechanical, physical, and chemical
properties of RBCs 159,160. Fourier transform infrared (FTIR) and Raman spectroscopy are
broadly used for characterizing the DC of polymerizable RBC materials 161. Both quite
similar techniques produce molecular changes in vibrational energy state by subjecting them
to excitation radiation in selected spectral regions 161. However, FTIR and Raman
spectroscopy differ in the means by which photon energy is transported to the molecules.
Thus, FTIR is an absorption technique, while Raman spectroscopy represents a scattering
method. Since the bands in FTIR spectra are due to polar functional groups while the bands
in Raman spectra are due to nonpolar functional groups, FTIR and Raman spectroscopy are
complementary techniques 161. The process of measuring DC (methacrylate-based RBCs) is
mainly based on comparing the frequency band (1634 cm−1), which corresponds to C=C
vinyl group stretching vibration and those of an internal standard frequency band (1608
cm−1). That frequency band corresponding to very stable aromatic rings during the
polymerization process, to those frequencies of an unpolymerized RBC material 161. Even
though FTIR and Raman spectra are parent techniques, some studies are more efficiently
conducted in FTIR than in Raman spectroscopy, depending on the chemical formulation of
tested materials 161.
27
The literature showed that differential thermal analysis (DTA) using a split fiber-optic
light source was a convenient and a valuable method for investigating polymerization
performance of light-activated RBCs 162. However, this approach introduced by McCabe
showed some drawbacks with different composition of commercial RBCs, empowered with
nano-technology 162. The literature revealed that the most meaningful evaluation of DC of
dental resin materials is obtained by utilizing FTIR and Raman spectroscopy. However, the
relative microhardness (bottom/top ratio) can provide reasonably accurate estimates 163.
Rueggeberg and Craig reported that microhardness values exhibit more sensitivity in
identifying minuscule changes in the DC after the network is cross-linked, in comparison to
FTIR 164. The literature has shown that radiant exposure (J/cm2) and irradiance (mW/cm2) of
the LPUs are highly correlated to DC of RBC materials 164. However, the study conducted by
Selig showed that high irradiance values had a significantly high effect on DC, compared to
time increase of RE 165. The high irradiance could enable a more substantial number of free-
radicals due to a higher rise in exothermic heat. The subsequent temporal viscosity of a
polymeric matrix decreases, allowing the polymer chains increase mobility, consequently
resulting in a much higher DC 165, 166, 167. Thus, the concept of exposure reciprocity law,
which proposes the reciprocity between the incident irradiance and exposure time, suggests
that observed DC values will be similar if the same amount of radiant exposure is delivered
to the RBC surface, however, it is not applicable to the majority of contemporary RBCs 152,
155.
28
1.5.2 Degree of crosslinking
Dimethacrylate monomers produce a highly crosslinked network after induced
photopolymerization. Glass transition temperature can indirectly measure the degree of
crosslinking, while the frequently used softening test can also determine the degree of
crosslinking in a relatively simple way 168. The degree of crosslinking, which is closely
related to the function of the DC, is an influential determinant of photopolymerization
efficiency, which has clinically significant implications for the mechanical properties and
ultimately the longevity of RBC restorations 168. During the photopolymerization reaction,
the conversion of C=C double bonds into primary covalent C-C bonds, which allow high
mobility and free rotation of polymer chains results in a highly crosslinked structure.
However, the double bonds conversion is never complete and always contains considerable
quantities of unreacted double bonds 169. Therefore, these residual double bonds can
influence the degree of crosslinking, by reacting with propagating radicals to form a cross-
linked, primary, or secondary cycle 169. The primary cycle is created when a radical reacts
with an unreacted double bond on its kinetic chain, while the reaction of the free radical on a
different kinetic chain, which becomes crosslinked, is called a secondary cycle. The primary
cycles create microgels and lead to heterogeneity in the polymer network, with co-existence
of loosely crosslinked and more highly crosslinked microgel regions. The formation of the
cycles enhances a higher local conversion rate, with almost no influence on the chain
mobility 169.
The literature reports a strong correlation between the degree of crosslinking and
fracture toughness, elastic modulus, solvent resistance, and glass transition temperature of
RBCs 168, 171. Studies have shown that polymerization modes had an essential influence on
29
the DC and mechanical properties of RBCs. Thus, at the same time, polymerization modes
have an impact on a degree of crosslinking which subsequently lead to the formation of more
linear polymer networks 168-171. The pulse delay or slow start mode has often been correlated
with the creation of a polymer structure with a lower degree of crosslinking, which is more
affected by an organic solvent attack during the softening tests 172. Furthermore, a slow start
polymerization technique generates a few cycles per area, which creates a more linear
polymer structure with relatively few crosslinks. In contrast, a rapid start continuous
polymerization technique creates a multitude of growth cycles per area, resulting in a
polymer with a higher degree of crosslinking 172, 173.
These results supported the sentiment, previously seen in DC that incident irradiance
and not radiant exposure has a tremendous influence on the degree of crosslinking 154,155. The
increased concentration of crosslinks has been correlated with improved physical properties
and stability of polymers 168. It has been established that DC does not provide a complete
characterization of polymer structures, due to polymers with similar DC may exhibit a
different degree of crosslinking 168, 171. The assessment of the degree of crosslinking can be
performed by measuring a resin glass transition temperature, using the Arrhenius plot 175.
Within the dimethacrylate monomers, a degree of crosslinking influences the reduction of
molecular mobility, which increases the transition temperature 175.
The characterization of the crosslinked network structure presents a challenge due to
the structural heterogeneity that develops in the polymer during photopolymerization.
Although some studies have used the ethanol solution softening test, a hardness measurement
is taken before and after ethanol immersion, as an alternative method of evaluation of the
degree of crosslinking. Furthermore, some researchers advocate acetone solutions for
30
softening test 176-178. It has been recognized that crosslinked methacrylate networks swell
when exposed to organic solvents. Apparently, the covalent bonds forces between the
polymer chains are exceeded by the forces of attraction between solvent molecules and
components of the chains. Thus, a higher degree of crosslinking is fundamental for better
mechanical properties and enhanced resistant to solvents 178.
1.5.3 Mechanical properties of RBC materials
The relationship between mechanical properties of RBCs and their intraoral
performance has been a subject of numerous studies. For instance, Ferracane showed that
mechanical properties such as Fracture Toughness (FT) and Flexural Strength (FS) of RBCs
are strongly correlated to the clinical performance of RBC restorations 179. Likewise, a strong
inverse correlation exists, between wear and FT, and wear and FS, and a weaker inverse
correlation between wear and hardness 180. Moreover, for the quicker and meaningful
characterization of mechanical properties, the identification of one parameter can lead to the
discovery of other properties, based on these correlations 180, 181, 182. For example, by
obtaining hardness data and retrieving the correlation between hardness and elastic modulus
(EM), it could be possible to estimate the EM of any RBC material 183. However, comparing
results of different studies and the variability among them makes it difficult to make a
meaningful conclusion, due to the differences in their experimental methodology. For
example, the FS values are usually enhanced when a higher cross-head speed of the universal
testing machine is applied 184.
31
The effect of filler (load, composition, size, etc.) on mechanical properties of RBCs
has been widely demonstrated. It has been observed that as a result of increased filler
content, the modulus of elasticity and surface hardness increase accordingly, while
volumetric shrinkage decreases 183,184. Some researchers are considering the more biomimetic
approach, formulating RBCs with the closest possible mechanical properties of dentin, in
which Young's modulus (20–25 GPa) and ultimate tensile strength (UTS) values in the range
of 52–105 MPa 185. According to ISO 4049 international standards requirements, FS and EM
are to be measured by a three-point bending test on 25mm×2mm ×2mm sample 186.
However, these standardized dimensions are not clinically realistic, considering the teeth
diameter and length average. Due to the overlapping irradiation required to polymerize the
entire portion of the specimen, may render the reliability of this testing procedure somewhat
questionable 187. The FT is an important mechanical property, which characterizes the
relative resistance to crack propagation from the surface or inherent flaws in the RBC
material 188. Innovative technologies currently employed, such as fiber or whisker
reinforcements have produced very significant enhancements in FT of RBCs, however, not to
the level of high toughness ceramics or casting alloys, and this may be what is required to
render the materials substantially fracture resistant in the clinical settings 189.
The testing procedure of FT and FS are similar, with the exception that FT specimens
have a notch, and the same overlapping irradiation protocol is applied with this testing
method. Diametral tensile strength test (DTS) has been designed to test brittle materials by
utilizing a cylinder-type specimen that is subject to compressive load across its diameter. It
has been observed that materials which exhibit plastic deformation would produce false DTS
values 16. Although DTS test is not a part of the ISO 4049:2009 standards, it was recognized
32
as technically reliable and a straightforward test for the assessment of RBC mechanical
properties 189. However, DTS test can provide useful information about the effectiveness of
RBC polymerization and in conjunction with other measuring methods can provide
meaningful data on the mechanical properties of given materials 189. Vickers and Knoop
microhardness tests are routinely employed for testing of RBCs' mechanical properties.
Hardness (H), represents an important parameter in the characterization of the surface
properties of the material on a microscopic scale, which is established as the ratio of the
applied load (P), to the contact area (A), which caused subsequent indentation impression:
H = 𝑃
𝐴 = β
𝑃
𝑑2 (1)
Where d represents the size of indentation, and β represents a constant related to the
geometry of a given indenter 318, 319. Vickers and Knoop are the most frequently employed
microhardness tests for characterization of RBC materials by utilizing a pyramid-based
indenter. The Vickers hardness number (VHN) is determined by using the contact area of the
indentation impression and the parameter d in Eq. (1), which is described as an average of the
two diagonals of the resultant square-shaped impression 320. Thus, the β constant for VHN
calculation is 1.8544. The Knoop microhardness test uses a pyramid-based indenter with the
length to width ratio of 7:1 and with corresponding face angles of 172.5 degrees for the long
edge and 130 degrees for the short edge. The Knoop hardness number (KHN) is calculated as
applied load (measured in kilograms-force) divided by the square length of the long diagonal
of the indentation in mm, and the constant of indenter associating the projected area of the
indentation to the square of the length of the long diagonal, equal to 0.07028 321.
33
Although the measurement of a residual surface plastic impression is the basis for the
hardness measurement, however, this surface property test represents an elastic-plastic
surface deformation process. Thus, the relative importance of reversible (elastic) and
irreversible (plastic) components of surface deformation induced by the hardness indenter
could be evident in the elastic recovery during unloading of the indenter. Therefore, this
elastic depth recovery following the maximum indentation should be taken into
microhardness calculation. The literature has shown, especially concerning RBC materials,
the inequality between the plastic zone beneath the indentation and the surrounding elastic
matrix exists. The elastic recovery is highly related to the surface geometry of an indenter,
which differentiates the elastic recovery of Vickers indentation in comparison to Knoop
indentation. The elastic recovery attributed to Vickers indentation usually develops along the
depth of the indentation, while the diagonal length of the surface impression remains
unchanged, in contrast to the Knoop indentation, where the elastic recovery appears as a
result of a minor diagonal contraction of the indenter 319, 320. The Knoop microhardness
computation takes the length of the longer diagonal only into the calculation of KHN, thus
making this method preferred for its reliability over other methods 318, 319, 321. A linear
correlation exists between Knoop microhardness and the mechanical properties of the RBC;
and Knoop microhardness and the irradiance of LPUs 180, 183, 190.
34
1.5.4 Shrinkage and shrinkage stress
Despite significant developments in the field of adhesive dentistry, the occurrence of
RBCs’ polymerization shrinkage and shrinkage stress remains a clinically relevant issue 192.
During the polymerization reaction, the distance between monomer molecules becomes
reduced by the transitional changes from loosely bounded Van der Waals forces, into tightly
linked covalent bonds of the polymer network, which produces the shrinkage stress. Cavity-
restoration interface exhibits the maximum shrinkage stress force, with degradation of that
interface resulting in subsequent microleakage and gap formation 193. The consequences of
gap formation may increase the possibility of tooth hypersensitivity and an adverse pulpal
reaction, with a high probability of detrimental effects on restoration longevity.
Stress represents a complex interaction among the following: the resin viscosity, the
volume shrinkage, the polymerization rate, the DC, modulus development, and network
structural evolution 193. However, each of these properties cannot be individually
manipulated and studied without having a significant impact on the others. Thus,
polymerization shrinkage stress is multifactorial, being related to cavity configuration and
RBCs properties.
The cavity configuration properties are defined by the geometry and size of the
preparation and its C-factor. The C-factor, or cavity configuration factor, represents the ratio
of bonded to non-bonded preparation wall areas, which have also been a subject of intense
debate in the literature 193. Empirically, the higher C-factor, the higher the stress level
generated. According to the literature, when a rigid (non-compliant) testing system is
utilized, a direct relationship between the contraction stress and C-factor value was reported,
while results of the studies using a less rigid, more compliant set-up, showed an inverse
35
relationship 194,195. However, from a clinical standpoint, the C-factor seemed to be a valid
parameter for the comparison of restorations with identical shapes and volumes, as reported
in experimental settings 196.
The material properties associated with RBCs’ polymerization shrinkage stress
include filler content, resin-matrix composition volumetric shrinkage, and elastic modulus
and viscous flow 197. The literature has shown that RBCs with a high filler content exhibit
low volumetric shrinkage and high stiffness 195-197. Furthermore, strain capacity is inversely
correlated to the filler content of RBCs 198. For instance, microfilled RBCs usually
experience lower elastic modulus, a higher strain capacity and similar volumetric shrinkage
properties in comparison with hybrids, due to their relatively lower filler volume percentage,
which is in a range from 35% to 50% 199.
Viscoelastic properties define another essential factor in contraction stress
development of RBCs during photopolymerization, the flow capacity at initial stages of the
polymerization, and the elastic modulus generated during the polymerization reaction 200.
Composite flow is influenced by the structure of individual molecules, crosslinking, the
filler/matrix interfacial characteristics, reaction kinetics, and the C-factor. Flow can happen
quickly when the RBC is allowed to shrink, which means that flow can be increased when
the preparation C-factor is low 201. Polymerization shrinkage is positively correlated to the
DC. Thus, an increase in the DC leads to higher polymerization strains because more
covalent bonds and more highly cross-linked networks are formed 202. The higher DC values
of RBCs correlate with improved mechanical and chemical properties, color stability and
biocompatibility. Therefore, the stress reduction cannot be accomplished at the expense of
adequate DC.
36
Various methods are aimed to measure polymerization shrinkage stress. Shrinkage
strain-rate and stress are measured during the polymerization reaction with a tensiometer,
based on the cantilever beam deflection theory 203. Various devices and methods have been
used for the measurement of polymerization shrinkage regarding volumetric and linear
shrinkage and the cuspal displacements. The reasonably straightforward, indirect techniques
to measure shrinkage include the microleakage assessment test and finite element analysis,
which utilize a three-dimensional micro-CT data 203. For maximum accuracy and clinical
relevance, the instrument compliance should be as close as possible to the features of a
natural tooth 203. Clinical performance of Bis-GMA based RBCs could be enhanced by
maximizing the DC and elastic modulus, decreasing volumetric shrinkage (maximizing
viscous flow), and by minimizing the polymerization stress 203. Another stress reduction
mechanism readily available is to increase the compliance of the preparation walls by
applying an intermediate, low-modulus layer, using a low-shrinkage flowable RBC material
203.
1.5.5 Depth of polymerization
Achieving sufficient photopolymerization of RBC materials is a critical parameter to
maximize mechanical properties of the restoration and to obtain a desirable clinical outcome.
The depth of polymerization (DP) of light-activated RBCs depends on the filler composition
and resin formulation, resin shade and translucency, photoinitiators content, the intensity and
spectral distribution of the LPU, and finally on the irradiation protocol. There are two DP
assessment techniques: direct and indirect. The indirect method for DP assessment includes
scraping tests 204, the dye uptake tests 205, and the surface microhardness tests 206. The direct
37
method measures the polymerization efficacy by utilizing spectroscopy, as a part of the DC
assessment, which has not been routinely used due to the cost associated with this
complicated methodology.
The simple scrape test has been modified and became a part of ISO 4049 standardized
testing procedures 186. The test utilizes a 4 mm diameter stainless steel mold, filled with the
RBC, and polymerized with the LPU from a single side only. At the non-irradiated side of
the specimen, a plastic spatula is used to scrape any material that is soft (inadequately
polymerized), leaving remaining hard and apparently polymerized material. The measured
remaining thickness of the RBC specimen is divided by 2 and is referred to as the DP scrap
test value, expressed in mm. This technique, while simple, it has several disadvantages. The
method does not provide an accurate estimate of the polymerization quality in any region of a
given material, especially in the deeper layers adjacent to the soft, slightly polymerized resin
which has been removed. The results of some studies found that scraping methods severely
over-estimated DP as compared to relative hardness testing or DC analysis 180. The results
verified that DC appeared to be the most sensitive test for DP. A good correlation exists
between the Knoop microhardness testing and infra-red spectroscopy 158-163. Microhardness
testing is the most popular method for investigating factors that influence DP, because of its
relative simplicity and accuracy.
38
1.5.6 Biocompatibility
Although RBC products are known to be highly stable structures, they are susceptible
to degradation, because of insufficient polymerization. Several components could be released
from RBC restorations into the oral environment 207. Thus, biocompatibility is not a direct
way to assess the photopolymerization efficiency. However, it is an essential prerequisite of
all materials that are placed into a biological system to be bio-friendly to the environment.
RBCs can affect the oral cavity in at least two ways: through the elution of bioactive
substances and by a quick and sharp temperature increase during polymerization. Sufficient
polymerization renders superior mechanical, physical, and chemical properties of RBCs,
which prevents and limits leaching of potentially harmful components from the materials 207.
The cytotoxicity of RBCs has been attributed to the leaching of unpolymerized
monomers, such as products of an incomplete polymerization and consequently the by-
products of the resin matrix degradation processes in the oral cavity 208. The most commonly
used monomers in RBCs compositions include Bis-GMA, TEGDMA, and UDMA, all of
which have been identified as cytotoxic molecules 209, 210. Further, literature has shown
particular concern with TEGDMA, which seems to be a mutagenic compound in mammalian
cell assays. It appears that these cytotoxic molecules induce gene mutations, by covalent
binding to the host DNA 211, 212. Several factors influence the release of unbound substances
from polymerized RBC materials 213. The literature shows that DC values are highly
correlated with the amount of leachable components from any given RBC material 213. The
solvent composition, which is applied for the extraction, affects kinetics and mechanism of
the elution process 213. The chemical characteristics, as well as the size of leachable
39
substances, moderate diffusion through the polymer network. The current concept presumes
that smaller molecules have enhanced mobility and exhibit a faster elution rate 213.
Chemical degradation of RBCs can be attributed to hydrolysis and attack of salivary
enzymes in the oral cavity 213. Studies showed that percolated monomers enhance bacterial
growth and are responsible for glutathione depletion, which is a critical factor in pulp or
gingival cell apoptosis and production of reactive oxygen species (ROS) 214. Additionally,
leached substances are associated with a variety of allergic reactions 214. Therefore, the
toxicology results from the cell cultures have revealed that monomers released from the
RBCs generate ROS and, therefore, affect the redox balance in the oral tissues cells 208-212.
The occurrence of diffusion of small molecule monomers throughout the polymer network is
more easily accomplished, due to their small and flexible structures, polar ends, and flexible
and smaller suspended groups 215. TEGDMA is considered the most frequently eluted
monomer among RBC materials, because of its monomer molecular structure and higher
solubility 213,214, 215. Compared with TEGDMA, however, Bis-GMA monomer is reported to
be much less soluble because of its hydrophobic nature and reduced diffusion in liquid
environment 216. Furthermore, the hydrophilic monomers such as HEMA, which is very
abundant in bonding agents and TEGDMA, predominantly found in RBCs, are the monomers
capable of diffusion through the dentin, endangering the pulp vitality 213, 215.
The magnitude of monomer diffusion is displayed in a millimolar/Liter (mmol/L)
range, which is the clinically significant concentration that could cause detrimental effects on
pulpal homeostasis 217. The literature showed that diffusion rate increases when the
remaining dentinal pulpal coverage thickness is less than 1mm, and the cavity has been
conditioned with acid before the bonding procedures 218. In addition, experimental research
40
showed that HEMA pulpal concentrations could be in the range of 1.5–8 mmol/L, whereas
TEGDMA concentrations could be in the range of 4 mmol/L, respectively 219, 220. These
concentrations have been found to cause significant cytotoxicity through mechanisms
associated with oxidative stress, ROS production, and depletion of intracellular glutathione
with the consequences of cell death, mainly via apoptosis. Long-term exposures to nontoxic
concentrations of HEMA (0.05–0.5 mmol/L) and TEGDMA (0.05–0.25 mmol/L)
significantly delay physiological migration, odontogenic differentiation, and mineralization
process of human deciduous teeth pulp stem/progenitor cells in a concentration-dependent
manner 221. Furthermore, just a single exposure to higher HEMA concentrations of at least of
2 mmol/L and TEGDMA at least of 1 mmol/L have caused inhibition of the formation of
reparative dentin entirely 220, 221.
The methodology used to evaluate eluted compounds from RBC materials into
biological fluids is based on chromatography, which is an analytical procedure that isolates
molecules related to differences in their structure and composition 221. High-performance
liquid chromatography (HPLC) is now one of the most influential tools in analytical
chemistry, capable of separating, identifying, and quantifying the compounds that are present
in any sample, dissolved in a solvent 221. However, reversed-phase liquid chromatography
appears to be the most preferred method in a dental materials analysis 222. Improvements in
the properties of RBCs to reduce and prevent some leachable compounds are continually
being studied. Thus, improvements in the photopolymerization quality could minimize all
detrimental consequences to the pulp health.
41
1.6 Evolution of Light Polymerization Units (LPUs)
The LPUs have become the essential instruments in the contemporary dental practice
because a large number of dental procedures requires some photopolymerization. These units
emit a beam of light having a specific spectral output that activates photoinitiators in a wide
range of photopolymerizable materials. Since their introduction into the dental profession in
the 1970's, the evolution of LPUs started from UV polymerization and developed to visible
light polymerization. During this time, LPUs became smaller, battery-powered, and capable
of delivering a high amount of energy in a brief time.
From the historical perspective, polymerization technology was adapted from the
chemical industry and modified for the use in dentistry 33, 34, 35. The first LPUs were designed
to emit ultraviolet light 36. The typical polymerization time was the 40s, but a 60s exposure
provided enhanced results 37. Initially, the technology was far from perfect, but it was
revolutionary for that era. The limitations associated with UV polymerization included
limited light penetration through the RBC material (limited depth) and health hazards related
to UV lights exposure on the eyes 38, 39, 40. However, UV polymerization was short lived
when Dr. Bassoiuny at the University of Manchester School of Dentistry, in 1976, placed the
first visible light polymerized RBC restoration. The Imperial Chemical Industries of England
developed the novel technology and modified for dental use 41, 42, 43. The new visible light
photoinitiation system was based on the photoinitiator camphorquinone along with a tertiary
amine co-initiator. This system underwent through several modifications over the years.
However, it is still in use with today's modern RBC materials 44, 45.
The initial light sources used to photopolymerize the new RBC were based on quartz–
tungsten–halogen (QTH) lamps developed with the Johnson and Johnson partnership. The
42
new venture launched the FotoFil restoration system and QTH dental light source 46.
Although this new technology had the capability to adequately photopolymerize a 2 mm
thick increment of RBC material, the QTH technology could not eliminate the hazard for
ocular damage without an appropriate eye protection 47, 48. The QTH LPU used an
incandescent quartz light bulb consisting of a tungsten filament surrounded by a halogen gas,
which emitted a beam of filtered white light which provided a broad spectrum between 400
and 500 nm. Although these light sources were manufactured with relatively low-cost
technology, the short lifespan of the QTH lightbulb with a gradual degradation of the filter,
compromised their light output consistency. The QTH LPU exhibited a low energy
performance because the filter system wasted a considerable amount of radiation, which
transferred the majority of the unwanted radiation into heat. Therefore, a cooling mechanism
was needed with these light sources. The QTH became the mainstream LPUs, for many
years. Even after the introduction of LED LPUs in the 1990s, the QTH LPUs underwent a
series of modifications and improvements, in such a way that the bulb power increased from
35W up to 100W. The QTH LPUs have been typically used for the 40s to 60s polymerization
exposure, for a 2 mm thick increment of the RBC 223. The inconvenience for the cooling
timeout was a disadvantage. Furthermore, the excessive heat produced by these LPUs
reduced their clinical effectiveness and impacted their lifespan significantly.
In the late 1990s, the next generation of LPUs was developed, with the most
promising technology at the time: the argon laser units and plasma-arc (PAC). The principal
reason for their introduction to dental practice was to enhance the effectiveness, and to
increase the magnitude of the light output and to shorten the exposure time 223.
43
The argon laser (Light Amplification by Stimulated Emission of Radiation), with an
active medium of argon gas, unlike QTH LPUs, does not employ filters. It produces a
collimated (focused, non-divergent) beam of coherent energy, when applied to a particular
target, resulting in a more consistent radiant exitance over the tip-to-target distance 224, 225.
Unfortunately, the major output wavelengths were out of the absorption range of CQ, and
thus, this type of laser was nowhere near as effective as it could have been, if the output had
been a slightly shorter wavelengths. Although this novel technology showed some
exceptional improvements (coherent beam of light energy), it experienced some drawbacks.
For instance, the light beam should not be allowed to be directed to the eyes, as the eye
exposure would result in an immediate retinal damage 226. Although the size, weight, and
portability of newer argon laser LPUs have been improved dramatically in recent years, they
often take considerably more space than QTH LPUs. Laser LPU generate a substantial
amount of heat, the cooling fan tends to be noisy, and these LPUs were very expensive. Thus,
these LPUs never gained popularity. However, laser polymerization demonstrated an
immediate higher DC than that obtained with the QTH LPU. The laser LPUs enhanced RBCs
polymerization, which resulted in improved physical properties and bond strengths 227.
The Plasma arc (PAC) LPUs, use technology that uses plasma for the light source.
The PAC, initially designed in France, consisted of two tungsten electrodes separated by a
small distance, embedded in a high-pressure xenon gas chamber. When electric current
passes through the plasma, it generated the emission of the UV light with a broad spectral
range 228. The PAC LPUs uses sophisticated bandpass filters to curb UV and IR radiation,
while the light is delivered by utilizing a three-foot-long liquid light guide 229. Early PAC
LPUs produced spectral output wavelengths between 370 to 450 nm (low spectrum units) or
44
between 430 to 500 nm (high spectrum units), and a typical radiant exitance was around
2,000 mW/cm2. The early PAC light (the Apollo) produced broad, banded, white light. The
different tip ends had filters that only passed light of shorter or longer wavelengths to pass
either wavelengths more near violet or more near blue. The clear tip passed all wavelengths,
emitting white light that was used to enhance tooth bleaching
The PAC LPUs were designed to produce a 3s exposure, while an upgraded version
of these LPUs was launched with four adjustable polymerization modes with exposure up to
10s 229. Although PAC LPUs exhibited an adequate RBCs' polymerization, faster than other
LPUs, they were frequently associated with a higher risk of pulpal damage 229, as result of
their increased heat production compared to the QTH LPUs. Furthermore, due to a relatively
high cost, this technology had limited utilization in dental practices.
The LED LPUs are the most popular and the most current source of
photopolymerization in dental practices. The light-emitting diode technology uses two
semiconductor crystals, type "n" and type "p" with different electron density 230. When
electrical current passes through these crystals, light is generated at the "np" junction, with
the wavelength determined by composition difference between the semiconductor substitutes.
The LED LPUs produce high efficiency and medium-intensity light, which does not require
filters nor fans. Unlike QTH light sources, LED LPUs generate light within a narrow spectral
range. For instance, gallium nitride diodes produce light with wavelengths between 450 and
490 nm with a peak at 460 nm. LED light sources often require less power to operate, which
gave them the ability to be smaller in size, portable, and battery operated. The LED LPUs
have shown another advantage: extended service life compared to any other light types. For
instance, the halogen bulbs' life expectancy, under best condition, can last a hundred hours,
45
whereas the LEDs can last thousands of hours 230, which could be a reason behind the
enormous LED LPUs popularity among dental practitioners. The spectral output of the first
developed LED LPUs was not tailored to match the maximal absorption of CQ
photoinitiator. However, it happened to be a close match. The evolution of LED LPUs can be
observed as a function of the power increase, the ability to polymerize alternative
photoinitiators, and the LPUs beam profile homogeneity 243. The first generation of LED
LPUs started in the early 2000s, with the launch of the LuxøMax (Akeda Dental A/S,
Lystrup, Denmark), a product based on prototype patented in the US 231. It was a first ever,
large, pen-like, NiCad battery- powered LPU, having seven discrete LEDs. The concept of
the first generation of LED LPU design was to use multiple single-emitting LEDs, with each
LED being able to produce 30 to 60mW of radiant power, embedded into an axial array to
provide sufficient spectral power output to start activation and subsequent
photopolymerization 232. Other manufacturers followed the lead and launched their versions
of LED LPUs.
Performance of the first LED LPUs was not in line with the high expectation, as they
were relatively low-powered, compared to the traditional QTH LCUs at that time. The early
results of LED LPUs performance, in comparison with QTH LCUs, as well as issues with
NiCad batteries, led to some unanticipated, negative predictions for LED LPUs future 233.
However, a high success achieved in the blue LED semi-conductor research boosted the
efficiency of the first generation commercially available LED LPUs. Therefore, in late 2001,
a launch of the EliparTM FreeLight (3M/ESPE, St. Paul, MN) dramatically changed the
outlook of LED LPUs market, by increasing the efficiency and the performance of LED light
sources 234. The new EliparTM FreeLight had an innovative design, with a 10 mm light probe,
46
incorporating 19 discrete LEDs in an array and was capable of generating a radiant exitance
of 400 mW/cm2. The success achieved by the incident irradiance level increase was partly
contributed to the use of a tapered, fused glass fiber light guide. According to the
manufacturer's claim, the radiant exitance level produced by new LED LPUs was equivalent
to the 800 mW/cm2 value generated by the conventional QTH LPUs.
The second generation of LED LPUs started in the early 2000s, due to advances in
the LED chip industry and incorporation of the new 1-W chips into the LED arrays, which
enabled the production of more powerful, single LED devices 235. In addition, the chip
manufacturers started fabricating chips specifically within the wavelength required for the
polymerization of RBC materials. Examples of these single more powerful LED emitters
were the Luxeon LXHL-BRD1 in 2004 235, which produced 140 mW (1 W chip) or 600 mW
(5 W chip) of blue light. Moreover, the more recent LED Engin Inc., LZ4-00B200 in 2012,
which produced 4,200 mW or 5,600 mW of radiant power depending on the chip 236. The
launch of the 2nd generation LED LPUs officially started with EliparTM FreeLight
2(3M/ESPE, St. Paul, MN), an improved version of EliparTM FreeLight. The new LED LPUs
successfully increased 2.5 times radiant exitance 237, by utilizing the tapered waveguide (the
so-called “turbo-tip”) to boost emitted irradiance 237. Nickel metal hydride batteries were the
power source that enabled the breakthrough. Other manufacturers started launching different
designs: a first in this group, L.E. Demetron II® (Kerr Dental, Orange, CA) began in 2005 238.
This unit had a conventional pistol grip design, with a built-in cooling fan, which was a
compromise to heat sink design. However, like other manufacturers, the light guide exhibited
a tapered turbo-tip to boost the incident irradiance. Later, manufacturers started to
experiment with light probe design, which led to the launch of the SmartLite PS® (Dentsply,
47
Milford DE), an ergonomic-pen-style light, which was small (24cm long), lightweight
(100g), and silent (no fan) 239. The manufacturer claimed that device could generate a radiant
exitance up to 950 mW/cm2. The advantage of this design allowed LPUs to produce a more
unidirectional light, with a substantial reduction in light loss, without a light guide between
the light source and the material to be polymerized. In addition to an increased irradiance
level, this innovation led to lower power consumption, longer battery life, and most
importantly, the significant reduction in heat production. This innovative technology helped
the industry to resolve the disadvantage associated with tapered light guides: more divergent
light beam and a smaller light tip.
In late 2010, this problem seemed to be resolved with the innovative design of
EliparTM S10 (3M ESPE. St. Paul MN). This light featured a 10 mm diameter, and a parallel
glass fiber light guide 240. The manufacturer claimed that the LPU could generate the radiant
exitance up to 1,200 mW/cm2. The second generation of LED LPUs, have experienced
tremendous improvements, a substantial radiant power increase, which enabled sufficient
photopolymerization in shorter exposure time. All previously mentioned LED LPUs were
designed to produce a single peak of spectral irradiance to match the maximum absorption
spectra of CQ photoinitiator system, they are referred as single-peak, or monowave LED
LPUs 243.
The third generation of LED LPUs provided an extended light spectrum in the violet
range, to match the range of alternative photoinitiators, as well as the CQ photoinitiator
spectra. Thus, the third generation of LED LPUs have the blue-light chips with a spectrum
from 420 nm to 510 nm and the violet-light chips with a spectrum from 380 to 420 nm. The
new LED generation was marked by significant improvements in power conversion, which
48
drastically reduced the internal but not external heat generation. Also, the LPUs showed a
substantial power increase, ability to control their exposure time, radiant exitance values and
the polymerization protocol. This new LED technology referred to as "multiwave," was
produced by some manufacturers, although Ultradent initially applied for the patent, Ivoclar-
Vivadent was allowed to patent this broad-spectrum technology as Polywave® (Dual-peak)
LED LPU. The majority of the manufacturers continued development of the second
generation of LED LPUs. The first multi-peak (multiwave) LED LPU became available in
2003 when the Ultralume 5® (Ultradent Products, South Jordan, UT) came to the market 241,
(Figure 1.6.1).
Figure 1.6.1 Ultralume 5® (Ultradent Products, South Jordan, UT) source:
http://www.airforcemedicine.af.mil/Portals/1/Documents/DECS/Product_Evaluations
/Equip/Lights/Curing/UltraLume_LED%205.pdf
This unit was a corded, pen-shaped LPU, powered with a 5W blue LED central chip
surrounded by 4, low power violet LEDs, capable of producing a broad spectrum, with high
peaks around 403 nm and 453 nm and the manufacturer claimed a radiant exitance of 1,200
mW/cm2.The LPU featured no fiber optic light guide. The results from a study evaluating
polymerization performance of 4 LPUs, by conducting a microhardness test, showed that the
49
Ultralume 5 achieved the highest KHN of all RBC materials tested, outperforming all of the
2nd generation LEDs and QTH LPUs 242. In 2004, the first product of the Bluephase® family
(Ivoclar-Vivadent, Amherst, NY), with a radiant exitance of 1,100 mW/cm2 came to the
market 243.
An improved product, the Bluephase 20i, exhibited a more traditional gun-shape with
a pistol grip, and standard fused-glass fiber light guide. The new multiwave LPUs from the
Bluephase family could generate radiant exitance up to 2,200 mW/cm2, which brought back
the more advanced cooling features to their design. Furthermore, these LPUs featured highly
sophisticated, programmable polymerization modes for the variety of clinical applications, an
extended spectrum with two spectral peaks at approximately 410 nm and 470 nm 244.
The Valo® (Ultradent Products, South Jordan, UT), represented the hallmark of the
multiwave light sources approach 245. A wand-shape design of the Valo featured two blue
emitters (around 460 nm), and one violet (about 445 nm), capable of producing a spectral
output from 395 nm to 480 nm 245. The Valo is made from a solid bar of tempered, high-
grade aerospace aluminum, which provides this light source with unsurpassed durability and
superior heat dissipation. The ergonomic and streamlined design enables this unit to easily
access intraoral locations to provide optimal light for polymerization, without sacrificing
patient comfort. The unit featured an optimally collimated, almost homogenous beam profile,
which had not been seen by any LPU before. The LPU exhibited programmable
polymerization options: Standard Power (1,000 mW/cm2), High Power (1,400 mW/cm2), and
Xtra Power (3,200 mW/cm2).
The launch of a multiwave LPU, the ScanWave® (Acteon, Mettmann, Germany),
which has not gained much popularity in North America, showed an entirely innovative and
50
sophisticated product 246. The LPU used 4 different wavelength LED emitters (405nm /
440nm / 460nm and 480nm), capable of producing spectral output coverage from 390 nm to
505 nm. The ScanWave® incorporated a laser aiming system using a wavelength of 655nm,
which allowed a precise light delivery for the light probe. According to the manufacturer's
claim, the LPU is capable of generating a radiant exitance of 2,200 mW/cm2 over a 5.5mm
light probe diameter. The unit featured very sophisticated programming options, where the
manufacturer claims that LED LPU can optimize polymerization while minimizing the heat
generation within the target 246. The ScanWave® is equipped with a system for the detection
of possible unit operation anomalies. Thus, the third generation LED LPUs represented the
examples of state-of-the-art light sources, able to generate low or high power in a variety of
clinical scenarios that had never been seen before.
While the LED LPUs have become the gold standard for the polymerization in the
contemporary dental practice, they have some inherent drawbacks, particularly concerning
the non-homogeneity of their irradiance beam profiles. This drawback is because irradiance
does not represent a single value, but rather the mean of all irradiated light intensities. Thus,
with a non-uniform beam profile of LED LPUs, the irradiance value can range from 300
mW/cm2 to 3,000 mW/cm2, which can make a significant difference in polymerization
efficiency and the RBC's surface hardness profiles 242. Spectral non-homogeneity of
multiwave LPUs beam profiles relates to different wavelengths of embedded LEDs within
the LPUs light beam profile. Newly developed LED LPUs with much more uniform beam
profile have been designed to overcome the shortcomings of the previous generation of light
sources could be identified as the fourth generation of the LED LPUs. The examples are
51
Bluephase Style (Ivoclar-Vivadent, Amherst, NY) and DeepCure-S (3M ESPE, St. Paul,
MN).
1.7 Intrinsic Factors Affecting Photopolymerization Efficiency
RBC restorative materials represent one of many success stories in advanced
biomaterials research, as evidenced by their use increasing exponentially in the last decade.
However, simplifying numerous factors that affect their polymerization efficiency is not an
easy task because commercial RBCs represent a variety of chemical formulations that are not
readily revealed, due to the trade secrets. The following factors were researched extensively:
the photoinitiator systems, alternative monomer chemistry, and a novel formulation of fillers
and their coupling agents. The fundamental understanding of photochemical reactions is
critical to establish correlations among the RBCs, to modulate desired photopolymerization
outcome.
The photochemistry represents the analysis of chemical reactions and physical
behavior that occur under the influence of visible and ultraviolet light. Two fundamental
principles are the foundation for understanding photochemical transformations. The first law
of photochemistry, the Grotthuss-Draper law, states that a photon has to be absorbed by a
chemical material for a photochemical reaction to take place 11. Therefore, the presence of
light alone is not enough to trigger a photochemical reaction; the light must exhibit the
correct wavelength match to be absorbed by the reactant species 11. The second law of
photochemistry, the Stark-Einstein law, states that, for each photon of light absorbed by a
chemical system, only one molecule can be activated for the photochemical reaction to occur
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11. Light must be present, and it must then be absorbed. In other words, there is a one-to-one
correspondence between the number of absorbed photons and the number of excited species.
However, not all absorbed photons can produce the same number of reactive species. The
ability to accurately determine the number of photons leading to a reaction enables the
efficiency or quantum yield of the photochemical reaction 11. The practical implication of
photochemistry is of immense importance, as it is the basis of photopolymerization of RBC
restorative materials. LED LPUs deliver light energy to the surface of RBC material. The
capacity of LPUs to deliver a sufficient quantity of photons within the appropriate spectra is
the critical step in a photochemical process. In regard to the previously mentioned, the
photoinitiator system is elevated to a state of higher energy, which represents the
photoexcitation state, a fundamental requirement for photopolymerization. For instance, if a
system does not absorb the light of a particular wavelength, no photochemistry will occur,
regardless of the duration of the light exposure. Thus, the prime objective of successful
photopolymerization would be an adequate quantity of light to be delivered to the surface of
any given RBC material, and the closest match between the spectra of incoming light and a
corresponding RBC photoinitiator system.
1.7.1 Photoinitiator systems
The dimethacrylate-based RBC material is the most frequently employed chemistry
formulation among the commercially available RBCs, and it has been a focus of extensive
studies since its introduction into general practice 154. Due to the RBCs' exponential clinical
53
acceptance, clinicians focused their expectation for fast polymerization, superior mechanical
properties, and greater longevity of the restorations on the photopolymerization concept.
A photopolymerization reaction is a complicated process induced by the LPU’s
irradiated light over the RBC surface, which in essence the photo-reactive systems absorb
light irradiation, and the monomers in the molecular structure become excited resulting in the
formation of a polymer structure. The free radicals that trigger the polymerization process are
formed by exposing a photo-unstable compound, the photoinitiator, to the appropriate light
source's quantities and wavelengths, causing the photolysis 5. The radicals have unstable
(reactive) groups of atoms with at least one unpaired electron in their methacrylate-based
radical groups.
The classification of photoinitiators is based on the wavelength range of light used for
activation or according to the mechanism employed for the photolysis. The light sources used
in dentistry for the photopolymerization produce a spectral emission in a range from 380 nm
to 515 nm. This spectrum is apportioned into the violet light range, from 380 nm to 420 nm,
and the blue light, from 420 nm to 515 nm 247. Because the photon energy is inversely
proportional to its wavelength, the photons in the violet light spectrum exhibit more energy
than those in the blue light spectrum 247. Radicals that initiate a polymerization reaction are
either formed by using the bond fusion or by contacting the hydrogen atom (H transfer) to a
second compound, the so-called “co-initiator” 247. Regardless of the mechanism used, it is of
the greatest importance that the photoinitiator reaches an excited state. Many photoinitiators
contain carbonyl groups as light-absorbing species. The majority of RBCs contain
methacrylic acid esters, which are used as radically polymerizable monomers. Depending on
their number of polymerizable methacrylate groups, they are classified into monofunctional
54
(n = 1; e.g. methyl methacrylate), difunctional (n = 2) and multifunctional (n > 2)
methacrylates 247. During the radical photopolymerization of monofunctional monomers,
linear polymers are developed. The rate of the radical formation is dependent on irradiance of
the LPU, the photoinitiator quantum efficiency, the extinction coefficient of the
photoinitiator and its concentration, and the thickness of the layer through which light passes
through 257. Useful photoinitiators are defined by high quantum efficiency and a high
extinction coefficient, i.e., the capacity to show a rapid absorption of the light in the
wavelength range applied 257.
There are two types of photoinitiator systems currently employed among commercial
RBC materials: Norrish Type I and Norrish Type II systems. Camphorquinone (CQ), a
Norrish Type II system developed by Dart and Nemcekin in 1972, became the most common
photoinitiator system for dental applications 1. CQ has a visible light absorption range from
360 to 510 nm, with a maximum absorption peak at 468 nm. This Type II system of
photoinitiator reacts by bimolecular hydrogen abstraction from suitable co-initiators,
preferentially aromatic amines, such as ethyl-4-dimethylaminobenzoate (EDMAB), aliphatic
amines, such as 2-(dimethylamino) ethyl methacrylate (DMAEMA), or a common
piperidines derivate, such as 1,2,2,6,6-Pentamethylpiperidine 247, 248. The first step in this
photopolymerization reaction is high-speed electron transfer radical. During a much slower
proton transfer process, two radicals are created, of which only an amine-based radical
initiates a photopolymerization reaction 196. The photochemical radical formation is a
diffusion-controlled process. In the case of high viscosity formulations, the efficiency of type
II photoinitiator is considerably decreased. Several attempts have been made to combine a
photoinitiator with the co-initiator in one molecule, either in a low molecular dimer or a
55
copolymer 248. The study results which utilized a low molecular dimer showed an increase of
up to a factor of 2 in the rate of a photopolymerization reaction. However, on the contrary, in
the study where the copolymer has been used, a decrease in reactivity was observed 248. The
free radicals generated in this process attack the C=C bonds of monomers, resulting in the
formation of new radicals with a much longer chain. This process propagates through a
cascade of chain reactions, which includes an auto-acceleration and, auto-deceleration,
coupled with high polymerization rates, along with diffusion-controlled termination,
followed by the free radical entrapment 248. The Type I photoinitiators exhibit greater
absorption efficiency and quantum yield compared with Type II photoinitiators 248.
Although CQ systems exhibit a very high acceptance among the RBCs
manufacturers, there are some drawbacks associated with their usage. Small amounts (ppm)
of yellow-colored CQ have a profound influence on color properties of RBC materials. It has
frequently been observed, that, under the effect of LPU's irradiation leachable by-products
could be generated, with the capacity to produce visible discoloration under restorations 249.
Also, the α-diketone group, derived from CQ, has a peak absorption in a visible range, which
could initiate RBCs' polymerization under ambient light, significantly reducing a working
time of the materials 250. Furthermore, some concerns regarding the biocompatibility and
toxicity of used amine compounds have been proposed 213. As a possible solution for
previously mentioned concerns, adding a small amount of specific polymeric or
macromolecular photoinitiator aromatic tertiary amine, with a bisphenol-A skeleton to CQ
chemistry, has been proposed 251. In addition, the initiator CQ can be substituted by
polymeric radical photoinitiators bearing the photo-sensitive CQ groups in the side chain 253.
56
The Type I photoinitiator system, is based on the photochemical cleavage of
aldehydes and ketones into two free radical intermediates, which initiate polymerization
without the need for an additional amine 248. Type I photoinitiators exhibit much faster
photopolymerization kinetics under short irradiation times, improved functional group
conversion, reduced monomer elution, and improved mechanical properties 180. The second
law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed
by a chemical system, only one molecule can be activated for the photochemical reaction to
occur 11. However, this fundamental principle applies to photoinitiators with a low free
radical production efficiency, as seen with camphorquinone (CQ) 11. For instance, as much as
four reactive radicals could be produced from one molecule of Irgacure 819, compared to
only one obtained from CQ 253, 255. Thus, photon absorption efficiency (PAE) is a very useful
parameter to determine the most efficient photoinitiator with corresponding LPU’s spectral
irradiance 254. Therefore, photoinitiators absorption spectra should be highly correlated with
the spectral emission profiles of LPUs. The type I systems, currently used with commercially
available RBC include:
• Phenylpropanedione (PPD) (1-phenyl-1, 2-propanedione), with a maximum
absorption spectrum in the range of 398 nm 249.
• Lucirin TPO Diphenyl (2, 4, 6-trimethylbenzoyl) phosphine oxide, with a maximum
absorption spectrum in the range of 381 nm 254.
• Irgacure 819 (bis-(2, 4, 6-trimethylbenzoyl) phenyl phosphine oxide), whose
maximum absorption spectrum is in the range of 370 nm 255.
57
• Ivocerin ® represents a blend of germanium-based compounds such as benzoyl
trimethyl germane (Ge-1) or dibenzoyl diethyl germane (Ge-2), whose maximum
absorption spectra are 411 nm and 418 nm respectively 256. The bis-(4-
methoxybenzoyl) diethyl germane Ge-3, with the broad absorption bandwidth to
460nm, which exhibits a maximum absorption spectrum at 408 nm 256.
The literature reports a quantum efficiency of the light-induced cleavage for Ge-2 is 0.85,
while that for Irgacure 819 is 0.59 256. In addition to the photoinitiator concentration and the
irradiation protocol, specific characteristics related to the chemistry of the molecule itself can
also affect polymerization initiation. The photoinitiator molecule must have a high molar
extinction coefficient, which is defined as the absorption per unit length divided by the molar
photoinitiator concentration of the solution 257.
Efficiently absorbed energy has enormous influence on a photoinitiation process.
Therefore, absorbed irradiance should be reported as well, to make a meaningful comparison
among the photoinitiators 257. Type I photoinitiator systems exhibit a higher molar extinction
coefficient when compared with Type II systems 257. The probability for CQ to absorb
photons at its maximum absorption peak is significantly lower than for any Type I system,
excluding Ivocerin, which has a much higher order of magnitude than any other Type I
photoinitiator system 257. The Type I systems do not need an amine co-initiator. Furthermore,
they also exhibit exceptional color stability, and they are employed in many commercially
available RBCs' bleach shades 258. Several studies report good results with mixtures of CQ
and other photoinitiators, resulting in more dependable polymerization 259.
Besides other factors, viscosity alone may significantly affect the polymerization
efficiency of a given photoinitiator during the evaluation process. The result of a well-
58
controlled study validated the assumption that the initiation system exhibited the significant
influence on the conversion and a degree of network crosslinking of highly-filled RBCs 258-
260. Those studies highlighted the advantages of using MAPO photoinitiator systems (Type I)
in a highly-filled dimethacrylate-RBCs with the assumption that a sufficient Bis-GMA
content (>40 mol %) and an adequate light spectrum were applied 259. Moreover, the radiant
exposure values for the proper polymerization of tested systems ranged from 3 J/cm2 for
MAPO systems, having values around 20 J/cm2 were commonly reported for CQ systems 259.
The MAPO photoinitiator systems, with Bis-GMA fractions of 40 mol% or more, have
experienced an increased trapped radical concentration, a reduced monomer elution, and
superior mechanical properties, in comparison to the CQ photoinitiator systems 259.
1.7.2 Viscosity, monomers, and fillers
Abundant evidence exists to support the assumption that initial RBC viscosity plays a
significant role in the reaction kinetics and final DC by affecting the mobility of each
monomer, and its ability to react 197. It is universally recognized that the initial viscosity of
RBC and its composition, pre-determine the extent of polymerization, through mobility
restrictions, with the onset of auto-acceleration and the beginning of diffusion-controlled
termination being changed to earlier stages of the conversion 197, 261.
The local viscosity is primarily influenced by the monomer composition and a filler
load. The polymerization efficiency is profoundly affected by the monomer molecular
structure. The viscosity exponentially increases as the percentage of a filler volume increases
but decreases significantly with a temperature increase 288. Although the majority of
59
commercial RBCs are very sophisticated in their chemical formulation, they are primarily
composed Bis-GMA (2, 2-bis-[4-(methacryloxy-2-hydroxy-propoxy)-phenyl]-propane), or
its derivates blended with TEGDMA (Triethylene glycol dimethacrylate) as a diluent. The
function of inorganic fillers is to improve mechanical properties and to reduce
polymerization shrinkage and to minimize the thermal expansion coefficient 197-200.
The viscosity of RBCs depends on the composition and ratio of the resin matrix, the
content and shape, size distribution and silane treatment of the inorganic filler, interlocking
between the filler particles, and interfacial interaction between the filler particles and the
matrix resin 171. Bis-GMA is the main monomer that is frequently used for the production of
commercial RBCs. Bis-GMA shows a relatively low polymerization shrinkage (around 6%),
rapid hardening by free-radical polymerization, low volatility, and acceptable mechanical
properties after the polymerization 260. Bis-GMA has a very high viscosity at ambient
temperature, which requires its dilution with a low viscosity monomer, to obtain an organic
matrix that can be mixed with an adequate amount of inorganic fillers 260. However, dilution
with low viscosity monomers (mostly TEGMDA) leads to a significant deficiency. Because
viscosity is correlated with the molecular weight of monomers, and because low molecular
weight monomers have higher polymerization shrinkage, it can be assumed those different
formulations of Bis-GMA/TEGMA could have a significant influence on polymerization
efficiency 199.
Although RBCs that have been analyzed exhibited the same total inorganic filler
weight content, their rheological behavior is very different from one to another 199. For
instance, 60 wt% of macrofillers alone do not have a considerable influence on the
viscoelastic properties of the material, except that they increase the viscosity of the material
60
in the way, which a slight shear-thinning behavior appears. However, by increasing the
microfiller content, a more complex viscosity increases 261. For instance, adding 60% of
macro-filler to a 50/50 Bis-GMA/TEGDMA mixture changes the viscosity from 0.2 Pa s to
1.4 Pa s at 1 rad/s 261. Regardless of RBCs formulation, RBCs will experience deformation
when applied to the cavity walls, and that phenomenon is called thixotropy. Thixotropy is
always anticipated from shear-thinning mechanism 261. Thixotropy occurrence is always
observed because of the finite time required for flow-induced changes in the microstructure,
and the process is entirely reversible. Therefore, in clinical situations, RBCs are
photopolymerized in a state which is very far from their rheological state of rest. It could
further indicate that RBCs should ideally be polymerized after the restructuring phase. From
that point of view, and considering the present results, it has been recognized that the time
needed for the RBCs to reach equilibrium after their deformation is exceptionally long. It has
been shown that RBCs need approximately 3,600 s to reach 80% of their initial viscosity, and
from 7,200s to 14,400s to return to their state of equilibrium 261. However, that would be
impossible to implement in clinical practice. Fillers have a significant impact on
polymerization efficiency, by increasing a filler volume fraction, the maximized DC has been
acknowledged 160.
The RBCs viscosity change is related to an increase in filler volume percentage, filler
size and, morphology 261. The viscosity of RBCs is substantially influenced by a resin matrix
chemistry, the shape, and size, as well as the content of a filler particle. Furthermore, the
viscosity is dependent on interlocking between the filler particles and the interfacial
interactions between the filler and a resin matrix 262. It has been shown that the resin matrix
exhibits major influence on polymerization shrinkage stress than the filler content. Therefore,
61
further research has been conducted with the aim of reducing stress by modifying the resin
chemistry, without the reduction of a filler content 263.
The incorporation of a novel class of mono vinyl (meth) acrylate monomers that
contain secondary and tertiary functionalities such as urethanes, carbonates, cyclic acetyl,
morpholine, cyclic carbonates, hydroxy/carboxy, oxazolidones, and aromatic rings,
significantly enhance the polymerization kinetics and significantly improve mechanical
properties of conventional RBCs 263. The incorporation of reactive organic nano gel-
polymers into the standard dimethacrylate matrices have been reported to delay vitrification
and elastic modulus development 264. Nanogels provide a stable, transparent solution of
swollen, dispersed or overlapped particles. During the polymerization reaction, nano gels
enhance a physical entanglement and potential chemical crosslinking between nano gel
structures and the resin network, which consequently reinforce mechanical properties of the
final network structure 264.
Temperature has a significant, inverse correlation to viscoelasticity changes of RBCs,
in the way that temperature increase will lead to an exponential decrease of RBCs viscosity
265. This phenomenon was explained by the Arrhenius equation, which represents the
relationship between the temperature and the viscosity of the fluid 265. The research showed
that a relatively weak molecular bond exists in a high-concentration suspension of the filler
particles in a resin matrix. For example, if a shear force is generated, the resultant shear flow
can induce the rolling of the fillers, which can increase the interstitial distance between the
filler particles, or between the filler and a resin matrix, which results in individual bonds
between molecules to become broken. Further increase of shear rate, an array of particles
becomes directional, which leads to diminished interactions between particles. As a result,
62
viscosity decreases with the shear rate increase. This phenomenon represents ‘shear thinning,'
which can further explain why a rapid, slight tapping after the RBC placement, can enhance
RBC flow and better adaptation to the preparation walls 263.
Although mechanical properties of RBCs, which exhibit viscoelastic behavior, have
often been characterized using static tests, these tests were primarily focused on the elastic
component of RBCs, without the valuable information about the viscous domain of the
material tested. Therefore, static tests are considered appropriate for characterization of the
ultimate strength of RBCs 266. Additionally, the results of commonly used tests with static
loading, including the creep test, stress relaxation test, and steady shear test, are in essence
with a low level of clinical relevance, because masticatory loading conditions are dynamic.
There are two test methodologies for RBCs viscoelastic properties characterization exist:
irreversible (destructive) and reversible (non-destructive). Although static and dynamic
destructive tests such as tensile and dynamic mechanical analysis (DMA) were extensively
conducted to measure viscoelastic properties of RBC materials, due to their irreversible
nature and problems associated with specimens' standardization, have been replaced with
non-destructive tests. A non-destructive methodology has the ability for a quick and accurate
estimate of RBC’s viscoelastic properties, without any specimens' damage. Therefore, the
majority of DMA tests that are accepted for characterization of RBC's viscoelastic properties
employ this methodology. For example, a dynamic oscillatory shear test is the most often
used test to characterize the linear viscoelastic properties of RBC restorative materials 261, 264.
The analysis employs the resonant vibration, which primarily generates specimens’
mechanical vibration in any of selected modes: torsional, transverse, or longitudinal, over a
specific range of frequencies 266.
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1.7.3 Optical properties
The optical properties of RBCs and their photopolymerization reaction are
interdependent, in the way that material constituents affect light transmission, which further
influences the polymerization depth. Light penetration of RBC materials is defined by the use
of the Beer–Lambert's law 287. According to this law, if a beam of monochromatic radiation
passes through a medium, its power attenuates based on the following equation:
P=P0 (1-RF) exp (-αd) (1)
Where P0 is initial optical power, RF is the total Fresnel reflectance coefficient, α is
the attenuation coefficient, and d is the thickness of the sample. However, in a function of the
RBC sample, the attenuation of LPUs' irradiance in the RBC sample follows Lambert's Law,
expressed in differential formats:
−𝑑𝐼
𝑑𝑥 = µI (2)
Where ‘ I’ represents the irradiance at depth ‘x’ in the RBC specimen, and ‘µ’ is the
linear attenuation coefficient (dimension L−1). Furthermore, the RBC material exhibits
various refractive indices and interfaces, including trapped air bubbles, and thus µ needs to
be further divided into two components, due to a true absorption ‘Ʈ’ and scattering ‘δ’. It
was discovered that the scattering coefficient is relatively consistent among the tested volume
fractions. Based on Eq. (2) the further rearrangements to show the transmittance (T) of the
RBC specimen, is often expressed as the optical density D of the RBC specimen:
64
D=µx log 10(e) (3)
It can be recognized, based on Eq. (3) that optical density is a mere linear function of
the depth of the RBC sample (assuming uniformity). However, taking into the consideration
that most frequently used LED LPUs, do not generate a monochromatic light spectrum, some
further adjustments should be made, however, the basic principle of the law is very relevant
to RBCs polymerization. Thus, the law describes the loss in optical power due to reflections,
absorption and scattering upon LPUs light transmission through the RBC materials 267.
As previously mentioned, LPUs do not produce a monochromatic light beam, as their
spectral irradiance depending on the type of LPUs, which can vary from 380 nm to 515 nm.
The portion of that LPUs spectrum referred to as a violet spectrum, usually interacts with a
particular type of RBC materials by producing Rayleigh scattering. The Rayleigh scattering
law states that the percentage of the light that will be scattered is inversely equivalent to the
fourth power of the wavelength 268. Small particles will scatter in a much higher percentage
of short wavelength (violet spectrum) than long (blue light) wavelength. Because the
mathematical association comprises the fourth power of the wavelength, even a small
wavelength difference can mean a substantial difference in scattering coefficients.
The following contributing factors are correlated with RBCs light attenuation: the
surface gloss, the resin matrix composition (resin volume fraction), filler composition (filler
volume fraction), the content of pigments, the presence of some chromatic by-products, and
refractive indices of monomers, silane, and the filler particles 269. The surface light reflection
has two functional components, the diffuse component, which results from a light penetrating
65
surface, following the multiple reflections and refractions and also re-emerging at the
surface. The spectral element is a surface phenomenon, which can be represented as a
function of the incidence angle and the refractive index (RI) of the material, the surface
roughness, and a geometrical shadowing function 270. The color coordinates of RBCs (value,
chroma, and hue) are related to the scattering and absorption characteristics, light reflectivity,
and translucency of the material 270. Translucency is one of several factors that determine the
optical features of the material and refers to the partial passage of light through a particular
structure. Therefore, the presence of different degrees of translucency in any RBC material is
a determining factor in the quality of esthetic restoration.
The RI of RBCs plays a significant role in their optical properties. Therefore, studies
have confirmed that transparency, opacity, and surface gloss values were profoundly
correlated with the RI value 271. Furthermore, a decreased transparency value will be
correlated to increased value for the opacity and surface gloss, while at the same time, the RI
value will be increased 271. Concerning polymerization depth, several factors can interfere
with the light transmission throughout the RBC material. The pigments and photoinitiators
can absorb the light photons 271. The light interference with pigments and a variety of shade
additives has been documented. The darker shades would experience a lower DC at the same
irradiance, in comparison to lighter shades 272. The inverted relationship between the
photoinitiator’s molar absorptivity of the incident light and polymerization depth has been
established. Thus, the higher molar absorptivity will present the lower polymerization 257.
The most of RBCs are formulated with camphorquinone (CQ) as a profoundly yellow
photoinitiator, which has a profound influence on the color of RBC materials. During the
photopolymerization process, depending on quantity and spectral emission of the LPU, the
66
color transformation of CQ changes to the transparent under favorable polymerization
conditions. However, a certain degree of yellow remains within the material insufficiently
polymerized. Thus, the further conversion of CQ continues under the influence of ambient
light. Therefore, the “bleaching” represents the conversion of CQ from yellow color to
almost transparent as a result of “adequate” polymerization 271.
The relationship of filler particle size and attenuation of light has been a subject of
extensive research. The following features of filler particles such as filler size, geometry,
density, and silane treatment, have an enormous effect on reduction of light intensity.
Therefore, an increase in filler content will accelerate light absorbance 267. Furthermore, light
scattering will be increased, with a significant difference in refractive indices of the matrix
and the filler particles 267. The RBCs light intensity reduction has been explained by the
Rayleigh equation, which demonstrates relationships between the following factors: the
thickness of a specimen, filler volume content, particle radius, refractive indices of the
particles, refractive indices of the matrix, and wavelength of incidental light 271. The
following relationships can be observed:
• In the case when the refractive index of the matrix is smaller than the particle's
refractive index, light transmittance reduction will occur with the increase of a filler
particle radius and filler volume fraction, while an increase in transmittance will
happen with an increase of incidental light wavelength;
• In the case where the matrix refractive index is higher than the particle's refractive
index, an increase in transmittance will occur with an increase of a filler particle
radius and filler volume fraction, and a reduction in transmittance will happen with an
increase of wavelength of incidental light;
67
Furthermore, the effect of each of the factors is differentiated, because a filler particle
radius is elevated to the third power, the wavelength of an incidental light to the fourth power
and a multiplier factor of 3 is attained to the filler volume fraction 272, 273. However, this
complex theoretical model has some meaningful, practical explanation. For instance,
Rayleigh scattering is contributed to the higher attenuation of LPUs' incident light in a
particularly short wavelength spectrum (under 420 nm), in comparison to the longer
wavelengths spectrum (above 420 nm). Furthermore, an incident light wavelength hitting the
equal or a smaller filler particle size, produces scattering of a higher magnitude. A
considerable extent of scattering would be expected when a filler particle size approaches to
the half of LPUs' wavelength, resulting in that the extent of light scattering is inversely
proportional to the fourth power of an incidental light wavelength 273. Based on these
findings, the LPUs with a violet spectrum should not be used for the bulk-fill composite
polymerization, because the violet spectrum has induced a substantial scattering effect and
reduced polymerization efficiency. Furthermore, the thickness of a specimen plays a
significant role in light attenuation.
1.8 Extrinsic Factors Affecting Photopolymerization Efficiency
RBCs' polymerization quality is determined by a variety of factors, which could be
classified into the two groups: intrinsic and extrinsic factors. The intrinsic factors are
frequently associated with RBCs properties. However, extrinsic factors are related to features
of LPUs: the radiant power and irradiance, the spectral emission, light beam profile, distance
from the light probe, the exposure time, and irradiation modes.
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1.8.1 Light polymerization units (LPUs) and emission spectrum
LED LPUs are the most common photopolymerization light sources in a
contemporary dental practice because a large number of dental procedures require
photopolymerization. The LED LPUs offer several advantages over other light sources: a
relatively high radiant exitance, enhanced mobility, a low operational cost, and an affordable
price structure. The two main designs are a gun-looking shape with a traditional light probe
or a turbo-probe, and a wand-type style, which has an LED source at its distal end sealed
with a fixed lens. However, the Bluephase Style is a wand-style unit having its emitting chips
still within the body of the wand, and not at the tip end. The advantage of the wand type
LPUs is easy to access to posterior teeth, especially in pediatric patients or in patients with
limited mouth opening. Furthermore, these types of LPU’s have a well-collimated light
guide, contrary to a turbo light guide that provides a significant irradiance decrease as a result
of an increased distance between the light tip and the RBCs 242.
The first and foremost property of LED LPUs is their ability to deliver adequate
quantities of energy to the RBCs, to achieve maximum polymerization. Thus, LPUs need to
have an appropriate radiant power output (mW) as well as spectral irradiance (mW/cm2/nm),
with as close as a possible match to the maximum absorption spectrum of the photoinitiators
present in the RBCs. It is well- recognized that monowave LED LPUs can deliver irradiance
spectra in the range to the maximum absorption spectrum of CQ (468 nm), while multiwave
LPUs deliver a broad range, with usually two peaks of spectral irradiance, suitable for the
activation of CQ and the alternative photoinitiators 287. However, a reduced polymerization
69
depth has been observed with bulk-fill RBCs, polymerized with multiwave LED LPUs,
proposedly attributed to the significant Rayleigh scattering effect between a filler particle
size and shortwave visible light 273, 274.
Despite conflicting views in the literature, some manufacturers have been working on
Norrish Type I photoinitiators, particularly with the PPD, Lucirin-TPO and the latest one,
Ivocerin® a germanium-based photoinitiator. Absorption spectra of those photoinitiators are
predominantly in the violet spectrum range: between 380 to 420 nm. Furthermore, due to
their absorption quantum yield and superior polymerization kinetics, these photoinitiators
have been attributed to the significant DC improvements and increased the degree of
crosslinking, resulting in much better mechanical properties of RBCs 273, 275. The
combination of Type I and Type II photoinitiators in commercial RBC formulations is
associated with significantly better mechanical properties that of CQ when used by itself 276.
The results were further validated by successful use of various blends of the CQ with of
Lucirin-TPO and Ivocerin® in Ivoclar-Vivadent commercially available RBCs 276, 277, 278.
The uniformity of the LPUs beam profile has been a subject of much research lately.
The spectral analysis of the LPUs irradiance beam profile showed enormous differences in
their light distribution across the light tips. Those LPUs without a uniform irradiance
distribution are associated with the production of hot spots (high irradiance) and cold spots
(low irradiance) on the RBCs surface 279. As a result, materials polymerized with these LPUs
exhibit areas with adequate and inadequate polymerization. Furthermore, a strong positive
correlation has been found between microhardness and elastic modulus values and beam
profile irradiance values 279. This issue could potentially have more detrimental clinical
70
results with the use of multiwave LPUs when using a non-uniform spectral irradiance beam
profile of the violet and blue light spectra.
1.8.2 Radiant exposure, irradiance, and irradiation time
There is much of a debate about the accuracy of reported values for LPUs' output in
the literature, as different studies report their findings using their terminology, which has
created a controversy 280. Because LPUs used for photopolymerization of RBCs produce
electromagnetic radiation of a specific wavelength range, primarily from 360 nm to 515 nm,
sufficient radiometric quantities should always be stated.
The radiant power is the correct term to describe the output from the LPU and is
expressed in units of mW. The next important feature of the LPU is the radiant exitance,
which characterizes the radiant power output from an LPU emitted thought out the area of
the light probe tip, expressed in mW/cm2. However, the most critical term regarding the light
source, which is often incorrectly used in the literature, is the incident irradiance. This
parameter represents a radiant power incident on a surface of the RBC material, expressed in
mW/cm2. The incident irradiance can be used to characterize the radiant exitance at 0 mm
target distance. The radiant exposure is the product of the irradiance and the exposure
duration: expressed in J/cm2. Due to a nonhomogeneous beam profile, the incident irradiance
represents a mean of all values irradiated throughout the tip surface, which is substantially
affected by the change of a tip diameter size. The tip diameter has been a center of a
controversy. There is a substantial difference, between an optical tip diameter and an
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effective tip diameter, which the former represents the actual, light-emitting area of the light
probe end.
A data set founded on the erroneous measurement of an optical instead of an active
tip diameter would potentially compromise the results of any study 279. In addition, a correct
tip diameter measurement should be comprehended to any LPU's operator. The clinically
relevant issue occurs with the Class II restorations, whether the LPUs' diameter of the light
probe irradiating the surface of Class II proximal box portion has enough of the beam
diameter coverage to irradiate the restoration. In this situation, an optical diameter of the
LPU tip should be at least 2 mm greater than the assumed surface of the restoration so that
the polymerization could be achieved in a single exposure 279, 280.
The International Organization for Standardization (ISO) has also adopted a set of
standards, mainly for testing LPUs performances, known as 10650-1 (ISO 2004) and 10650-
2(ISO 2007, updated September 2015) standards 281, 282. The ISO 10650 standard for
measuring the LPUs’ output, uses a laboratory grade power meter to measure the radiant
power output, however, that device is not readily available to dentists. The most common of
all available devices to measure the light emission from LPUs, frequently employed in dental
offices, are hand-held dental radiometers. However, it has been published that dental
radiometers were not reliable sources for the accurate measurements of LPUs' performances.
The irradiance measurements from hand-held dental radiometers are affected by a correct tip
diameter, the light beam profile and the LPUs emission spectrum 283. Furthermore, these
devices are calibrated for a particular brand and a model of LPU. Thus, measurements of
other brands of LPUs with it may not render accurate results, which would cause a significant
discrepancy 283.
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The radiant exposure, a product of incident irradiance and an exposure time is a
critical factor that controls mechanical properties of polymerized RBCs. The radiant
exposure required for any given RBC depends on two factors: a material's formulations and
the LPU's properties. Thus, these variations could be in a range from 6 J/cm2 to 48 J/cm2 as
reported in the literature 284.
Another issue that caused considerable debate over the last decade was the reciprocity
law. In photography, reciprocity has the inverse relationship between the intensity and
duration of light that determines the reaction of a light-sensitive material 285. Thus, the
exposure reciprocity law (adopted from photography) has been proposed based on the
concept that a given quality is directly subordinate on its exposure dose, but independent of
the individual values of the irradiation and exposure time 151, 152. In the wake of some
advanced LPUs that can generate more than 4,000 mW/cm2, ultra-fast polymerization
becomes an attractive feature for a significant clinician’s chairside reduction, while
researchers are trying to answer a question of the validity of the exposure reciprocity law in
dentistry.
Recent studies demonstrated that the majority of RBCs should not be expected to
follow the reciprocity law 152. As per increased irradiation, mostly above 1,500 mW/cm2, the
overall dose required to achieve an adequate conversion is also increased, and this
polymerization behavior is resultant from the dominant bimolecular termination mechanism
for the free radical polymerization. It has been confirmed that final conversion of RBCs, as
well as their ultimate shrinkage stress values, were not dependent upon a dose of light energy
delivered, but instead were dependent on the irradiance and a corresponding polymerization
rate 151, 285.
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1.8.3 Light exposure
Contemporary LPUs are usually equipped with programmable polymerization modes
that can deliver radiant energy in different forms: continuous, pulse-delay, or stepped mode.
The continuous mode delivers the same amount of exitant irradiance from the start to the end
of the polymerization cycle. The pulse-delay mode initiates light emission by a short flash of
emitted light, followed by a programmable time delay before the final exposure is performed.
In a step-polymerization mode, an initial cycle starts with a low exitant irradiance, for a
precise period, then is promptly followed by a much higher exitant irradiance. The ramp-
mode starts with zero amount of exitant irradiance, and then the amount of exitant irradiance
reaches the maximum almost immediately, continuing the same level for the rest of the cycle.
Polymerization with a pulse delay and a stepped mode are ordinarily regarded as "soft-start"
modalities. Soft start exposures typically do not start at zero, but instead initiate at a very low
level and apply that low level for a specified time period (step), or they gradually increase
output over time, until full output is generated (ramp). The soft-start polymerization modes
have been associated with a significant reduction in gap formation, due to a lower level of a
polymerization stress formation at the cavity margins because of the contraction stress 286.
Research has shown, however, that this polymerization type has been associated with
a reduction of DC, in addition to a significantly lower degree of crosslinking, which caused
substantial mechanical properties decrease 150, 168. Literature reports that a 40s radiant
exposure has not produced any statistically significant differences among the polymerization
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modes used, however, by using a ramped soft-start polymerization mode, a substantial
reduction in a contraction strain and a corresponding polymerization temperature has been
recognized 286. Consequently, with the latest RBCs' developments, in particular, with the
introduction of low-shrinkage materials and the alternative photoinitiator systems, the
relevance of this issue will probably become less important.
1.8.4 The effect of temperature on photopolymerization
Temperature has a significant effect on RBCs polymerization. It has been reported
that for each one 0C temperature increase, the corresponding polymerization reaction
increases by 1.9%. However, an initial RBCs' temperature increase from 25 0C to 35 0C,
duplicates the corresponding rate of the DC, which also correlates to an elevated and swift
polymerization stress build-up, compared to the RBCs at ambient temperature 287, 289.
Photopolymerization generates an elevated temperature due to a chemical exothermal
reaction and the incidental radiant energy from the LPUs source. The extent of generated
heat depends on the incident irradiance values, the thickness of the RBC sample, and the
corresponding polymerization rate. The LED LPUs with an extraordinary high radiant power
can generate a sudden temperature spike, which can cause a sharp temperature increase near
the pulp, as much as 5.50C, the value that is considered harmful for the pulp 290, 291. Further, a
recent in-vivo study revealed, a high, positive correlation (r2= 0.92) between the radiant
exposure and the pulpal temperature increase 291. This correlation was observed, especially in
the cases where the radiant exposure reached 80 J/cm2 which increased a 5.5 0C pulpal
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temperature 291. Depending on the tooth and patient age, the level of 5.50C pulpal
temperature increase can lead to the irreversible pulpal damage. It is interesting to note that
some RBCs, due to their much higher monomer content, exhibit an elevated exothermal
polymerization reaction, as seen in flowable RBCs 291. Thus, clinicians should avoid using
LPUs with high radiant power output, particularly with flowable RBC, due to a possible
pulpal damage.
Preheating RBC just before its placement into the preparation has recently gained
popularity among dental practitioners 292. This procedure results in viscosity reduction with
the following potential clinical outcomes: enhanced marginal adaptation, improved handling,
an increase in polymerization rate, an increase in the DC values, and improved mechanical
and physical properties. However, the exact relationship depends on the chemical
formulation and the intrinsic properties of RBCs 292. It would be recommended for the
refrigerator-stored RBCs to be preheated to at least to 450C, before the placement into the
oral cavities 292.
1.8.5 Light guide tip positioning
Adequate LPU’s light probe positioning has a crucial influence on the quantities of
light energy received by the restoration during the radiant exposure. All types of LPUs, but
the laser, deliver a non-coherent light beam profile. Furthermore, there is loss of incident
irradiance, with an increased tip-to-object distance. Manufacturers always report the
irradiance of their LPUs at 0 mm distance, without any consideration to a clinically relevant
distance in the molar region, which can be more than 7 mm 293.
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The Class II restorations, regarding location accessibility, exhibit challenging
locations for proper light energy delivery. The challenges associated with
photopolymerization in the posterior region are attributed to the rather small inter-occlusal
distance, resulting in a significant degree of a light probe positioning difficulty, in addition to
lack of visualization, rendering inadequately polymerized RBCs. An increased distance,
between the tip of the LPU light guide and the surface of the RBC restoration from 0 to 10
mm has caused a 23% reduction in incident irradiance, which was associated with a
substantial decrease in the DC at the bottom of a 2 mm thick RBC specimen 293. Good
clinical practice fosters the closest possible positioning of the LPU light probe tip to the
material being polymerized. Practitioners should always pay full attention and make sure that
the LPU's tip is positioned at the right angle to the material surface. The light probe should
be stabilized to prevent any movements during the entire polymerization cycle. In addition,
practitioners need to have a clear view of the light probe to correct any misalignment of the
light probe. The constant monitoring of the polymerization process involves a two-hand
polymerization technique. The one-hand polymerization technique usually results in poor
stabilization of the light probe tip or its misalignment, which subsequently renders an
unsatisfactory clinical outcome.
It is well-known that LED LPUs produce electromagnetic radiation in a range from
violet to blue light wavelengths. LPUs exhibit the enormous potential for ocular damage in
the range from 435 nm to 440 nm, because these blue light wavelengths are efficiently
transmitted to the retina, where they can express local harmful effects 294. Thus, clinicians
should be encouraged to use appropriate orange blue-blocker eye protection, which renders
an adequate level of safety for watching for the appropriate light probe position during the
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entire photopolymerization cycle. Furthermore, dental practitioners are required to protect
their patients and employees from ocular hazards associated with LPUs 324.
1.9 Review of studies on RBCs Longevity and Effectiveness of LPUs
Although RBC placement is a technique-sensitive procedure that requires clinicians’
full attention, polymerization by LPUs may not receive the same consideration as it should 4.
Many factors are affecting the longevity of RBCs. The main factors are the RBCs' property
and the clinical skills of the operators, as well as LPUs' performances. Those factors are
elaborated in selected studies on the RBCs’ longevity and LPUs’ effectiveness.
1.9.1 Longevity of RBCs direct restorative materials
Due to an increased demand for esthetics, as well as environmental concerns
regarding mercury in amalgam restorations, an exponential use of RBCs, has been observed,
as either a primary RBC placement or as amalgam replacement 1-4. Although amalgam has
been a direct restorative material in continuous service for more than 150 years, presently it
is not considered a material of choice for posterior restorations, due to an exponential
increase in RBC placements that was reported in many industrialized countries 4. However,
in developing countries, amalgam remains the restorative material of choice, at least for
posterior restorations, perhaps because of economic issues.
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The abundance of published studies on the clinical performance of RBC restorations
exists, however, a direct longevity comparison of different types of restorations among
different studies is almost impossible for numerous reasons 296. The following studies
published in the last 13 years are about to present more knowledge on performances and
longevity of posterior RBC restorations.
The first comprehensive study, (Manhart et al., 2004) 295, which has evaluated
longevity and annual failure rates of Class I /II restorations, restored with amalgam, direct
RBC, compomers, glass ionomers and derivative products, composite and ceramic inlays,
and cast gold, since the 1990s to 2003. The analysis has shown that an average annual failure
rate (AFR) for posterior restorations were: 3.0% for amalgam restorations, 2.2% for direct
RBCs, 3.6% for direct RBC with inserts, 1.1% for compomers, 7.2% for regular glass
ionomer (GI) restorations, 7.1% (for tunnel GIs, 6.0% for ART GIs), 2.9% for composite
inlays, 1.9% for ceramic restorations, 1.7% for CAD/CAM ceramic restorations, and 1.4%
for cast gold inlays /onlays. The factors that were associated with amalgam restorations
failure included secondary caries, a high incidence of bulk and tooth fractures, in addition to
cervical overhangings and marginal discrepancies. The principal mode of RBC restoration
failure included secondary caries, bulk fractures, marginal deterioration, and wear. This study
showed that amalgam and RBC restorations exhibited similar annual failure rate.
An international study that evaluated retrospective longitudinal data for posterior
RBC restorations, which covered a period from 1996 to 2011, with a minimum observation
period of five years included a total of 34 Class I/II studies (Demarco et al., 2012) 297. The
results revealed AFR for vital teeth ranged from 1% to 3%, while at the same time AFR of
non-vital teeth 2% to 12.4%. Interestingly, the study was the first to indicate that differences
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in RBCs’ filler characteristic may have some influence on restoration longevity. The
principal factors accredited to failure were secondary caries and restoration/tooth fracture,
however, patient caries-risk factor, the clinical setting of the study, and the socioeconomic
characteristics of the study population expressed the effect to a certain degree.
A Systematic Review and Meta-analysis (Opdam et al., 2014) investigated nine
prospective and three retrospective studies on direct Class II or Classes I and II RBC
posterior restorations in the permanent dentition 298. The analysis consisted of a total of 2,816
restorations (2,585 Class II and 231 Class I). It was concluded that the overall RBCs annual
failure rate at five years and ten years was 1.8% and 2.4%, respectively. It was reported that
secondary caries and bulk fractures were frequently associated with the restoration failure.
The authors observed that a yearly failure rate was significantly higher in high-caries-risk
patients (4.6%) compared to low-caries-risk patients (1.6%) 298. The findings revealed that a
patient caries-risk status has a substantial influence on long-term survival of posterior RBC
restorations.
A 10-year retrospective study investigated longevity of 2-surface/3-surface Class II
restorations (molar/premolar), using 4 microhybrid RBCs (Lempel et al., 2014) 299. A total of
225 adult patients (86 males, 139 females) with 701 RBC restorations were evaluated by two
operators using the modified USPHS criteria. The average AFR of RBCs tested was 0.52%,
with the range from 0.08% to 0.71%. According to the study authors, a 10-year survival rate
of 97.86% could be associated to a well-motivated patient group (university clinical practice
affiliation), which exhibited high socio-economic standards and were subjected to periodical
examinations, oral hygiene, and instructions regarding dietary habits. Similarly, the main
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reasons for the failures in this study were secondary caries, fracture, and endodontic failure.
Furthermore, similar survival rates were observed between molar and premolar restorations.
Results of a randomized controlled 27-year follow-up study (Pallesen et al., 2015) of
three RBCs in the Class II restorations, showed acceptable longevity over the 27-year
evaluation period 300. Thirty participants, 25 females and five males (mean age 38.2 years,
range 25–63), received at least three Class II restorations of moderate size. The evaluation
was carried out with modified USPHS criteria at baseline, 2, 3, 10 and 27 years. The 27-year
overall success rate was 56.5%, with an annual failure rate of 1.6%. The failed RBC
restorations were analyzed for the restoration failure cause. It was discovered that secondary
caries (54.1%), occlusal wear (21.6%), and bulk fracture (18.9%) were reasons for RBCs
failure. In addition, extended longevity was recognized with the 3-surface restorations, in
comparison to 2-surface restorations, however, the difference was not statistically significant.
A 2014 Cochrane systematic review of longevity of direct RBC restorations versus
amalgam restorations in permanent posterior teeth, presented a different picture 301. The
authors included seven clinical trials, separated into two groups (a two and a five split-mouth
trials group) in the systematic review. The analysis of a two-trial group included 1,645 RBC
restorations and 1,365 amalgam restorations (921 in children), while the five split-mouth
trials analysis included 1,620 RBC restorations and 570 amalgam restorations (unspecified
number of children). The results demonstrated that in the 5-7 years’ follow-up of RBCs
exhibited a significantly higher risk of failure, compared to amalgams, with a risk ratio (RR)
1.89, and an increased risk of secondary caries RR 2.14. However, without any substantial
evidence of restoration fracture risk, which was recorded to be RR 0.87. Nevertheless, the
authors reported some problems associated with the analysis: all seven trials were
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experiencing a high-risk of bias, and there were problems with data recording in the five
split-mouth trials. Moreover, analyzed data were perceived as "low-quality evidence." The
authors concluded that posterior RBC restorations exhibited a decreased longevity and a
higher incidence of secondary caries than the amalgam restorations. However, the frequency
of bulk fractures is similar for both materials, during the observed period of 5 to 7 years. It
was concluded that RBC restorations have a shorter lifespan in comparison to amalgams,
however, long-term, and superior quality randomized clinical trials should be conducted to
elucidate the current state of the RBC restorations longevity.
An Austrian study (Beck et al., 2017) 302, analyzed RBC longevity of the Class I/II
prospective studies and clinical trials that occurred from 1996 to 2015, which included a total
of 88 studies. Studies were classified based on the observation period (all studies vs. short-
term vs. long-term studies). The average AFR was 1.46% for short-term studies, and 1.97%
for long-term studies. The difference in failure rates between materials tested was not
significant. Further analysis showed the influence of operators, isolation techniques and
bonding systems on the overall failure rate to be not significant, while the filler-size effect
was statistically significant. The results of short-term observation studies (>5 years), the most
common reasons for failure were fractured restorations, followed by marginal defects and
secondary caries. In more prolonged study periods (<5 years) secondary caries and tooth-
fracture were the predominant reasons for failure.
A recent international study (Borgia et al., 2017) 303, examined clinical performance
of RBC restorations placed in Class I/II molars/premolars over the course of 5 to 20-year
observation period. The study analyzed 105 restorations, from which 101 RBCs were placed
on vital teeth, and only 4 restorations were placed in endodontically treated teeth, restored
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without a post. The subjects consisted of 40 women and 21 men. The evaluation used
modified Ryge criteria (marginal adaptation and discoloration, and surface morphology, with
a 3–level assessment criteria). The observed mean survival time of restorations that remained
functional was 11 years and seven months. At the time of the examination, 103 (98%) RBCs
were still in function: 62 premolars and 41 molars. Two (2%) RBCs had failed. It was
observed that 98 RBC restorations (95.1%) were rated as clinically successful. It was noted
that there was a significant difference between RBC clinical performances of premolars and
molars (60.2% vs. 39.8%), and corresponding restoration size and location. Clinical
performances among hybrids, microhybrids, nanohybrids, and microfilled or midfilled RBCs
were evaluated. The analysis revealed no statistically significant difference among all RBCs
used. However, Heliomolar (Ivoclar-Vivadent), an inhomogeneous microfilled RBC,
exhibited the longest mean survival time.
A Finish study (Palotie et al., 2017) 304, analyzed the longevity of 2-surface and 3-
surface posterior restorations according to the tooth type, restorations’ size, and the
restorative material used in Public Dental Services in Helsinki City. A total of 5,542
restorations were placed in 40% premolars and 60% molars at the baseline. These 2-surface
and 3-surface posterior RBC and amalgam restorations were followed indirectly from 2002
to 2015. Out of a total number of restorations, 93% were RBC, and 7% were amalgam. Sixty-
one percent of the restorations were placed in female patients, and 39% were placed in male
patients, and recorded patient age was between 25-30 years. AFRs of restorations were
calculated separately by tooth type, restoration size, and the material used. The most
extended median survival time and the smallest failure rates were found for teeth in the
maxilla, for premolars, and for 2-surface restorations. The median survival time of all
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restorations was 9.9 years. It was noted that restoration replacements happened less often in
the maxilla (AFR 4.0%) than in the mandible (AFR 4.7%). The median survival time of
RBCs was greater for 2-surface than for 3-surface restorations. The overall AFR across 13
years was 4.3%, as observed to be smallest for 2-surface RBC restorations in premolars and
largest for 3-surface RBC restorations in molars. Furthermore, the mean AFR in 13 years
showed that the values of 2-surface amalgams and 2-surface RBCs in molar teeth were
similar (4.7 vs. 5.0). However, the mean 13-year AFR for 3-surface amalgams was
statistically significant in comparison with 3-surface RBCs in molar teeth (5.2 vs. 7.1). The
authors concluded that longevity of posterior RBC multi-surface restorations is comparable
to the longevity of amalgam restorations.
1.9.2 Effectiveness of LPUs in dental practices
Although there was a plethora of laboratory studies on LPUs' performance, however,
a small number of studies that evaluated LPUs in dental practices have been published. The
first ever published study that characterized LPUs' performance in Britain's dental practices
was (Dunne et al., 1996) 305, a survey of QTH LPUs effectiveness and a comparative
evaluation of light testing devices. The authors characterized the light output from 49 LPUs
in clinical use. Also, they measured the effect on depth of polymerization (DOP) of RBCs
caused by a range of light outputs, and they assessed the relationship between radiometer
meter readings and DOP of RBC in a human tooth model and a Heliotest. The evaluation of
LPUs was carried out by an analog “Lamp Checker” radiometer. Although the manufacturer
of the radiometer considered optimal light output to provide a meter reading to be within the
range 5.0 to 7.0, the mean meter reading produced by the 49 LPUs using a lamp Checker
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radiometer was 4.4 (+/-2.4 SD), range 0.3 to 10.0. The cumulative study results (DOP and
Lamp Checker) revealed that more than 50% of tested LPUs have been underperforming.
Following the previous study findings, another study (Burke et al., 1997) 306 brought
new light about the QTH LPUs effectiveness in dental practices in Great Britain. These
findings were based on a survey that tested LPUs effectiveness used in general dental
practices by conducting the Heliotest (Ivoclar-Vivadent) and Z100 RBC (3M ESPE).
Heliotest is, in essence, a modification of a simple scrape test. The test utilizes a 3 mm
diameter split plastic mold, which is filled with the RBC, and polymerized with the LPU
from the top side only. At the bottom side of the specimen, any soft material is removed. The
height of the remaining hard, polymerized material is measured top-to-bottom and expressed
in mm, which represents the Heliotest result. The LPUs characterization was in a function of
DOP assessment (Heliotest and Z100). The mean incremental depth stated as being used by
the respondents was 2.6 mm. It was found that 43.5% of the participants' LPUs (n = 63)
produced DOP below that value. Although reported results could exhibit inaccuracies to a
certain degree, the findings were in line with the previous study.
An Australian study, (Martin et al., 1998) 307 assessed the adequacy of QTH LPUs
output, their pattern of usage and the maintenance, based on the survey conducted by 3M
Health Care representatives who paid visits to approximately 4% of dental practices in
Australia. All 3M representatives were trained so that data collection could be standardized.
The incident irradiance of LPUs was tested using an analog radiometer (Demetron Research,
Danbury, CT, USA). The irradiance recordings were assigned into three categories: 400
mW/cm2 and above (sufficient intensity), 201-399 mW/cm2 (marginal intensity; where
additional exposure time would be required), and 200 mW/cm2 and less (low intensity). A
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total number of units assessed was 214. Based on the minimum irradiance passing criterion
of 400 mW/cm2 that was established for this study, it was found that 47.7% of LPUs tested
showed substandard performances. Moreover, the age of LPUs tested, ranging from 6 months
to 19 years. The participating dental practitioners exhibited a general lack of awareness of the
need for maintenance of their LPUs.
Another international study (Pilo et al., 1999) 308 published results from the Middle
East with similar findings. The incident irradiance of 130 LPUs from 107 dental offices was
assessed with analog (P/N 10503) and heat (P/N 10517) radiometers (Demetron Research
Corporation, Danbury, CT, USA) according to manufacturer's instructions. Also, Knoop
hardness values of 50 randomly selected LPUs were taken to calculate relative hardness
(bottom/top ratio). The irradiance of tested LPUs ranged from 25–825 mW cm2. The study
design followed manufacturer’s recommendation that incident irradiance value of 300
mW/cm2 was recognized as a passing standard. The results of this study revealed that 55% of
LPUs tested were underperforming. However, data obtained by the heat radiometer showed
that only 24% of tested LPUs were underperforming. When hardness data applied, 46% of
LPUs tested were found to be inadequate, which was in line with previous findings.
A South African study (Solomon et al., 1999) 309 conducted assessment of QTH
LPUs used in private dental practices, and in state and, trade union dental clinics. The
irradiance of each LPU was measured using an analog radiometer Cure Rite (Efos,
Mississauga, ON). The mean reported values ranged from 22 to 448 mW/cm2. The authors
reported that 45.7% of LPUs tested functioned below the proposed standard (300 mW/cm2).
The following studies conducted in Canada and Brazil exhibited similar results,
regardless of the study design. The Toronto study (El-Mowafy et al., 2005) 310, which
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subsequently was succeeded by the Brazilian study (Santos et al., 2005) 311, evaluated a large
sample size that measured incident irradiance of QTH LPUs, using a dental radiometer in
conjunction with a Knoop microhardness test. The Toronto study evaluated incident
irradiance values of 214 QTH LPUs with an analog radiometer. The number of the
participating private dental offices was 100. In this study, an irradiance level of 400 mW/cm2
and above was deemed sufficient. It has been shown that 65 LPUs had irradiance values
below 400 mW/cm2, and 14 LPUs had irradiance values lower than 233 mW/cm2. Therefore,
a total of 30.3% of LPUs tested generated less than the sufficient irradiance values, which
was a significant improvement in comparison with the 1999 study net results, regardless the
difference in methodologies employed in those studies. The Brazilian researchers 311 visited
100 private dental offices in El-Salvador, Brazil and evaluated 120 QTH LPUs, using an
analog radiometer (Optilux 100, Kerr). The recorded exitant irradiance values ranged from
10 to 1,000 mW/cm2, with an average mean value of 255.8 mW/cm2. Based on a passing
criterion that was set at 300 mW/cm2, 56% of LPUs tested were considered underperforming.
The age of LPUs tested varied significantly, with a range from 1 to 21 years.
Other international studies evaluated several QTH and LED LPUs, with interesting
findings. First, an Indian study (Hedge, 2009) 312 investigated a pool of 200 LPUs, consisted
of 81 LED and 119 QTH LPUs. An L.E.D Radiometer (Kerr, Orange CA) was used. They
set up 400 mW/cm2 as a passing irradiance criterion. The results showed that only 10% of
LED LPUs and 2% of QTH LPUs were able to meet this criterion. These results were very
discouraging and alarming, since most of LPUs tested could not sufficiently polymerize
RBCs.
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Studies conducted in Saudi Arabia (Al-Shaafi et al., 2012) 313 followed by Jordanian
study (Maghaireh et al., 2013) 314 shed some new light on LPUs' effectiveness in private
dental practices in the Middle East. It is critical to note that the Saudi study had a mixed pool
of 140 LPUs (67 LED LPUs and 73 QTH LPUs), which was examined by a laboratory-grade
spectroradiometer, in selected urban and rural areas of Saudi Arabia. The passing criteria
were 300 mW/cm2 for QTH, and 600 mW/cm2 for LED LPUs. The results showed that urban
areas had a significantly higher number of LPUs that performed according to the passing
criteria. The passing scores reported in rural versus urban areas were 32% and 49%,
respectively, for QTH LPUs, and 66.7% and 70.5%, respectively, for LED LPUs. The results
showed that LED LPUs outperformed QTH LPUs by a large margin.
The Jordanian study 314 examined a total of 295 LPUs (154 LED and 141 QTH) using
the L.E.D Radiometer (Kerr, Orange CA) in 295 dental clinics. The researchers found that
79.2% of LED LPUs and 26.2% of QTH LPUs could generate incident irradiance at least of
300 mW/cm2. Again, these results showed that QTH LPUs exhibited severe performance
issues and that they should be replaced with LED LPUs.
Furthermore, the results from a Swiss study (Sadiku et al., 2010) 315, that used an
integrating sphere to evaluate a total of 220 LPUs, which more than 65% were LED LPUs,
showed that the total amount of insufficient and damaged LPUs was as high as 40%.
A Kenyan study (Kassim et al., 2013) 316 examined (irradiance, depth of
polymerization (DOP) and Vickers hardness) of 83 LPUs in private and public dental health
facilities in Nairobi, Kenya. The irradiance assessment was conducted using a Cure Rite
radiometer (Efos, Mississauga ON). LPUs tested included a total of 43 (51.8%) LED LPUs,
39 (47.0%) QTH LPUs, and only one (1.2%) Plasma Arc Curing (PAC) LPU. Thirty-five
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(42.17%) LPUs generated the irradiance levels in the range from 0 to 300mW/cm2, while
forty-three (51.8%) of LPUs tested produced the irradiance between 300 and 1,200 mW/cm2.
The authors did not differentiate the irradiance value differences among LED, QTH, and
PAC LPUs. However, they concluded that generally, any irradiance level above 600
mW/cm2 yielded acceptable surface microhardness but only intensities above 900 mW/cm2
resulted in adequate DOP (2 mm or more).
The latest published international study (Hao et al., 2015) 317 investigated the
incidence irradiance values, and LPUs` service information used in private dental offices in
Changchun City, China. The irradiance recordings were grouped into two categories: 300
mW/cm2 and higher (adequate), and 299 mW/cm2 and less (inadequate). A digital Cure Rite
radiometer (Dentsply Caulk, Milford, DE,) with a range of 0–2,000 mW/cm2 was used to
characterize the irradiance of LPUs tested. A total of 196 LPUs from 156 private dental
offices in Changchun City were tested. These units included 132 QTH LPUs and 64 LED
LPUs. Survey samples covered 31 brands of LPUs, 14 of which were foreign brands, and 17,
domestic. The mean irradiance of the 196 light units was 453.1 mW/cm2, and the distribution
of the irradiances varied widely from 0 (2 units) to 1,730 mW/cm2. The average irradiance
value of the QTH LPUs was 340.5 mW/cm2, while the LED LPUs had an average irradiance
of 682.5 mW/cm2. The results showed that 31.6% of all tested LPUs were performing below
the 300 mW/cm2 passing criterion. Also, 76 (57.6%) of QTH LPUs and 58 (90.6%) of LED
LPUs exhibited irradiance of 300 mW/cm2 or more. The study researchers concluded that
LED LPUs have a more extended clinical life in comparison to QTH LPUs.
A Norwegian study (Kopperud et al., 2017) 13, which examined light polymerization
procedures: LPUs` performance, clinicians’ knowledge, and safety awareness among
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participants in Norwegian Public Dental Service in 2015, presented some interesting
findings. The survey investigated dentists’ knowledge on practical use and technical features
of their light sources as well as their safety awareness. The authors invited a total of 1,313
dentists to participate in the study. However, only 748 dentists responded and agreed to
participate. In addition, 35 dentists from those who responded, stated they were not proficient
in restorative procedures and were excluded from statistical analyses. It is important to notice
that out of 713 participants in the study, 69.6% were females and 30.4% males. The results
showed that participants on a daily basis used the average of 57.5% of their time for RBC
placements, which ranged from 1 to 30 (mean 7.7, SD 3.6). The average radiant exposure per
a single increment of RBC was 27s. The longest individual mean polymerization cycle per
day was about 100 cycles longer than the shortest one. Furthermore, the majority of
participants (78.3%) were unaware of the irradiance value of their LPUs, while a
significantly higher percentage of dentists in this group had not performed regular
maintenance of their LPUs compared with all participants (17.1% vs. 3.3%). Moreover,
almost one-third of participants used inadequate eye protection during the light
polymerization. The findings revealed considerable variation among Norwegian dentists in
the Public Dental Service concerning participants' knowledge about light polymerization, the
LPU common clinical daily usage, the maintenance protocol implemented in their practices,
and safety protection during the polymerization. Although two-thirds of participants showed
adequate knowledge and practice in regard to photopolymerization, the number of dentists
that have not presented sufficient awareness about personal protection during the
polymerization, or lack of knowledge about their LPUs, is still very high. Furthermore, the
majority of the study participants (78.3%) were unaware of the irradiance value of their
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LPUs, thus rendering the radiant exposures uncertain. Based on these findings, the authors
concluded that a substantial number of practitioners in Norwegian Public Dental Service had
exhibited inadequate knowledge and technical skills for sufficient RBC polymerization. The
findings are somehow alarming because in Scandinavian countries amalgam placement is
almost eliminated and has been replaced with RBCs.
Although all of these studies reported that LPUs employed in dental practices include
a relatively large number that produce inadequate irradiance levels, it was evident that LED
LPUs have shown much better results than QTH LPUs. Although, geographic variations,
different methodologies, and differences in study designs, present challenges in making
meaningful comparisons, needless to mention that effectiveness of LED LPUs require further
investigation because the percentage of inefficient LED LPUs remains relatively high.
All the previously mentioned studies concluded that there was a persisting problem
associated with LPUs used in dental practices. In addition, the effectiveness of LPUs used in
dental practice depends on the LPUs performance as well as the operator's ability to deliver
light energy to the restoration. Therefore, there was a need to conduct a new study to
determine the combined effects of both, LPU efficiency and the adequacy of operator's light-
application technique, collectively.
To the authors’ best knowledge, this would be an essential and pertinent study, which
would shed some light on those two factors, closely related to the proper polymerization of
RBC restorations. For that reason, we focused our research on the LED LPUs used in
Toronto private dental practices, to characterize crucial factors that have been shown to have
profound effects on light polymerization of RBCs. Furthermore, the data obtained in this
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study will be compared with the QTH LPU survey conducted in Toronto, Canada, ten years
prior.
Chapter 2
Objective and Aims
92
The literature review made it clear that adequate RBC polymerization is highly
dependent on the capacity of LPUs to generate sufficient quantities of energy at appropriate
wavelengths and the ability of dental practitioners to deliver the right amount of energy
during the polymerization.
With state-of-the-art equipment, the goal of this project was to measure the effectiveness of
and characterize LED LPUs in collaboration with 100 dental practices in the Greater Toronto
Area.
The aims of this study were:
1. Quantify exitant irradiance and the 10s radiant exposure of LED LPUs in the
participating dental practices using the CheckMARC® system,
2. Assess LPU operators' technique and consistency of a10s radiant exposure with LED
LPUs using the MARC® Patient Simulator, and
3. Evaluate the mechanical properties of RBC samples polymerized with previously
tested LED LPU in the participating dental practices by conducting the Biaxial
Tensile Strength (BTS) test and Knoop b/t ratio hardness (RH) test.
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Chapter 3
Manuscript 1
Assessment of LED Light-Polymerization Units Used in
Toronto’s Dental Private Practices
94
Assessment of LED Light-Polymerization Units Used in Toronto’s Dental Private
Practices
Dave D. Kojic DDS, MSc. PhD-candidate
Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto, Toronto,
ON, Canada
*Omar El-Mowafy, BDS, PhD, FADM
Professor and Head, Department of Restorative Dentistry, Faculty of Dentistry, University of
Toronto, Toronto, ON, Canada
Richard Price DDS, MS, PhD
Professor, Department of Clinical Dental Sciences, Faculty of Dentistry
Dalhousie University, Halifax, NS, Canada
Wafa El-Badrawy, BDS, MSc.
Associate Professor Department of Restorative Dentistry, Faculty of Dentistry, University of
Toronto, Toronto, ON, Canada
Gildo Santos, DDS, MSc, PhD
Associate Professor Chair - Restorative Dentistry, Schulich School of Medicine & Dentistry,
Western University, London, ON, Canada
*Corresponding author Omar El-Mowafy, BDS, PhD, FADM
Department of Restorative Dentistry
Faculty of Dentistry, University of Toronto
124 Edward Street, Toronto, Ontario M5G 1G6, Canada
95
Abstract
Objectives: To characterize the radiant exposure of LED light-polymerization units
(LPUs) in 100 private dental practices in Toronto and evaluate the effect of age on their
performance. Materials and Methods: 100 dental practices from the Greater Toronto Area
were recruited. Each LPU was assessed for age and light-guide condition, and its 10s radiant
exposure (RE) measured by the CheckMARC® (BlueLight Analytics, Halifax NS). A
Bluephase Style (Ivoclar-Vivadent, Amherst, NY, USA) unit served as a reference LPU. A
10s RE was multiplied by 2 to represent a 20s RE and compared to an arbitrary passing
criterion of 16 J/cm2. Data were statistically analyzed using Pearson correlation and
independent-samples t-tests at the 95% Confidence level. Results: 113 LPUs were tested.
LPUs age ranged from 1 to 5.5 years, and damage levels to light-guide were between 0 and 4
(scale 0/8). RE ranged from 3.8 to 59.6 J/cm2 (M=21.7, SD=8.6). A considerable number of
LPUs (26 LPUs) did not pass the 16 J/cm2 criterion. The independent-samples t-tests showed
that LED LPUs that did not produce the minimum energy level, were older, t (113) =5.16,
p<0.001, and their light-guide had damage, t (113) =7.17, p<.001. Conclusions: A wide range
of RE (3.8-59.6 J/cm2) was recorded among 113 LPUs tested. Seventy-seven percent of
LPUs were able to generate 16 J/cm2 RE. LPUs with accumulative damage above 2.5(0/8),
and at least 3.5 years in service could not produce a study-required RE. A strong, (r=-0.67,
p<0.001) negative correlation between LPUs' age and light-guide damage and LPUs' incident
irradiance was found. Clinical significance: 23% of LPUs tested failed to meet an arbitrary
RE value of 16 J/cm2, while 47.8% of LPUs tested performed below the reference LPU RE
value of 21 J/cm2.
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Keywords: Light Polymerization Unit (LPU); Irradiance; Radiant exposure;
CheckMARC® device.
97
Introduction
Due to an increased demand for natural-looking restorations, as well as health and
environmental concerns regarding the mercury in amalgam restorations 1, a rapid increase in
use of resin-based composite (RBC) materials have been observed, as either a primary RBC
placement or amalgam restoration replacement 2, 3. However, the literature has shown that
longevity of RBCs has been significantly shorter than that of amalgams 4. The most
commonly reported consequences associated with RBC failure are secondary caries and bulk
fractures 5, 6. The failure of RBC materials could be attributed to insufficient polymerization,
in particular in the Class II proximal boxes 7. Although RBC placement is a technique-
sensitive procedure that requires clinicians’ full attention, however, polymerization by Light
Polymerization Units (LPU) may not receive the attention it merits 7. Adequate RBC
polymerization is much more than a simple point-and-click procedure 8.
In contemporary dental offices, light-emitting diode (LED) LPUs have become the
standard units for RBC materials polymerization 9. The LED LPUs have not been exclusively
used for the polymerization of RBC materials, rather their use has been extended to a wide
range of clinical procedures, from fissure sealants to veneer cements. The LED light sources
offer several advantages over other light sources, such as a relatively high radiant exitance,
enhanced mobility, and affordable price structure 10. Currently, two main types of LED LPUs
are available. Single peak LED LPUs, which deliver their spectral irradiance in the closest
range to the maximum absorption spectrum of camphorquinone (CQ) photo-initiator
(ƛmax=468 nm), referred to as monowave LPUs 11. Multiwave LED LPUs (also known as
Polywave®, a trade name of the Ivoclar-Vivadent company), which are suitable for activation
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of CQ and the alternative photoinitiators (primarily PPD, Lucirin TPO, and Ivocerin), usually
deliver two peaks of spectral irradiance 11.
Because LPUs produce electromagnetic radiation of specific wavelengths, mainly
from 360 nm to 515 nm, radiometric nomenclature should be used 12. The output from LPUs,
which is accurately termed the radiant power, is expressed in mW. Another essential feature
of any LPU is the radiant exitance, which characterizes the LPU`s radiant power output
emitted throughout the area of the light probe tip, expressed in units of mW/cm2. The most
important feature of LPUs, often incorrectly reported, represents the incident irradiance,
which is the radiant light power delivered to the RBC material surface, expressed in
mW/cm2. The term irradiance can also be used to characterize the radiant exitance at 0 mm
light source distance 13. The radiant exposure is the product of irradiance applied to the RBC
surface and exposure time, and its value is expressed in J/cm2. Due to a non-uniform
irradiance beam profile 13, the value represents a mean of all values irradiated throughout the
tip of the light probe surface, which is considerably affected by a change to the diameter size
at the tip of the light probe 14.
Dental radiometers usually are used for measurement of the LED LPUs' irradiance in
most dental practices. However, some studies have warned about erroneous readings
associated with dental radiometers 15, 16. However, they can be used to rank LPUs on a
relative basis.
Adequate polymerization of RBCs is critical for their long-term clinical success. A
commonly held assumption that an equivalent of 16 J/cm2 of delivered radiant exposure to a
single 2 mm thick increment of RBC material is considered sufficient for adequate RBC
polymerization 17, 18, 19. However, the manufacturers' recommended minimum radiant
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exposures for RBC polymerization can vary to a great extent, depending on intrinsic
properties and chemical formulation, as well as the shade of the material and the radiant
exitance of the LPUs 19, 20, 21. Dental practitioners are not able to effectively measure the
radiant exposure at the RBC restoration. As a result, dental practitioners can either deliver
too much energy to the restoration, which can locally generate heat, or deliver too little
energy, which can cause inadequate polymerization 22, 23. Recent research has found that
contemporary LED LPUs with a radiant exitance of more than 1,000 mW/cm2, designed for a
10s exposure, often were habitually extended to 20s exposure 24. There is a plethora of
studies on performance of LED LPUs in private dental practices. However, based on earlier
studies, LPU assessments were conducted by using dental radiometers and with the
microhardness measurements for calculation of relative hardness 25- 30. A previous study
conducted to determine the state of LPUs in Toronto's private dental practices which was
based on quartz-tungsten-halogen (QTH) LPUs' assessment, was published in 2005 when
QTH LPUs were the primary source for photopolymerization 26. Current literature indicates
that LED LPUs have become the gold standard for the polymerization of RBC materials in
experimental research settings as well as the most frequently used light sources in private
dental offices 31. The percentage of clinically unacceptable LPUs found in different dental
practices varied considerably 25-30, from 10% to as much as 70%. However, these data should
be interpreted with caution, since the study designs differed significantly, and all of these
studies employed dental radiometers that had proven to exhibit inaccuracies 32.
This study aimed to examine the incident irradiance of LED LPUs in 100 private
dental practices in the Greater Toronto Area, and to evaluate the effect of age and damage of
light-guide of these LPUs' measured incident irradiance as recorded by a CheckMARC®
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device. The Check MARC® has a built-in laboratory grade UV-VIS spectrometer USB4000
(Ocean Optics, Dunedin, FL), with a spectral range from 365 to 515 nm and a laboratory-
grade cosine-corrected sensor. This device can efficiently measure incident irradiance up to
10,000 mW/cm2 (integration times are used to enable changes in intensity and prevent
saturation). A 16 mm diameter sensor is individually calibrated using a NIST-referenced
light source (HL-2000 Tungsten Halogen, Ocean Optics, Dunedin, FL). The objective was to
test the following research hypotheses. The first research hypothesis was that LED LPUs
tested can deliver 16 J/cm2 radiant exposure in a 20s exposure. The second research
hypothesis was that neither the age of the tested LPUs nor the damage to the light probe
would affect the 16 J/cm2 radiant exposure in a 20s exposure cycle.
Materials and methods
Upon receiving the University of Toronto Research Ethics Board approval, private
dental practices were recruited to participate in the study. The 2012 listing of dentists
published by the Royal College of Dental Surgeons of Ontario was used to identify dental
practices in the Greater Toronto area. Over 350 potentially qualified practices were initially
contacted via telephone to determine their eligibility to participate in the study. However,
only 250 qualified dental practices were provided with the study protocol, along with a letter
of consent. Only 100 dental practices were retained for the study.
The recruitment criteria for the study participants included the following: a general
practice physically located within the Metropolitan Toronto area, with proficiency in RBCs
placement. Testing of LED LPUs was conducted in participating dental practices using a
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commercial, laboratory grade light measuring device (CheckMARC®, BlueLight Analytics,
Halifax, NS), (Figure 3.1). The device was attached to a laptop computer. One test
administrator conducted all the testing in each of 100 participating dental practices. The
following LED LPUs information was obtained and entered into the CheckMARC® V2
software: the make and model of the LED, visual and tactile inspection of the light probe for
cracks, chips, or debris, use of a physical barrier, and sterilization method. The
CheckMARC® uses the planar active area of the light tip to calculate incident irradiance
measurements 33. The planar active area is defined by the area of the light tip from which
light exits. When a light tip has a curved lens or no flat surface, the area equivalent to the
opening plane is measured such is the case for the Valo LPU (Ultradent, Utah), which has a
9.7 mm diameter opening for the lens 33. Technically the curved lens has a more extensive
area but the plane at the tangential point of the lens to define the area has been used. The
terms internal or external diameter is not adequate to explain the area measurements for all
light types. Thus, to adequately use this device, proper identification of an external diameter
size was required. Several LPUs have more than one size of a light probe diameter. The
external diameter was measured using Mitutoyo digital caliper (Mitutoyo Canada,
Mississauga ON), in which the corresponding light probe diameter from the CheckMARC®
V2 software was selected.
The light probe level of damage and resin build-up was appraised by using an
evaluation scale (0-8), where 0 represents no damage, and 8 represents the maximum
involvement of the light emitting surface of LPUs. The damage on the probe surface was
typically observed as a localized crescent along the margins and represented the glass
chipping at the marginal area, due to a fall. The resin build-up was also observed at the
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periphery of the light probe, very close to the margin. In many instances, resin build-up was
localized at the top of damaged surface, overlapping each other. The light probe of each LPU
tested was wiped off with disinfecting wipes to remove any dirt, before commencing the
incident irradiance test. The image of the light probe was imported into the photo-editing
software (Adobe XD™, Adobe Systems, San Jose, CA). The circle represents the light
emitting area of the majority LPUs. The damage location resembled a crescent, and it was
always noticed at the margin of the LPU light-guide tip, not passing more than 5 mm towards
the center of the probe. The software has an option to cut the imported image circle into eight
equal sectors and to calculate the area that represented the damage. Furthermore, the software
has a tool which measures and calculates the area of damage along the edge of the light
probe, as shown in Figure 3.2. The grey elliptical areas in the figure represent the captured
physical damage on the surface of the light probe. However, this pattern did not work with
the SmartLite Max (Dentsply), which has an elongated ellipsoid light probe surface. In this
situation, the damage level was assessed by dividing the light probe surface into eight equal
sectors and then applying the previous methodology. The LPUs have been tested “as-is” to
obtain an accurate indication of their photopolymerization potential.
After entering the required information, the CheckMARC® device was attached to
the computer. The test administrator conducted the test by applying the LED LPUs light tip
directly to the Check MARC® sensor and completing a 10-second test cycle. This was
repeated 3 times. The mean of 3 measurements was displayed on the computer screen. The
measurements were taken in the standard mode of the LPU and without any protective
barrier. The Check MARC® software generated a LED LPU report, which included
irradiance (mW/cm2). A 10s radiant exposure was multiplied by 2 to represent a 20s radiant
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exposure. The rationale for using the 10s, and not 20s radiant exposure, was only for
efficiency purpose in a busy dental office. However, it is well-known that LPUs emit a
particular quantity of light after the termination of the process. Thus, the CheckMARC®
device could capture the amount of light energy beyond a 10s-recording cycle. Thus, when a
report was generated, it often showed the radiant exposure in 10.1s.
A computed 20s average radiant exposure of a reference LED LPU (Bluephase Style,
Ivoclar-Vivadent), which generated 21 J/cm2, was used as a positive control. The rationale to
use a positive control was to determine how the top of the line LPU (at the start of the
project) would be scored when comparing the tested LPUs.
• Statistical analysis:
Descriptive statistics were used to evaluate the irradiance of the LED LPUs tested. To
assess the association between the LPU light probe damage and age of the LPU effect on
radiant exposure, values in the function of the required energy of 16 J/cm2, the data were
statistically analyzed using the Pearson correlation (alpha of 0.001) and independent-samples
t-tests. Furthermore, monowave and multiwave LPUs were compared for the level of damage
and their age, using an independent-samples t-test and chi-square test. All statistical tests
were conducted using a pre-set alpha of 0.05. Windows-based statistical software (SPSS
version 22, SPSS Inc., IBM, Somers, NY) was used for statistical analyses.
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Results
The Q-Q plots were used for normality assessment, and the results of the incident
irradiance values of multiwave and monowave LPUs data distributions are presented in
Figures 3.5 and 3.6.
The Levene's test was employed for testing of homogeneity of variance across the
comparison groups. The test was not significant, (p=0.298), thus, homogeneity of variance
across the comparison groups was assumed. The power analysis using the G Power program
for the independent and paired t-tests revealed that the sample size of 113 was capable of
detecting a medium effect size with the independent samples t-tests (Cohen's d=.53), and a
small effect size with the paired-samples t-tests (Cohen's d=.23). Additionally, for the Chi-
square test of independence, guidelines from an article (Oyeyemi et al., 2010) 34 was used for
the power analysis. According to these guidelines, the study sample size of 113 can detect
only large effects with the power of .80.
The pool of tested LED LPUs consisted of 32 multiwave and 81 monowave LPUs. In
the multiwave group, the Valo (Ultradent, South Jordan, UT) was the most common (57.6%),
while Demi Plus (Kerr, Orange, CA) was the most common (26.3%) in the monowave group.
The interpolated radiant exposure values ranged between 3.8 and 59.6 J/cm2 (M=21.7,
SD=8.6), using a 20s radiant exposure. The average computed 20s radiant exposure was 22.3
J/cm2 (SD=9.7) for monowave LPUs and 20.4 J/cm2 (SD=4.5) for multiwave LPUs. The
minimum criterion (16 J/cm2) was reached by the majority of LED LPUs (n=87, 77.0%).
Figure 3.3 shows the relationship among all LPUs tested with a cut-off value of 16 J/cm2.
105
The LED LPUs tested were between 1.0 and 5.5 years of age (M=2.6, SD=1.2) and
had damage levels (scale 0 to 8) ranging from 0 to 4 (M=1.6, SD=1.2)
The results of the independent-samples t-tests showed that the LED LPUs that were unable to
produce the minimum acceptable level of radiant exposure, were older (t (111) =5.2, p<0.001,
95% CI for MD: .78; 1.75), and suffered more damage to the light-guide (t (111) =7.2,
p<0.001, 95% CI for MD: 1.11; 1.96).
The average age of LED LPUs that were unable to meet the minimum level of radiant
exposure was 3.6 years (SD=1.0), while the average age of the LED LPUs that met this
criterion was 2.3 years (SD=1.1). Similarly, the average damage level of the LED LPUs that
failed to meet the minimum level of radiant exposure was 2.8 (SD=0.9) as compared to 1.3
(SD=1.0) for those LED LPUs that met this criterion. The age distribution of LED LPUs that
were able/unable to meet the minimum radiant exposure criterion, depending on their
damage levels is presented in Figure 3.4.
A Pearson correlation analysis was used to explore the relationship between the
radiant exposure versus LPU's age and light probe damage level. The results revealed a
strong, r=-0.67 (p<.001) negative correlation between the LPUs' radiant exposure and age
and damage of LPUs.
Monowave and multiwave LPUs were analyzed according to their age and level of
damage. The average age of monowave LPUs in the sample was 2.7 years (SD = 1.2), and
the average age of multiwave LPUs was 2.6 years (SD = 1.2). This difference was not
statistically significant, (t (111) = 0.15, p = 0.884, CI 95% for MD: -0.46; 0.53), Cohen’s
d=0.03 indicating a small effect size. Similarly, the two types of LPUs were not significantly
different concerning the level of damage, χ2 (4) = 5.43, p = 0.246, Cramer’s V=.22 indicating
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a small effect size. Specifically, 17.7% of monowave LPUs and 17.6% of multiwave LPUs
had no damage, and 6.3% of monowave and 5.9% of multiwave LPUs had the highest level
of damage (level 4).
Discussion
Parametric statistical tests used in this study relied on the assumptions of normality
and equal variances among the comparison groups. Following these assumptions is especially
critical when the sample size used in the study is small. Considering that the sample used in
this study consists of 113 cases, parametric statistical tests were employed. However, to
investigate the potential impact of the violation of the normality assumption on the results of
statistical tests, nonparametric tests were also conducted. Should the results of the parametric
and non-parametric tests deviate, then the results of the non-parametric tests should be
reported. However, it was discovered that the results of non-parametric tests and parametric
tests were always the same. Therefore, for simplicity of the presentation, parametric tests
were used.
Three relevant parameters express crucial influence on a clinically acceptable level of
RBC photopolymerization: the chemical formulation, the operator's clinical skills, and
the LPU's characteristics. The literature shows that the longevity of RBCs has been
significantly shorter than that of amalgams 4, while RBC's failure is often associated with
insufficient polymerization 7. Although the operator's clinical skills have the most critical
107
influence on RBC's survival, LPU's ability to produce sufficient quantities of light energy at
appropriate light spectra is fundamental for the RBC restoration's clinical success.
The first research hypothesis, which stated that LED LPUs in the participating dental
practices would be able to deliver 16 J/cm2 of light energy, was rejected. The minimum
criterion (16 J/cm2) was reached by the majority of LED LPUs (n=87, 77.0%), but not by all.
The relationship among all LPUs, tested with a cut-off value of 16 J/cm2, can be seen in
Figure 3.3. The pool of tested LED LPUs consisted of 81 monowave and 32 multiwave
LPUs. The average computed 20s radiant exposure was 22.3 J/cm2 (SD=9.7) for monowave
LPUs and 20.4 J/cm2 (SD=4.5) for multiwave LPUs.
The second research hypothesis, which stated that LPUs` age and its light-probe
damage would not affect radiant exposure, was also rejected. It was determined that the LED
LPUs that were unable to deliver at least of 16 J/cm2 were older (t (111) =5.2, p<0.001, 95%
CI for MD: .78; 1.75), and exhibited more light-probe damage (t (111) =7.2, p<0.001, 95% CI
for MD: 1.11; 1.96). The age distribution of the LED LPUs that were able/unable to meet the
minimum radiant exposure criterion, depending on their damage levels, is presented in
the Figure 3.4. Furthermore, a strong, r=-0.67 (p<.001) negative correlation between the
LPUs' radiant exposure and its age and level of damage was recognized. The study revealed
that 23% of LED LPUs used in the participating private dental practices performed below the
study-required 16 J/cm2 passing criterion, which was in accord with previously published
results 25-31.
Some comparisons can be drawn between the 2005 Toronto study 26 and the current
study concerning performance differences between QTH and LED LPUs. Back in 2005,
QTH LPUs were deemed as a gold standard for photopolymerization and LED LPUs just
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started to gain popularity among dental professionals. The 2005 Toronto study used an
analog radiometer, the best tool available at that time, to quantify irradiance of QTH LPUs.
However, ten years later, the current study used more accurate laboratory-grade
spectroradiometer to complete the same task. Although 100 dental practices were selected,
the sample size consisted of 113 LED LPUs. The goal was to evaluate only the LPUs that
were used daily, rather than all LED LPUs that may have been available in the participating
offices. Furthermore, the current study intended to characterize the performance of LED
LPUs in the same geographic region and to see the real differences in use among dental
practices ten years after the previous study. Instead of incident irradiance measurement, a
calculated radiant exposure of all LED LPUs tested was conducted, following the
recommendations in the literature, for a minimum requirement of 16 J/cm2 that is considered
as an adequate radiant exposure for the majority of RBC materials 17. The study results were
quite surprising compared to those of the previous studies 25-31. The current study revealed
that 23% of LPUs tested were below the study-required passing criterion, much lower than
previously reported the study average of 46% (range 12‑95%) 25-31. From
the underperforming LPUs group, only three (11.5%) multiwave LED LPUs were unable to
meet the minimum requirement, while 23 (88.5%) of the underperforming LPUs were
monowave. It was observed that multiwave LPUs outperformed monowave LPUs by a large
margin. Furthermore, by calculating the ratio of substandard LPUs to the total number of
LPUs tested (multiwave LPUs [3/32] versus monowave LPUs [23/81]), it was discovered
that multiwave LPUs were three times less likely to perform below the study-required
standard than monowave LPUs. Although the average computed 20s radiant exposure was
similar for both LPU groups, 22.3 J/cm2 (SD = 9.7) for monowave LPUs and 20.4 J/cm2
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(SD = 4.5) for multiwave LPUs, double values of SD for monowave LPUs were observed.
The double values of SD for monowave LPUs suggest that a significantly higher range of
extreme radiant exposure values contributed to values in this group. A possible explanation is
that multiwave LPUs represented 3rd-generation LED LPUs, which were introduced
relatively recently in dental practice. Thus, the LPUs' age at the time of measurement was
significantly lower.
In comparison with a 20s radiant exposure produced by a reference LED LPU
(Bluephase Style, Ivoclar-Vivadent), which generated 21 J/cm2, 47.8% of LED LPUs tested
exhibited lower radiant exposure values. The substantial reduction in reported rate of
inadequate LPUs could be attributed to technical advancements in LED light source
technology, to the significantly higher accuracy of the spectroradiometer, and to
standardization in testing by a single administrator. It is possible that QTH LPUs tested in
previous studies could have exhibited some technical issues with the equipment used in the
study, which may influence the accuracy of the data reported. Furthermore, the accuracy of
irradiance readings depends on the adequate measurement of the light probe. The external
diameter was measured using Mitutoyo digital caliper (Mitutoyo Canada, Mississauga ON),
in which the corresponding light probe diameter from the CheckMARC® V2 software was
selected. Thus, the accuracy to assess the radiant exposure of tested LPUs was at the highest
level possible. Using the integrating sphere to measure the radiant power and then to divide
this by the tip area is prone to errors because a small error in measuring the active tip
diameter will lead to a substantial error in the irradiance value. For example, if the radiant
power is 800 mW and the active tip diameter is 7.5 mm, the area is 44.2 mm2. The incident
irradiance is thus 1,811 mW/cm2. If the tip diameter is 7.0 mm, the area is 38.5 mm2. Thus,
110
for just a 0.5 mm error (0.25mm on each side), the incident irradiance is now 2,079 mW/cm2,
or approximately 10% more, even though the radiant power is the same.
Other factors may have contributed to the degradation of LED LPUs besides age and
damage of the light probe. Some additional factors could be related to the condition of the
battery or the diminished power of the LED elements. Not only external, but the internal light
probe damage was examined. During the assessment of the light probe for possible damage,
the light probe was removed (when possible, depending on the type of LPU) and carefully
inspected for any internal cracks, which might affect the light transmission through the light
probe. When possible the light patency of the probe was examined for an unobstructed
reading of any printed material, from the light probe tip to its end. Thus, internal shatter
inside the light probe, although not easily detected, could cause a justified concern for the
capacity of the LPU to deliver expected quantities of light energy. Furthermore, it is
important to note that more than 80% of the participants had no practical knowledge of how
to inspect the light probe for the light transmission patency properly.
In this study, the damage level and age of the LPUs have been included in the
function of the study-required radiant exposure of 16 J/cm2, which differs from the design of
the previous studies 25-31. Figure 3.4 shows that the age distribution of the LED LPUs had not
met the passing criterion that depended on the damage level. As can be seen from this figure,
the minimum level of radiant exposure can only be guaranteed for new LPUs. Even one-
year-old LED LPUs might not meet this criterion if their damage level is 2 or above. None of
the LPUs that had a damage level of 4 met the minimum criterion. Overall, the results
suggested that other factors might also contribute to the radiant exposure of LED LPUs as
both groups of LPUs (those that met and those that did not meet the minimum criterion)
111
exhibited overlapping age and damage level intervals. Further analysis of LED LPUs that
were not able to generate the required 16 J/cm2 of radiant exposure is shown in Table 3.1. As
can be seen from the table, LED LPUs that failed to generate the required energy exhibited a
mean age of 3.6 years and light probe damage of 2.8/8 together with their average radiant
exposure of 12.03 J/cm2.
The limitations of the current project were associated with the LED LPUs' assessment
study design, which used the CheckMARC® device for a 10s radiant exposure and
subsequently converted recorded values for a 20s radiant exposure. It would have been better
if the study design had included a 20s radiant exposure measurement. The primary reason for
choosing 10s versus 20s exposure time was based on the fact that choosing the latter would
double the experimental time spent in the participating dental practices as this project was a
part of an extensive study that required several test procedures in the same dental practice.
Future research using the same methodology should focus on different types of dental
practices. For example, it would be interesting to know the performance of LED LPUs used
in Toronto's public dental clinics and compare this data with the results of the current study.
Furthermore, the assessment of LPUs used by dental professionals in rural areas would shed
more light on the possible difference in the quality of dental care between urban and rural
private dental practices.
Regardless of the current study's results, the LED LPUs tested in the participating
dental practices in the Greater Toronto Area continued to exhibit a relatively high percentage
of substandard LPUs, which is a cause for justifiable concern regarding sufficient RBC
polymerization.
112
Conclusions
Within the limitations of this study that characterized the LED LPUs in the
participating dental practices in the Greater Toronto Area regarding their capacity to deliver
at least 16 J/cm2 of radiant exposure in the 20s, the following conclusions can be reached:
1. 23% of LED LPUs tested did not meet the 16 J/cm2 criterion.
2. Multiwave LPUs showed two and a half times better performance than
monowave LPUs.
3. A strong, r=-0.67 (p<.001) negative correlation between the LPUs' radiant
exposure and age and level of damage of LPUs was observed.
4. It was found that LPUs older than 4 years tended to exhibit a higher level of
damage.
5. Rigorous inspection of LPUs is required for early detection of any problems
associated with routine use.
6. Failure to meet the study criterion of 16 J/cm2 in the 20s implies that 23% of
the LED LPUs tested will not allow the practitioners to produce adequate
RBC polymerization.
113
Acknowledgements
The current study is part of a Ph.D. thesis submitted to the Faculty of Dentistry,
University of Toronto. This project was financially supported by grants from the Faculty of
Dentistry Research Institute of the University of Toronto, and the American Academy of
Esthetic Dentistry. The CheckMARC® device was loaned and technically supported by
BlueLight Analytics for which the authors are grateful. The Bluephase Style was graciously
provided by Ivoclar-Vivadent. There are no words to express immense appreciation to the
project research assistants, Ms. Ana Burilo and Ms. Ivana Orlovic for their extensive
contribution to the study.
114
Figure 3.1 The CheckMARC® spectroradiometer
Figure 3.2. Schematic illustration of light-tip damage index (physical damage and/or
irremovable resin build-up) used to assess on the light probe surfaces of LPUs tested. Note
that observed damage levels during the LPUs’ assessments were in the range from 0/8 to 4/8.
115
116
Figure 3.3 Calculated 20s radiant exposure of 113 LPUs with a minimum passing
criterion cut-off of 16 J/cm2. Note that 26 LPUs failed to reach the passing criterion.
Figure 3.4 Box and Whisker plots: LED LPUs which did not meet (above) and did
meet (below) a passing criterion. Circles and stars represent the outlier data points.
The circles are the outliers that are closer to the center of the distribution, and the
stars are the outliers that are farther from the center of the distribution. It’s interesting
to note a wide range of values at zero damage in respect to the age of LPU, only
found with LPUs that met the passing criterion.
117
Table 3.1 Listed LED LPUs that failed to reach a minimum of 16 J/cm2 of
radiant exposure in the 20s. The listing includes the LED LPUs’ age, light
probe’s damage level, and 20s radiant exposure results. The LPUs were
classified by the type of LPU: where d: dual-peak (multiwave), s: single-peak
(monowave). The LPUs listing is presented in descendent order according to
RE values.
118
LPU brands Age Damage J/cm2 Type
Demi Plus 3 2 15.6 s
SmartLite IQ2 4 4 15.4 s
Flashlight 1401 4 2 15.2 s
Valo Cordless 3.5 3 15 d
Demi Plus 2 1 14.8 s
D-Lux 1.5 2 14.8 s
SmartLite MAX 2.5 2 14.6 d
Flashlight 1401 4 2 14.6 s
SmartLite MAX 2 2 13.2 d
LED Turbo Victor 4 3 13.2 s
LE Demetron 4 3 12.8 s
Elipar FreeLight 4 3 12.6 s
Demi Plus 4.8 3 12.4 s
Bluephase 16i 3.5 4 12.2 s
Coltolux LED 4 3 12.2 s
SmartLite IQ2 5.5 4 12.2 s
LE Demetron 4 3 11.2 s
Demi Plus 4 2 10.2 s
Coltolux LED 3 2 10.2 s
LE Demetron 4 4 10.2 s
D-Lux 2.5 1 9.8 s
Demi Plus 4.5 3 9.8 s
Coltolux LED 4.5 4 9.8 s
Ultra LED 4 4 8.6 s
Coltolux LED 3.5 3 8.4 s
ART L5 3.5 4 3.8 s
119
Figure 3.5 Q-Q plot showed the multiwave LPUs incident irradiance observed
values against normally distributed data (represented by the line).
Figure 3.6 Q-Q plot showed the monowave LPUs incident irradiance observed
values against normally distributed data (represented by the line).
120
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Chapter 4
Manuscript 2
Photopolymerization application effectiveness
among private dental practices in Toronto, Canada
126
Photopolymerization application effectiveness among private dental practices in
Toronto, Canada
Dave D. Kojic DDS, MSc. PhD-candidate
Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto, Toronto,
ON, Canada
*Omar El-Mowafy, BDS, PhD, FADM
Professor and Head, Department of Restorative Dentistry
Faculty of Dentistry, University of Toronto, Toronto, ON, Canada
Richard Price DDS, MS, PhD
Professor, Department of Clinical Dental Sciences
Faculty of Dentistry Dalhousie University, Halifax, NS, Canada
Wafa El-Badrawy, BDS, MSc.
Associate Professor Department of Restorative Dentistry
Faculty of Dentistry, University of Toronto, Toronto, ON, Canada
Gildo Santos, DDS, MSc, PhD
Associate Professor Chair - Restorative Dentistry
Schulich School of Medicine & Dentistry, Western University, London, ON, Canada
*Corresponding author
Omar El-Mowafy, BDS, PhD, FADM
Department of Restorative Dentistry
Faculty of Dentistry, University of Toronto
124 Edward Street, Toronto, Ontario M5G 1G6, Canada
127
Abstract
Objectives: To determine the ability of dental practitioners to deliver 8 J/cm2 radiant
exposure (RE) in 10s to the MARC® Patient Simulator’s (PS) (BlueLight Analytics, Halifax
NS) simulated restorations. Methods: Dental practices from the Greater Toronto Area were
recruited. Light-emitting-diode (LED) Light Polymerization Units (LPU) were assessed for
their ability to deliver the study-required energy. Dental practitioners used their office's LED-
LPUs to deliver light-energy to anterior and posterior PS restorations. Following
photopolymerization instructions, participants were retested using the same LED-LPUs. Data
were statistically analyzed using mixed ANOVA, Chi-square tests, and paired-samples t-tests
with a pre-set alpha of 0.05. Results: Before instructions, 73.6% of participants delivered
required-energy to anterior restorations, and 32.2% of participants delivered required-energy
to posterior restorations. Following instructions, 94.3% of participants delivered required-
energy to anterior restorations, and 73.6% of participants delivered required-energy to
posterior restorations. Conclusions: Although LPUs could generate 8.0 J/cm2 in 10s, the
percentage of dental practitioners who failed to deliver the study-required energy, even after
specific instructions, was 5.7% for anterior restoration and 26.4% for posterior restoration.
Clinical significance: It was revealed that a large number of dental practitioners failed to
deliver the study-required 8 J/cm2 in 10s to posterior restoration. This finding is alarming as
it relates well to early failure of Class-2 RBC restorations due to insufficient polymerization.
Keywords: Light Polymerization Unit (LPU); Irradiance; Radiant exposure; Dental
spectroradiometer; the MARC® Patient Simulator.
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Introduction
Given the demand for natural-looking restorations, in combination with health and
environmental concerns regarding mercury in amalgam restorations 1, 2, resin-based
composites (RBC), have become the material of choice for many restorative procedures 3.
However, sufficient RBC polymerization requires application of adequate quantities and
appropriate spectra of light energy delivered to the surface of RBCs 4. Although RBC
placement is a technique-sensitive procedure that requires clinicians’ full attention,
polymerization by Light Polymerization Units (LPU) may not receive the same consideration
as it should 5, 6. It is well-known that an inadequate polymerization has a profound impact on
RBCs longevity, as those unpolymerized residues exhibit health hazards due to their nature to
leach into oral fluids, which has been reported in many toxicology studies 7, 8, 9.
Dental practitioners are facing many challenges in their clinical practices when it
comes to the polymerization of RBCs, especially in the posterior areas, where correct light
probe positioning and alignment involves a significant degree of difficulty, due to limited
access. It has been confirmed that an angular change in the light probe positioning would
cause a significant impact on energy delivered to the restoration, with a corresponding
reduction in RBCs polymerization efficiency 10 – 13. Dental practitioners usually do not have
the appropriate resources to evaluate their LPUs, but to use dental radiometers. However,
recent studies have warned about erroneous readings associated with dental radiometers 14, 15,
16. Furthermore, dental radiometers measure incident irradiance (mW/cm2) at the tip of the
surface of the light probe, and thus a small change in the tip diameter will dramatically
change the actual incident irradiance values.
129
Although LPUs irradiance measured with spectroradiometers are considered the most
accurate measuring devices, this information cannot provide clinicians with the answer, of
how well their RBC restorations were polymerized 17, 18. Manufacturers of commercially
available LPUs report that irradiance values were taken at 0 mm distance between the light
probe and the measuring device, which is not a clinically relevant distance. For instance, the
measured distance in the molar region between the molar cusp tip and the bottom of Class II
proximal box could exceed 8 mm, a distance that would significantly reduce the light energy
received by the RBC 19-23.
Sufficient polymerization of RBCs depends on several key factors: the capacity of
LPUs to generate light energy (appropriate quantities and spectra), intrinsic properties of
RBCs, and the technique followed by the dental practitioners to deliver the light-energy to
the restorations 24, 25. The goal for any dental practitioner is to efficiently and adequately
polymerize RBC restorations within a clinically acceptable time frame. However, the exact
quantity of required light energy for adequate RBCs' polymerization remains unanswered. A
commonly held assumption is that an equivalent of 16 J/cm2 of received radiant exposure
delivered to a single 2 mm thick increment of RBC material was considered sufficient for a
satisfactory RBC polymerization 26. Nonetheless, in other research reports, a range between 6
and 24 J/cm2 was suggested as a minimum energy requirement for a sufficient
polymerization of majority commercially available RBC materials 27, 28.
Manufacturers` recommended minimum radiant exposure values for the
polymerization of their RBCs can vary to a great extent, depending on the intrinsic properties
of the RBC and its chemical formulations, and shade, as well as, LPU’s irradiance 24, 25.
Dental practitioners are not able to efficiently measure the radiant exposure of their LPUs. As
130
a result, dental practitioners can either deliver too much energy to the restoration, which can
locally generate heat, or deliver too little energy, which can result in insufficient
polymerization 25, 26. Because LPUs produce electromagnetic radiation of specific
wavelengths, sufficient radiometric quantities should be used in reporting the features of
LPUs 29. The radiant power is the correct term used to describe the output from LPUs, and its
measured value is denoted in units of mW. The radiant power emitted throughout the area of
the light probe tip represents the radiant exitance of the LPU, expressed in units of mW/cm2.
The incident irradiance, an essential term of the LPU performance, often incorrectly reported,
accounts for the radiant power incident (energy flux) delivered to the RBC surface, expressed
in units of mW/cm2. A radiant exposure, which represents a product of irradiance and
exposure time, is expressed in units of J/cm2.
With the introduction of the MARC® Patient Simulator (PS) (BlueLight Analytics,
Halifax NS), researchers have received a new device to accurately measure radiant exposure
received by simulated restorations in a mannequin head. The MARC® PS device incorporates
a laboratory-grade UV-VIS spectroradiometer (USB4000, Ocean Optics, Dunedin, FL) and
two, laboratory-grade cosine-corrected sensors 44. The sensors are 4 mm in diameter, which
is the diameter of the ISO depth-of-cure mold. Light captured by the sensors is forwarded to
the spectrometer through a bifurcated fiber-optic cable. The MARC® PS system simulates the
patient using the mannequin head and typodont. The maxillary typodont is equipped with two
light sensors: an anterior one, located between central incisors, 1mm below the buccal
surfaces of central incisors (Class III simulated restoration), and a recessed posterior sensor
(Class I simulated restoration), located in the maxillary left second molar. The light detector
131
is located at the base of a Class I preparation, 2 mm lower than the cavosurface margin and 4
mm below the cusp tip 44.
Studies assessing the photopolymerization techniques of dental practitioners utilizing
the MARC®PS have shown a massive difference among the participants to deliver the light
energy to the simulated restorations 30-35. The idea to explore the efficacy of LPUs in dental
practices was a subject of several studies. However, to date, no study was able to measure
and compare polymerization techniques of dental practitioners in the participating dental
practices 36-43.
The purpose of this study was to evaluate the operator's technique in delivering light
energy to the MARC® PS simulated restorations, using LED LPUs in the participating
private dental practices in the Greater Toronto Area. The objectives were to test two research
hypotheses. The first hypothesis was that a 10s radiant exposure delivered by dental
practitioners to the MARC®PS (anterior and posterior restorations) would produce energy
equal to or greater than 8.0 J/cm2, which was reported to be half the minimum requirement,
quoted in the literature 26. The second research hypothesis was that, following the
individualized photopolymerization instructions, dental practitioners would deliver more
radiant energy to the MARC®PS simulated restorations, and will improve the consistency of
energy delivered in a 10s radiant exposure cycle.
Materials and methods
Following the University of Toronto Research Ethics Board approval, the private
dental practices were recruited. The 2012 listing of dentists published by the Royal College
132
of Dental Surgeons of Ontario was used to identify dental practices in the Greater Toronto
area. Over 350 targeted practices were initially contacted via telephone to determine their
eligibility to participate in the study. A total of 250 qualified dental practices were presented
with a letter explaining the study purpose, along with a consent form. Only 100 dental
practices met the study criteria and agreed to participate, with 113 dental practitioners that
took part in the study. The recruitment criteria for the study participants included a general
dental practice with proficiency in RBCs placement located within the Greater Toronto Area.
Study acceptance was granted exclusively for participants who had never experienced
MARC®PS testing before. The assessment of photopolymerization techniques of participants
to deliver radiant energy was measured using a commercial testing device (MARC®PS) and
was completed in the participating dental practices. Software (MARC®PS version 3.4)
provided real-time radiant exposure data display and a calculation of energy delivery within
user-defined spectral ranges and exposure time to reach 8.0 J/cm2.
The study was a part of a broad project, which examined a total number of 113 LPUs
for their ability to deliver at least of 8 J/cm2 of radiant exposure during a 10s exposure. The
assessment was completed by using a commercial, laboratory-grade light measuring device
(CheckMARC®, BlueLight Analytics, Halifax, NS). A total number of 87 LPUs was found
capable of delivering required energy. Therefore, 87 participating dental practitioners with
their LPUs were assessed for their ability to deliver the study-required 8 J/cm2 of radiant
exposure in 10s to the MARC®PS.
However, to acquire all relevant information for this study, the participating dental
practitioners (n=113) (female and male dentists, and dental assistants) were asked to deliver a
10s radiant exposure, using their LPUs, to the MARC®PS simulated restorations. The
133
practitioners were asked to use their LED LPUs in the way they perform the polymerization
on a patient. The participants were tested using the MARC®PS by choosing to use eye
protection (hand-held screens, protective glasses, or no protection at all). Participants applied
LED LPU’s light tip directly to the MARC®PS anterior sensor (Class III) and completed a
10-second test cycle, performing this test three times. The same procedure was performed on
the posterior sensor (Class I). After providing individualized instructions for proper
photopolymerization, participants were retested, using the same protocol and the same LPU.
The following instructions were given to participants after their initial testing: demonstration
of proper hand positioning during light exposure, mandatory use of protective glasses, proper
positioning and stabilization of the light probe, and visual inspection at each stage of the
process. The test results were shared with the participants after the test procedure. A positive
control group consisted of three graduate dental students who were tested in the same way as
the participating dental professionals using a reference LPU (Bluephase Stylus, Ivoclar-
Vivadent).
Statistical analyses
Mixed-design analysis of variance (Mixed ANOVA) was used to test for differences
between three independent groups (male dentists, female dentists, and dental assistants)
while subjecting participants to repeated measures (before and after instructions). Following
this, Fisher's post hoc multiple comparison tests were conducted using a pre-set alpha of
0.05. Chi-square tests of independence were conducted using a pre-set alpha of 0.05 to
investigate whether delivering the minimum acceptable level of radiant energy depends on
the dental practitioner (male dentist, female dentist, or dental assistants). Paired-samples t-
134
tests were conducted to investigate the influence of individualized instructions (before/after)
on light energy delivered to simulated restorations by all participating dental professionals
(n=113), using a pre-set alpha of 0.05. The SPSS software for Windows, version 22 (SPSS
Inc., IBM, Somers, NY), was used for all statistical analyses.
Parametric statistical tests used in this study (such as paired t-tests, chi-square tests, and
ANOVA) rely on the assumptions of normality and equal variances among the comparison
groups. According to the central limit theorem, when the sample size is greater than 30 cases
per group, normal distribution of the sample means can be assumed if the sample data are not
normally distributed. Considering that the sample used in this study consisted of 87 (113)
cases, parametric statistical tests were used.
Results
The Q-Q plots were used for normality assessment. The average 10s radiant exposure
delivered by dental practitioners (n=113) to the MARC®PS anterior and posterior
restorations, before and post-instructional, are presented in Figures 4.7 to 4.10.
Equality of variances assumption across the comparison groups was tested with the
Levene's test. The test was not significant, (p=0.816), thus, the equality of variances was
assumed. A power analysis using the G Power program for independent and paired t-tests
revealed that the sample size of 113 could detect a medium effect size with the independent
samples t-tests (Cohen's d=.53), and a small effect size with the paired-samples t-tests
(Cohen's d=.23). A power analysis for Mixed ANOVA used a reference (Green et al., 2010)
45, which states that each cell in design should have at least 15 cases to express the minimum
135
power of .80, and that criterion was met for the current study. Additionally, for the Chi-
square test of independence, a reference (Oyeyemi et al., 2010) 46 was used for the power
analysis. According to the guidelines, a study sample size of 113 can detect only large effects
with the power of .80.
The majority (73.6 percent) dental practitioners delivered the study-required energy
to the anterior restoration before instructions. Eighteen participants (20.7 percent) did not
deliver required energy before instructions but were able to deliver it post -instructional.
Only three participants (3.4 percent) were not able to deliver this energy at all. The change in
the number of dental practitioners able to deliver (before and after instructions) the required
energy minimum to anterior restoration, as measured by McNemar test, was significant
(p<.001).
Only twenty-eight dental practitioners (32.2 percent) were able to deliver the study-
required energy to the posterior restoration before instructions. An additional thirty-six more
participants (41.4 percent) were able to deliver required energy post-instructional. The
change in the number of dental practitioners able to deliver (before and after instructions) the
study-required energy to the posterior restoration, as measured by McNemar test, was
significant (p<.001). The average levels of radiant exposure delivered by dental practitioners
to the MARC® PS anterior and posterior restorations, before and post-instructional, are
presented in Table 4.1, Figure 4.2, and Figures 4.3 to 4.6.
To investigate whether delivering the acceptable level of radiant energy depends on a
dental practitioner (n=113) (male and female dentist, or dental assistants), chi-square tests of
independence were conducted. The tests disclosed that the experience of dental practitioners
to deliver the study-required energy is not related to the restoration location. Thus, for
136
anterior restoration before instructions (2(2) =2.57, p=0.277, Cramer’s V=0.17), post-
instructional (2 (2) =4.95, p=0.084, Cramer’s V=0.24), or posterior restoration before
instructions (2(2) =1.15, p=0.562, Cramer’s V=0.12), or post-instructions (2 (2) =3.65,
p=0.162, Cramer’s V=0.21) were recorded.
The average levels of radiant energy delivered to anterior and posterior restorations
before and after instructions for all dental practitioners (n=113) regardless the ability of their
LPUs to deliver the study-required energy are presented in Table 4.1. Paired-samples t-tests
were conducted to investigate whether the average energy level delivered by dental
practitioners was changed post-instructional. The results of these tests were significant for
both anterior (t (112) = -10.26, p < .001, 95%CI for MD: -3.14; -2.13) and posterior
(t (112) = -12.26, p < .001, 95%CI for MD: -3.17; -2.29) restorations.
Mixed ANOVA was used to analyze whether practitioners' consistency of energy
delivery was changed post-instructional. The results of this analysis showed a significant
effect of time (baseline or post-instructional), Wilk's Ʌ=0.91, F (1,110) =10.42, p=0.002. On
average, regardless of location (anterior/posterior), and type of dental practitioner, the
consistency of energy delivery improved after the instructions: M (before) =1.2 compared to
M (after) =0.9 (95%CI for MD: 0.115; 0.480). However, the effect size of this difference, as
measured by partial ɳ2, was small (.09).
The average light-energy that the positive control group participants delivered to the
MARC®PS restorations was: 9.8 J/cm2 to the anterior restoration, and 9.9 J/cm2 to the
posterior restoration before instructions, and 12.6 J/cm2 to the anterior restoration, and 11.1
J/cm2 to the posterior restoration, post-instructional.
137
The average 10s radiant exposure (J/cm2) that dental practitioners (n=113) delivered
to simulated MARC® PS restorations, using monowave (n=81) and multiwave (n=32) LED
LPUs is presented in Figure 4.11. No significant difference in performances between
monowave and multiwave LPUs was observed, except in only one cluster, at posterior
restoration, post-instructional (p=.022).
Discussion
The first research hypothesis was that the light energy delivered by dental
practitioners (n=87) in the Greater Toronto Area private dental practices, during the 10s
radiant exposure would be equal or higher than 8.0 J/cm2, was rejected. The majority, (73.6
%) of participants delivered the study-required energy to anterior restoration and only 32.2%
of participants delivered required energy to posterior restoration, before instructions.
Following instructions, 20.7 % of participants deliver the study-required energy to anterior
restoration, and 41.4 % of participants delivered required energy to posterior restoration.
Based on the MARC®PS results, it is clear that photopolymerization skills of some dental
practitioners at the baseline, before they received individualized instructions, were not at the
level that would allow them to pass the study-required criterion. Thus, the percentage of
participants that delivered required energy to anterior restoration was 70.2%, while only
32.2% participants were able to deliver the study-required energy to posterior restoration.
Consequently, the percentage of participants that failed to deliver required energy to the
MARC® PS simulated restorations was 29.8% to the anterior and 67.8% to the posterior
restoration, respectively.
138
The second research hypothesis was that, following individualized instructions on
proper photopolymerization, participants (n=113) would deliver more radiant energy to
simulated restorations, and they would improve the consistency of energy delivered during a
10s radiant exposure, was accepted. Because of individualized instructions, participants’
average for energy delivered to the anterior restoration increased by 22.6% and to the
posterior restoration by 30.4%. Furthermore, they improved the consistency of delivered
energy (smaller SD) during a 10s radiant exposure, 25.9% on average, regardless of a
restoration location or type of dental professional: M (before) =1.2 compared to M (after)
=0.9.
Dental practitioners (n=113) were asked to perform photopolymerization on the
MARC® PS simulated restorations, using their LPUs most frequently used for all daily
clinical procedures. The rationale for using all participating LPUs regardless of their capacity
to deliver the study-required energy was to discover how two different location of simulated
restoration changed the magnitude of delivered light energy. Further, to strengthen the data
on the advantage of individualized polymerization instructions on improved consistency and
quantity of energy delivered by dental practitioners, the participants had three attempts to
deliver light energy in a 10s radiant exposure cycle, first to anterior and then posterior
restoration using their choice of eye protection (or not at all). Participants were given
individualized instructions on proper photopolymerization, before they were asked to repeat
the test using mandatory eye protective glasses and the same LPUs, the anterior restoration
first, followed by the posterior restoration. The LPU in each dental practice was
characterized by the CheckMARC® spectroradiometer device to ensure the capacity of the
LPU to generate at least 8.0 J/cm2 of radiant exposure during a 10s-time interval.
139
Although results acquired by the CheckMARC® were accurate, the smaller diameter
size of the sensor at the MARC® PS (4 mm) increased the irradiance values depending the
differences between the LED LPUs' light probe diameter and the MARC® PS sensor. Due to
the inhomogeneity of the light-beam profile, where the LPU’s central beam exhibits higher
exitance irradiance values compared to the periphery, yielding a 4 mm the MARC® PS
sensor to capture inflated irradiance values, resulting in an increase of the MARC® PS
readings values by 10% to 20% on average. That could explain the CheckMARC® results
that twenty-six LED LPUs could not produce 8.0 J/cm2 of radiant exposure, while the
MARC® PS results showed that eight LED LPU's could deliver the study-required energy to
at least one of the MARC® PS restorations.
A positive control group consisted of three graduate dental students who were tested
in the same way as the participating dental professionals using a reference LPU (Bluephase
Stylus, Ivoclar-Vivadent). The control LPU's ability to produce a minimum energy
requirement was validated by the CheckMARC® (incident irradiance of 1,077 mW/cm2). In
addition, the graduate students that represented a control group had no prior experience with
the MARC® PS before were tested. The rationale to use a control LPU for the assessment of
photopolymerization skills of graduate students was that a single LPU would exhibit fewer
variations during the light energy production cycle, therefore, the focus was on the light-
delivering technique, not on the LPU per se. Thus, the quantity of energy delivered to the
MARC® PS represented precisely photopolymerization skills of subjects tested. The average
energy levels delivered by the control group in the 10s radiant exposure were 9.8 J/cm2 to the
anterior restoration, and 9.9 J/cm2 to the posterior restoration before instructions, and 12.6
140
J/cm2 to the anterior restoration, and 11.1 J/cm2 to the posterior restoration, post
instructional.
The average radiant exposure that dental practitioners delivered to the MARC® PS
truly represented the state of affairs in the participating dental practices in the Greater
Toronto Area. The average baseline radiant exposure values recorded using the MARC® PS
were lower in comparison with post instructional values because participants tried to avoid a
direct eye exposure to the LPU’s light source, which prevented them from correctly
positioning and stabilizing the LPUs on top of the simulated restoration. The participants that
were using the hand-held screens for eye protection exhibited insufficient stabilization of the
light probe over the simulated restoration, which resulted in substantially lower radiant
exposure values. In addition, some participants experienced jitters because they were
somewhat apprehensive. However, during the second and third attempt to deliver light
energy they gradually increased confidence, which was shown by improved values of
delivered energy. After individualized instructions on proper photopolymerization, which
also included mandatory use of protective glasses, the participants on average, delivered a
significantly higher energy level to the MARC® PS. The research conducted in the
participating dental practices measured polymerization techniques of practitioners who were
not entrusting the photopolymerization to their assistants. All LPUs were tested without
infection control because a variety of infection control methods were used among the
participating practices, and some practices were not using any infection control at all. For
that reason, the LPUs were tested without any protective barriers, to make meaningful
comparison among LPUs tested in all participating dental practices.
141
Post-instructional data showed a significant increase in delivered light energy, dental
practitioners on average, increased the energy delivered to anterior restoration by 22.6% and
posterior restoration by 30.4%. At baseline testing, most of dental practitioners employed a
single-hand polymerization, using the hand-held protective screens or no eye protection.
However, following the instructions and the mandatory use of eye protective glasses, dental
practitioners using the two-hand polymerization technique improved delivered energy to
simulated restorations by 26.5% on average.
A significant difference in delivered light-energy that was observed between the
anterior restoration and the posterior restoration may be related to the light probe design and
the limited practitioners’ ability to access the posterior restorations. There are many
difficulties in the posterior area access. A light probe design, which shows a notable
curvature on its end, usually stands for a substantial source of challenges in limited
interocclusal space, during the light application. Furthermore, the absence of visual control
during proper light probe positioning creates a struggle in keeping immovable light probe tip,
as well as keeping it at the right angle to the axis of the tooth. The MARC® PS results
revealed that the LPUs with a more curved light probe delivered, on average, less light
energy to the posterior restoration, in comparison to the LPUs with a less curved light probe.
The explanation for this phenomenon could be in the fact that a curved light probe usually
needs more space for its proper positioning.
This study tested the ability of dental practitioners to deliver light energy to simulated
restorations (anterior and posterior) using the MARC® PS. It was unveiled that the average
amount of energy delivered to each simulated restoration, was similar among the three
professional groups. However, female dentists were the most consistent in energy delivery
142
(expressed in lowered standard deviation values), followed by male dentists and dental
assistants, respectively. Although the pool of dental assistants was indisputably larger than
any other group, it can be safely assumed that the highest standard deviation values could be
associated with the larger pool size (Figure 4.2). However, the difference among the energy
values delivered by dental practitioners was not significant.
The current results presented in this study that proper light polymerization
instructions have significantly increased energy delivered to the simulated restorations on the
MARC® PS were in accord with the findings from the earlier studies 30-35. The unique
perspective of the current study was the ability to characterize incident irradiance of LPUs
tested, to assess the radiant exposure that dental practitioners delivered to the MARC® PS
restorations, as well as to make a comparison between the two LPU types, monowave, and
multiwave. The study results overwhelmingly refute the view that significant improvement
has occurred as a result of individualized instructions that dental practitioners received and
use of protective goggles and a two-hand polymerization technique.
Bias can occur in any phase of a research project, from planning, data collection, and
analysis to the publication phase. This study exhibited selection bias to some extent because
those dental practices with the possible lower quality of LPUs or questionable proficiency in
RBC placements refused to participate in this study. As mentioned before, over 350 targeted
practices were initially contacted via telephone/email to determine their eligibility to
participate in the study. Although it was concluded that 250 practices satisfy the criteria, 100
dental practices were only recruited with a total of 113 dental practitioners. Many factors led
to low study recruitment outcome.
143
The study exhibited standardized protocols for data collection and data entry. A
single administrator completed the entire study, which substantially reduced any possible
inter-observer variability and performance bias while, at the same time, enabling the test
administrator to evaluate internal validity during the entire research process efficiently.
Another potential limitation of this study was a possible conceptual problem with a
10s radiant exposure using the MARC® PS. If the passing criterion of 16 J/cm2 radiant
exposure during a 20s exposure was performed, the results might be more pronounced and
probably could bring a much more comprehensive margin of passed/failed polymerization
skills. Further, the 16 J/cm2 radiant exposure represented an arbitrary value that was more
commonly cited in the literature 26, 27.
The direction of future research could include a change in the research population,
thus to see how rural dental practitioners differ from the urban practitioners and possible
include the assessment of Dental Public Health professionals. The study findings will be
most useful to researchers, as well as to dental practitioners, as well as possibly to regulatory
agencies.
Conclusions
Within the limitations of this study, which used the MARC®PS to quantify energy
delivered to simulated restorations by dental practitioners, the following conclusions can be
reached:
144
1. Before instructions, 73.6% of participants delivered required-energy to the anterior
restoration, and 32.2% of participants delivered required-energy to the posterior
restoration.
2. Following instructions, 94.3% of participants delivered required-energy to the
anterior restoration, and 73.6% of participants delivered required-energy to the
posterior restoration
3. Participants on average increased energy delivered to the anterior restoration by
22.6% and the posterior restoration by 30.4% because of instructions.
4. Specific instructions and the mandatory use of eye-protective glasses demonstrated
that two-hand polymerization technique (post-instructional) is significantly better
than one-hand technique engaged at baseline.
5. Following specific instructions, and because of the two-hand polymerization
technique, dental practitioners improved the consistency of delivered energy to the
simulated restorations on average by 25.9%.
6. A significant percentage of dental practitioners (26.4%) were unable to deliver the
study-required energy to the posterior restorations, even when given instructions.
7. No significant difference in performances between monowave and multiwave LPUs
was observed, except in only one cluster, at posterior restoration, post-instructional.
145
Acknowledgements
The current study is part of a Ph.D. thesis submitted to the Faculty of Dentistry,
University of Toronto. This project was financially supported by grants from the Faculty of
Dentistry Research Institute of the University of Toronto, and an American Academy of
Esthetic Dentistry. The CheckMARC® device was loaned and technically supported by
BlueLight Analytics, for which the authors are grateful. The Bluephase Style was kindly
provided by Ivoclar-Vivadent. There are no words to express immense gratitude to the
project research assistants, Ms. Ana Burilo and Ms. Ivana Orlovic for their contribution to
the study.
146
Figure 4.1 The MARC® Patient Simulator system with a light probe
positioned to deliver light energy to the posterior (Class I) restoration. The
front sensor, representing the anterior restoration (Class III), between the
upper central incisors, is depicted in the picture
Figure 4.2 A 10s radiant exposure (Mean and SD) that all participants
(n=113) delivered to the MARC® PS restorations, separated by each
professional group, (Male and Female Dentists, and Assistants). No
statistically significant difference among 3 professional groups was found.
147
Table 4.1 Descriptive Statistics for a10s radiant exposure (J/cm2) delivered by
113 dental practitioners using their LPUs to the MARC® PS restorations, in 4
sequences: anterior and posterior restorations before instructions, and anterior
and posterior restorations post-instructional. The results were recorded by the
MARC® PS v.3.4 software.
J/cm2
Min
Max
Mean
SD
Anterior before 1.78 32.00 9.02 4.42
Anterior after 3.03 36.16 11.65 4.89
Posterior before 0.81 15.96 6.25 3.06
Posterior after 1.59 27.99 8.98 3.95
-
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
Ant_before Post_Before Ant_After Post_After
J/cm
2
M_Dentists F_Dentists Assistants
148
Figure 4.3 10s Radiant Exposure delivered by dental practitioners (n=113) to the
MARC® PS anterior restoration, before instructions, with respect to 8 J/cm2 cut-off
value.
Figure 4.4 10s Radiant Exposure delivered by dental practitioners (n=113) to the
MARC® PS anterior restoration, post-instructional, with respect to 8 J/cm2 cut-off
value.
149
Figure 4.5 10s Radiant Exposure delivered by dental practitioners (n=113) to the
MARC® PS posterior restoration, before instructions, with respect to 8 J/cm2 cut-off
value.
Figure 4.6 10s Radiant Exposure delivered by dental practitioners (n=113) to the
MARC® PS posterior restoration, post-instructional, with respect to 8 J/cm2 cut-off
value.
150
Figure 4.7 Q-Q plot showing the average energy quantities that dental practitioners
(n=113) delivered to the MARC®PS anterior restoration before instructions. The
observed values are plotted against normally distributed data (represented by the
line).
Figure 4.8 Q-Q plot showing the average energy quantities that dental practitioners
(n=113) delivered to the MARC®PS posterior restoration before instructions. The
observed values are plotted against normally distributed data (represented by the
line).
151
Figure 4.9 Q-Q plot showing the average energy quantities that dental practitioners
(n=113) delivered to the MARC®PS anterior restoration, post instructional. The
observed values are plotted against normally distributed data (represented by the
line).
Figure 4.10 Q-Q plot showing the average energy quantities that dental
practitioners (n=113) delivered to the MARC®PS posterior restoration, post-
instructional. The observed values are plotted against normally distributed data
(represented by the line).
152
Figure 4.11 a 10s radiant exposure (J/cm2) (Mean and SD) that dental practitioners
(n=113) delivered to simulated MARC® PS restorations, using monowave (n=81) and
multiwave (n=32) LED LPUs. *A significant difference was observed between two
LPU types in only one group, at posterior restoration, post-instructional (p=.022).
*
153
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159
Chapter 5
Manuscript 3
Relative Hardness of Resin Based Composites Polymerized by LED Light-Polymerization Units in
Toronto’s Private Dental Practices
160
Relative Hardness of Resin Based Composites polymerized by LED Light-
Polymerization Units in Toronto’s Private Dental Practices
Dave D. Kojic DDS, MSc. PhD Candidate
Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto, Toronto,
ON, Canada
*Omar El-Mowafy, BDS, PhD, FADM
Professor and Head, Department of Restorative Dentistry, Faculty of Dentistry, University of
Toronto, Toronto, ON, Canada
Richard Price DDS, MS, PhD
Professor, Department of Clinical Dental Sciences, Faculty of Dentistry Dalhousie
University, Halifax, NS, Canada
Wafa El-Badrawy, BDS, MSc.
Associate Professor Department of Restorative Dentistry, Faculty of Dentistry, University of
Toronto, Toronto, ON, Canada
Gildo Santos, DDS, MSc, PhD
Associate Professor Chair - Restorative Dentistry, Schulich School of Medicine & Dentistry,
Western University, London, ON, Canada
*Corresponding author Omar El-Mowafy, BDS, PhD, FADM
Department of Restorative Dentistry
Faculty of Dentistry, University of Toronto
124 Edward Street, Toronto, Ontario M5G 1G6, Canada
161
Abstract
Objectives: To assess relative-hardness (RH) of two brands of resin-based
composites (RBC), polymerized using Light-emitting-diode (LED) light-polymerization units
(LPUs) in private dental practices, Toronto, Canada. Methods: 100 dental practices were
recruited. In participating dental practices, disc-specimens (2mm-thick/4mm-diameter) (n=6)
were fabricated from TPH Spectra (Dentsply) (camphorquinone (CQ) photoinitiator) and
Tetric EvoCeram (Ivoclar-Vivadent) (CQ/Lucirin-TPO photoinitiators). Using the office's
LPU, specimens were polymerized for the 20s from the top surface, at 1mm (n=3) and 4mm
tip-to-target distance (n=3). Specimens were stored (distilled-water/370C/24h), dried for 1h,
before microhardness-testing. Knoop-hardness number (KHN) for each specimen was
recorded from 8 measurements (4lower/4upper), using microhardness tester with Knoop
indenter (Tukon 300, Bridgeport, CT), and applying 50g load/30s dwell-time. Calculated RH
ratios data were statistically analyzed using Pearson correlations, Chi-square tests, and
independent-samples t-tests with a pre-set alpha of 0.05. Results: Both RBCs polymerized at
1mm had average ratios greater than the .80 level of RH (ratio of mean lower/upper surface
hardness), but not at 4mm tip-to-target distance. A significantly higher percentage of LED
LPUs achieved the 0.80 ratio of hardness with TPH Spectra, compared to Tetric EvoCeram
RBC with both, 1mm (χ2(1) =27.32, p<.001) and 4mm tip-to-target distance (χ2(1) =15.48,
p<.001). Conclusions: The RH profiles of RBCs tested demonstrated a strong, inverse
correlation between the tip-to-target distance and average RH values. The incident irradiance
and LPUs’ beam dispersion, had a significant effect on Knoop hardness profile and
mechanical properties of RBCs.
162
Keywords: Light Polymerization Unit; Irradiance; Knoop hardness, Degree of
conversion; polymerization; Resin-Based Composite; CheckMARC®;
163
Introduction
There are several testing procedures available to characterize RBCs polymerization
efficiency. During a free-radical photopolymerization reaction, double bonds in the vinyl
groups of the RBCs are converted into single covalent bonds, thus further enhancing the
conversion of monomers into the linear and cross-linked polymers. The degree of conversion
(DC), a significant determining factor of the polymerization kinetics, is highly associated
with mechanical, physical, and chemical properties of direct restorative RBC materials 1, and
represents the ratio of converted single bonds to the total number of starting double bonds.
Several analytical methods are available for measurement of RBC conversion. For instance,
differential scanning calorimetry provides a measure of methacrylate conversion based on the
enthalpy of the exothermic polymerization process 2. The majority of analyses done to
evaluate conversion of RBCs have been based on the use of spectroscopy, which gives a
direct measure of reacted and unreacted methacrylate groups 2. Although these methods
exhibited an elevated level of accuracy and sophistication, they are always related with high-
costs 3-6.
The most efficient methods to determine a RBC polymerization level include DC,
depth of polymerization (DoP), the degree of crosslinking (DoC), and microhardness 7-10.
Researchers prefer to use reliable and easy to use microhardness tests, rather than more
elaborate tests, such as FTIR or Raman spectroscopy preferentially used for DC.
Furthermore, Knoop microhardness testing has proven to be a reliable and a relatively simple
method to characterize RBCs' mechanical properties after polymerization. The Knoop test
gained popularity among researchers, primarily due to its good correlation with DC, which
has been confirmed in numerous studies 11-16. Monomer conversion could not provide a
164
complete characterization of the polymer structure, because polymers with similar DC may
exhibit a different degree of crosslinking and consequently different mechanical properties 17.
The commonly held assumption before the introduction of bulk-fill RBC materials, was that
an equivalent of 16 J/cm2 of received radiant exposure to a single 2 mm tick increment of
RBC material is considered satisfactory for adequate RBCs polymerization 18. However,
manufacturers' recommended minimal radiant exposures for sufficient RBC polymerization
can vary to a great extent, depending on intrinsic properties and chemical formulation, as
well as on the material shade and the radiant exitance of the LPUs 19-21.
Hardness (H), represents an important parameter in the characterization of the surface
properties of a material on a microscopic scale, which is defined as the ratio of the applied
load (P), to the contact area (A), which resulted in the subsequent indentation impression:
H = 𝑃
𝐴 = β
𝑃
𝑑2 (1)
Where d represents the size of attributed resultant indentation, and β represents the
constant related to the geometry of a given indenter 22. Vickers and Knoop are the most
frequently employed micro-hardness tests for characterization of RBC materials by utilizing
a pyramid-based indenter. The Vickers hardness number (VHN) is defined by using the
contact area of the indentation impression and the parameter d in Eq. (1), is described as an
average of the two diagonals of the resultant square-shaped impression. Thus, the β constant
for VHN calculation is set to be 1.8544. Knoop microhardness test uses a pyramid-based
indenter with the length to width ratio of 7:1 and with corresponding face angles of 172.5
degrees for the long edge and 130 degrees for the short edge, as shown in Figure 3.1.
Furthermore, Knoop hardness number (KHN) calculation considers the projected area of the
165
indentation impression, and the parameter d, which is determined by the length of the long
diagonal of the resultant rhombic impression and β constant set to be 14.229. Based on Eq.
(1), it can be implied that microhardness represents an indentation area corresponding
variable. The literature has shown, especially with RBC materials that the inequality between
a plastic zone beneath the indentation and a surrounding elastic matrix exists. Therefore, that
phenomenon is described as the elastic recovery of the given material after the removal of the
indenter. The elastic recovery is highly related to the surface geometry of an indenter, which
differentiates the elastic recovery of Vickers indentation in comparison to Knoop indentation.
The elastic recovery attributed to Vickers indentation usually develops along the depth of the
indentation, while the diagonal length of the surface impression remains unchanged, contrary
to the Knoop indentation, where the elastic recovery appears as a result of a minor diagonal
contraction of the indenter 23. The Knoop microhardness computation only considers the
length of the longer diagonal into the calculation of KHN, and thus makes this method
preferential for its reliability over other methods 24, 25. A linear correlation between Knoop
microhardness and mechanical properties of RBC and Knoop microhardness and irradiance
of LPUs have been shown to exist 28.
Although the scraping test has been considered as a depth of polymerization measure
in the ISO 4049 standardization for RBCs, microhardness tests were perceived to be more
reliable 7. The test is based on top and bottom hardness measurements; however, it is
common to calculate the ratio of the bottom/top hardness, and the arbitrary minimal values of
0.80 would represent a ratio of sufficient RBCs polymerization 24, 27, 28. "Relative hardness"
term represents the average bottom-to-top hardness ratio 29.
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The project objective was to evaluate relative hardness (RH) of RBCs polymerized
using LED LPUs, which were in use in participating private dental practices in the Greater
Toronto Area. The test parameter for RH was the bottom-to-top Knoop microhardness
values. The study objectives were to test the following research hypotheses: the first
hypothesis was that the LED LPUs, from the participating dental practices, capable of
producing the radiant exposure of 16 J/cm2, would be positively correlated with the RH
values of RBCs tested. The second hypothesis was that 1mm and 4mm material-to-probe
distance, would not affect the average 0.80 RH values of 113 LED LPUs tested.
Materials and methods
From all participating private dental practices in the Greater Toronto Area, a total of
number of 113 LED LPUs were included. Specimens were fabricated using an opaque plastic
mold (Figure 5.1) from the A2 shade of two conventional RBC materials: TPH Spectra HV
(Dentsply, Milford DE) and Tetric EvoCeram (Ivoclar-Vivadent, Amherst NY). TPH Spectra
exhibits a camphorquinone (CQ) photoinitiator system, while Tetric EvoCeram has a mix of
CQ and a small amount of Lucirin TPO photoinitiators 30.
With each LPU, a total number of 12 specimens were prepared (6 per material) with 4
mm diameter and 2 mm thickness. The specimens were covered with celluloid matrices on
both sides and with a glass slide on the top. A 20s exposure was administered to the top
surface of the specimen (n=3) by applying the LPU available in the visited dental practice
LED LPU directly through a 1mm glass slide. A 2 mm-metal spacer with a 20 mm diameter
was placed on the top of the glass slide that covered the specimen, then 1mm glass slide
167
covered the metal spacer. This setup created a 4mm material-to-probe distance. A 20s
exposure of the top specimens ‘surface (n=3) by the same LED LPU was conducted. The
same protocol was implemented to the other brand of RBC. Following the light exposure,
specimens were stored in dark containers in distilled water medium at 370C for 24h before
microhardness testing. The next day, all specimens were dried for 1h, before commencing the
tests. Knoop Hardness Numbers (KHN) were obtained at 10X magnification from eight
surface measurements, as shown in Figure 5.2. Thus, each side of the specimens received
four indentations in North-South and East-West direction, using a microhardness tester
(Tukon 300, Wilson Instrument Division, Bridgeport, CT), and an applied load of 50g and,
during a 30s dwell time. RH ratio for each specimen was calculated from the specimen’s
bottom mean KHN values divided by the specimen’s top mean KHN values. A reference,
multiwave LPU (Bluephase Style, Ivoclar-Vivadent, Amherst NY) was used as a positive
control. The assessment of 113 LPUs in the participating dental practices was completed by
using a commercial, laboratory-grade light measuring device (CheckMARC®, BlueLight
Analytics, Halifax, NS).
• Statistical analysis
Descriptive statistics were used to evaluate irradiance values of LED LPUs tested.
The 10s radiant exposure data obtained by the CheckMARC® were multiplied by 2 to
produce 20s radiant exposure values, which were correlated to the minimum passing criterion
of 16 J/cm2. To evaluate whether the association between the RH values and LED LPU
radiant exposure values, concerning the minimum required the energy of 16 J/cm2 exists, the
Pearson correlation analysis was used, significant at the 0.001 level. Furthermore, the same
168
analysis was used to evaluate the relationship between the irradiance values of LPUs (n=113)
and obtained Knoop hardness b/t ratio of two RBC materials, polymerized at 1 mm and 4
mm material-to-probe distance. Chi-square tests of independence were used to examine a
possible relationship between LED LPUs with the capacity to deliver 16 J/cm2 energy and
the minimum level of RH (0.80) for each type of specimen. The results of these tests for all
four testing groups (two brands of RBCs and two tip-to-target distance levels) at the 0.05
level of significance were plotted. All statistical analyses were conducted using the Statistical
software (SPSS for Windows, version 22, SPSS Inc., IBM, Somers, NY).
Parametric statistical tests used in this study relied on assumption of normality of data
and equal variances among the comparison groups. Following these assumptions is especially
critical when the sample size used in the study is small. Considering that the sample used in
this study consists of 113 LED LPUs, parametric statistical tests were used. However, to
investigate the potential impact on the normality assumption violations, nonparametric tests
were also conducted. In case of discrepancies that arise by using non-parametric tests, the
results of the non-parametric tests would be reported. However, it was discovered that the
results of non-parametric tests and parametric tests were identical.
Results
The Q-Q plots were used for normality assessment, and the results of the average RH
values in four data distributions are presented in Figures 5.7 to 5.10. Equality of the variances
assumption across the comparison groups was tested with the Levene’s test. The tests were
169
not significant, Tetric EvoCeram at 1 mm (p=0.351), and 4 mm (p=0.829) material-to-probe
distance, and TPH Spectra at 1 mm (p=0.114), and 4 mm (p=0.067) material-to-probe
distance, respectively. Thus, the equality of the variances was assumed. A power analysis
using the G Power program for the independent and paired t-tests revealed that the sample
size of 113 was capable of detecting a medium effect size with the independent samples t-
tests (Cohen's d=0.53). Additionally, for the Chi-square test of independence, a reference
(Oyeyemi et al., 2010) 39 was used for the power analysis. According to the reference, the
study sample size of 113 can detect only large effects with the power of 0.80.
The descriptive statistics for RH levels of two RBC's brands obtained from the
specimens polymerized at 1 mm and 4 mm material-to-probe distance are presented in Table
5. 1. The average 0.80 level of RH (ratio of the mean bottom/top surface) was accomplished
for both RBC brands, polymerized at 1 mm material-to-probe distance. However, the
specimens polymerized at 4 mm material-to-probe distance were not able to produce an
average 0.80 level of RH regardless of the RBC brand. The percentage of dental practices
that reached the 0.80 level of RH for each type of specimens is displayed in Figure 5.3.
Significant differences were observed between 1mm and 4mm tip-to-target distances for the
two RBC brands, and their RH results (inter-brand and intra-brand).
The pool of LPUs tested consisted of 32 multiwave and 81 monowave units. The
average RH values of two LPU types and two RBC brands polymerized at 1 mm and 4 mm
material-to-probe distance are presented in Figure 5.4. The results of the independent-
samples t-tests did not show any statistically significant difference between the average RH
values of two RBC brands and a type of LPU used at material-to-probe (1 mm and 4 mm)
distance. The following results were observed for Tetric EvoCeram at 1mm material-to-probe
170
distance t (113) = -0.94, p= 0.351 (95%CI for MD: -0.03; 0.01), and at 4 mm material-to-probe
distance t (113) = 0.22, p= 0.829 (95%CI for MD: -0.02; 0.03), while the results for TPH
Spectra at 1mm material-to-probe distance were t (113) = -1.59, p= 0.114 (95%CI for MD: -
0.04; 0.00), and at 4 mm material-to-probe distance were t (113) = -1.85, p= 0.067 (95%CI for
MD: -0.06; 0.00).
The average RH ratios produced by the reference LPU (Bluephase Style, Ivoclar-
Vivadent) were 0.85 at 1 mm, and 0.80 at 4 mm material-to-probe distance for Tetric
EvoCeram, and 0.84 at 1mm and 0.83 at 4 mm material-to-probe distance for TPH Spectra.
Discussion
The first research hypothesis, which stated that the participating dental practices LED
LPUs, capable of producing a 20s radiant exposure of 16 J/cm2, would be positively
correlated with the Knoop hardness b/t ratio values of RBCs tested, was accepted. A positive,
weak-to-moderate, but significant (an average r=0.36, p<.001), correlation was found
between the average 0.80 RH level of the two RBC brands and a 20s radiant exposure of
LED LPUs capable of delivering at least of 16 J/cm2 radiant energy.
During the course of this project, wide variations of LED LPUs were tested, along
with possible temperature differences (different weather conditions), and other contributing
factors that may happen during the specimens' preparation in the participating dental
practices that could cause these results. For instance, the correlation factor is not consistent
among RBC materials and the material-to-probe polymerization distance. Although TPH
Spectra and Tetric EvoCeram are popular brands of the Bis-GMA based nanohybrid RBCs,
171
which exhibit the presence of pre-polymerized filler particles in their compositions, they
differ in a total filler load values, being 60-62 volume percentage for TPH Spectra and 53–55
volume percentage for Tetric EvoCeram. Furthermore, TPH Spectra exhibits a
camphorquinone (CQ) photoinitiator system, while Tetric EvoCeram has a mix of CQ and
Lucirin TPO photoinitiators 30. Thus, the effect of polymerization distance in combination
with chemical properties of RBCs tested resulted in a wide range of correlation values. Tetric
EvoCeram exhibited a moderate correlation factor of r= 0.43 at 1 mm material-to-probe
distance, but at 4 mm material-to-probe distance the correlation factor was reduced to r=0.33,
where in an opposite direction with TPH Spectra (r=0.44 at 4 mm material-to-probe distance
and r=0.26 at 1 mm material-to-probe distance) was detected. The magnitude of these
relationships suggests that other factors are contributing to RBC microhardness, such as
LPU’s spectral irradiance output (monowave vs. multiwave), as well as a beam profile
differences, apart from the confirmed LED LPUs capacity to deliver at least 16.0 J/cm2 of
energy.
The second hypothesis, which anticipated that 1mm and 4mm material-to-probe
distance, would not affect the average 0.80 RH values of the two brands RBC tested, was
rejected. The average 0.80 level of RH polymerized at 1mm material-to-probe distance was
obtained by 69.0% of Tetric EvoCeram specimens and 82.3% of TPH Spectra specimens,
respectively. However, at 4 mm material-to-probe distance, the average 0.80 level of RH
values was not achieved by any RBC material (0.77 for both materials). Furthermore, it was
discovered that at 4 mm material-to-probe distance, only 28.3% of Tetric EvoCeram
specimens and 37.2% of TPH Spectra specimens be able to obtain the minimum RH ratios.
Therefore, a recorded RH ratio drop over the 3 mm material-to-probe distance was 59% for
172
Tetric EvoCeram and 54.8% for TPH Spectra, respectively. The results are in line with
previous finding that light energy diminishes considerably with an increased material-to-
probe distance, which is observed as reduced microhardness profile values 35, 36, 37. The
results could be associated with the interactions of light polymerization energy with the
microscopic slides, Mylar strips, and a 2 mm span of plastic mold lateral walls. Furthermore,
a metal spacer which was used to produce 4 mm distance between the light probe and the
RBC specimens, could also influence the RH values. However, the current test set up design
was employed for its simplicity, quick set up time, and its ability to use in any participating
dental practice.
The participating dental practice LED LPUs capable of delivering at least of 16 J/cm2
radiant exposure, produced the 0.80 level of RH only by 61.1% Tetric EvoCeram specimens
polymerized at 1 mm material-to-probe distance and by 25.7% polymerized at 4 mm
material-to-probe distance. A similar pattern was observed with TPH Spectra, where the
requirement was achieved by 70.8% specimens, polymerized at 1mm material-to-probe
distance and with only 34.5% specimens polymerized at 4 mm material-to-probe distance. It
was evident that specimens at 4mm material-to-probe distance, regardless of RBC brand
performed notably below what was found when the light was held at an only 1mm material-
to-probe distance. Therefore, the percentage of LPUs that exhibited insufficient
polymerization at 4 mm material-to-probe distance was greater than 60%. The material-to-
probe distance of 4 mm or more is clinically relevant because most of the polymerization
locations in posterior teeth can exceed 7 mm material-to-probe distance 37.
173
The current study tested RBC specimens of A2 shade, because of the following facts:
A2 shade is the most commonly employed shade in clinical practice 30, and the composition
of pigments and other colorants has the minimum effect on photopolymerization 31.
The pool of tested LED LPUs consisted of 32 multiwave and 81 monowave LPUs. In
the multiwave group, the Valo® (Ultradent, South Jordan, UT) was the most common (38.7
%), while the Demi Plus® (Kerr, Orange, CA) was the most common (26.2 %) in the
monowave group. Due to the possible variations among participating light sources, the
assessment was carried on all LPUs by measuring their irradiance and radiant exposure with
a laboratory-grade spectrometer, the CheckMARC® system. The rationale was that all tested
LPUs had to be examined whether they had the capacity of delivering at least 16 J/cm 2
radiant exposure, to correlate the produced energy data with averaged 0.80 RH ratios of two
RBC materials.
The results of an analysis which compared Tetric EvoCeram (an alternative
photoinitiator system) and TPH Spectra (a conventional CQ photoinitiator system) with
multiwave and monowave LPUs, showed no statistically significant difference among those
four variables (see Figure 5.4). The multiwave LPUs outperformed monowave lights in all
tests except with the Tetric EvoCeram 4 mm material-to-probe distance, where the average
RH values of both LPUs type were identical. Interestingly, multiwave LPUs performed better
on TPH Spectra specimens, regardless of the material-to-probe distance. Although TPH
Spectra exhibited a higher filler load content and a conventional CQ photoinitiator system,
apparently these parameters did not produce significant Rayleigh scattering, often seen with
the multiwave LPUs violet light spectrum.
174
This project was the first study that correlated the minimum-required level of radiant
exposure of LED LPUs in the participating dental practices using the average 0.80 level of
RH to test two RBC materials. The majority of published studies test a selected number of
RBCs using yet a variety of LPUs 11-16. This study design differed significantly from all
previous studies. Microhardness values were taken 24h after exposure and stored in 37ºC
water to mimic the oral environment. One hour before the hardness testing, the specimens
were dried using a paper towel and left in the ambient temperature, after which KHN
measurements were obtained. The RBC materials complied with the Water Sorption (ISO
4049) standards 52. Based on results from a pilot study which used the same two brands of
RBCs, no statistically significant difference in microhardness was seen when each of the two
RBC materials during the 24h storage in either distilled water or dry containers.
A reference LPU (Bluephase Style, Ivoclar-Vivadent) produced sufficient hardness,
regardless of the polymerization material-to-probe distance or the RBC tested. In comparison
with RH values for Tetric EvoCeram obtained by the reference LPU (0.85 at 1 mm and 0.80
at 4 mm material-to-probe distance), 108 participating LPUs (95.6%) were below that RH
level at 1 mm material-to-probe distance, and 81 LPUs (71.7%) did not produce that RH
level at 4 mm material-to-probe distance. Similarly, in comparison with RH values for
Spectra TPH obtained by a reference LPU (0.84 at 1 mm and 0.83 at 4 mm material-to-probe
distance), 97 LPUs (85.8%) were below that RH value at 1 mm material-to-probe distance,
while 101 LPUs (89.4%) did not reach that RH level at 4 mm material-to-probe distance.
Furthermore, the reference LPU exhibits relatively homogeneous beam profile distribution of
its spectral irradiance, which is the part of the latest LED generation development. Thus, it
175
was observed that the reference LPU exhibited more uniform Knoop hardness profile with all
materials and polymerization material-to-probe distances.
Limitations of the current project were associated with the current set-up that the
polymerization light is passing through glass slides, Mylar strips, and bouncing from the
lateral walls of the metal spacer, which probably affect the RH values to a certain degree.
However, metal sectional matrices that are used in clinical settings may similarly influence
RBC polymerization. It would be better if the current set-up had included an opaque plastic
spacer, preferably from the same material as the mold used for the specimens’ production.
Future research should be focused on bulk-fill RBC evaluation, using a similar
method as was used in this project. Furthermore, to create a clinically relevant scenario,
using the same methodology, the irradiation distance could be increased to 6 mm, because
most of the polymerization locations in posterior teeth can exceed 7 mm material-to-probe
distance 37.
Conclusions
Within the limitations of this study, the following conclusions can be reached:
1. Relative hardness ratios of the two RBC brands demonstrated a weak-to-
moderate correlation between the light probe distance to the material surface
and 20s radiant exposure.
2. The 0.80 RH ratio of two RBC brands was achieved by the majority of LED
LPUs at 1 mm material-to-probe distance.
176
3. The 0.80 RH ratio of two RBC materials was obtained by a minority of LED
LPU's at 4 mm material-to-probe distance.
4. The percentage of LPUs that exhibited inadequate polymerization at 4 mm
material-to-probe distance was observed to be higher than 60%.
5. The average relative hardness reduction over a 3 mm material-to-probe
distance, expressed by the RH profile, was 59% for Tetric EvoCeram and
54.8% for TPH Spectra, respectively.
177
Acknowledgements
The current study is part of a Ph.D. thesis submitted to the Faculty of Dentistry,
University of Toronto. It was supported by grants from the Faculty of Dentistry Research
Institute of the University of Toronto, and an American Academy of Esthetic Dentistry. The
CheckMARC® device was loaned and technically supported by BlueLight Analytics, while
material donations were provided by Ivoclar-Vivadent and Dentsply for which the authors
are very grateful. In addition, the Bluephase Style was graciously loaned by Ivoclar-
Vivadent. The authors are immeasurably thankful to our research assistants Ms. Ana Burilo
and Ms. Ivana Orlovic for their contribution to this project.
178
Figure 5.1 Plastic mold used for the specimens’ production
Figure 5.2. Specimen ready for Knoop microhardness testing
179
Table 5.1 Descriptive statistics for RH ratios recorded for two RBC’s specimens
polymerized at 1mm and 4mm tip-to-material distance. The assumptions of
normality and equal variances among the comparison groups was assumed.
Min Max M SD
Tetric
EvoCeram
1 mm distance .58 .89 .80 .05
4 mm distance .41 .91 .77 .06
TPH Spectra
1 mm distance .37 .90 .81 .06
4 mm distance .31 .88 .77 .08
Figure 5.3 Percentages of LPUs that reached the .80 ratios of RH for each RBC
brand. Significant differences were observed between 1mm/4mm tip-to-target
distances for the two RBC brands, and their RH results (inter-brand and intra-
brand).
69.0
2
8.32
28.3
82.3
37.2
*
*
**
**
180
Figure 5.4 Average Knoop hardness b/t ratios (Mean and SD) produced by multiwave
(n=32) and monowave (n=81) LPUs for the two brands of RBCs (Tetric EvoCeram
and TPH Spectra HV), irradiated for 20s from 1 mm and 4 mm tip-to-target distance.
No statistically significant difference among the groups was found
Figure 5.5 Average RH ratios for Tetric EvoCeram polymerized at 1 mm and 4 mm
tip-to-target distance, using a total of 113 LED LPUs.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
RH 1mm Tetric RH 4mm Tetric RH 1mm Spectra RH 4mm Spectra
Har
ne
ss r
atio
multiwave monowave
181
Figure 5.6 Average RH ratios for TPH Spectra polymerized at 1 mm and 4 mm tip-to-
target distance, using a total of 113 LED LPUs.
Figure 5.7 Q-Q plot showing the average RH ratios of Tetric EvoCeram at 1 mm tip-
to-target distance observed values against normally distributed data (represented by
the line).
182
Figure 5.8 Q-Q plot showing the average RH ratios of Tetric EvoCeram at 4
mm tip-to-target distance observed values against normally distributed data
(represented by the line).
Figure 5.9 Q-Q plot showing the average RH ratios of TPH Spectra at 1 mm
tip-to-target distance observed values against normally distributed data
(represented by the line).
183
Figure 5.10 Q-Q plot showing the average RH ratios of TPH Spectra at 4 mm tip-
to-target distance observed values against normally distributed data (represented
by the line).
184
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Chapter 6
Manuscript 4
Assessment of Diametral Tensile Strength of Resin Based Composites Polymerized by LED Light-
Polymerization Units in Private Dental Practices in Toronto, Canada
191
Assessment of Diametral Tensile Strength of Resin Based Composites polymerized by
LED Light-Polymerization Units in Private Dental Practices in Toronto, Canada
Dave D. Kojic DDS, MSc. PhD-candidate
Department of Restorative Dentistry, Faculty of Dentistry, University of Toronto, Toronto,
ON, Canada
*Omar El-Mowafy, BDS, PhD, FADM
Professor and Head, Department of Restorative Dentistry, Faculty of Dentistry, University of
Toronto, Toronto, ON, Canada
Richard Price DDS, MS, PhD
Professor, Department of Clinical Dental Sciences, Faculty of Dentistry Dalhousie
University, Halifax, NS, Canada
Wafa El-Badrawy, BDS, MSc.
Associate Professor Department of Restorative Dentistry, Faculty of Dentistry, University of
Toronto, Toronto, ON, Canada
Gildo Santos, DDS, MSc, PhD
Associate Professor Chair - Restorative Dentistry, Schulich School of Medicine & Dentistry,
Western University, London, ON, Canada
*Corresponding author Omar El-Mowafy, BDS, PhD, FADM
Department of Restorative Dentistry
Faculty of Dentistry, University of Toronto
124 Edward Street, Toronto, Ontario M5G 1G6, Canada
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Abstract
Objectives: To evaluate Diametral Tensile Strength (DTS) of two resin-based
composites (RBC), polymerized using LED light-polymerization units (LPUs) in
participating private dental practices. Methods: A 100 dental practices from the Greater
Toronto Area, were recruited. Specimens were made of TPH Spectra (Dentsply),
(camphorquinone (CQ) photoinitiator) and Tetric EvoCeram (Ivoclar-Vivadent),
(CQ/Lucirin-TPO photoinitiators). A plastic mold was used to produce cylindrical specimens
(4mm-thick/6mm-diameter). Each specimen's surface received a 20s radiant exposure.
Specimens (n=5) were stored in dry/dark container (370C/24h). The DTS was conducted in a
universal testing machine (model 8501 Instron Corp), using a cross-head speed at 0.5
mm/min. Fracture load (N) was recorded, and the DTS was calculated. The DTS data of
Tetric EvoCeram and TPH Spectra were analyzed with a paired-samples t-test and a mixed
ANOVA. All statistical tests were completed using a pre-set, α of 0.05, and Pearson
correlations, using a pre-set, α of .001. Results: No significant difference in average DTS
values between the two materials (t (113) =-.72, p=.475) was found. The mean DTS observed
for Tetric EvoCeram was 40MPa (SD=5) and for TPH Spectra 39.9MPa (SD=4). The DTS
values were highly correlated between the two materials (r=.71, p<.001) and the
corresponding LPUs radiant exposure. Conclusions: Although DTS averages two RBCs
were not significantly different, moderately-strong correlations between the 16 J/cm2 of
radiant exposure in 20s and DTS levels of Tetric EvoCeram (r=.54, p<.001) and TPH Spectra
(r=.53, p<.001) were observed.
Keywords: Light Polymerization Unit; Irradiance; Radiant exposure; Diametral
Tensile Strength test; Degree of conversion; Resin-Based Composite; CheckMARC®;
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Introduction
Adequate RBC photopolymerization is a critical factor in predicting clinical success
and the longevity of direct restorations. Insufficient RBC polymerization is highly correlated
with a substantial reduction in their mechanical properties 1.
The efficient methods to determine the level of RBC polymerization are the degree of
conversion (DC), depth of polymerization (DoP), the degree of crosslinking (DoC),
compressive strength (CS) test, and the diametral tensile strength (DTS) test. Some
mechanical testing procedures have been examined to address the complexity of loading in
clinically relevant situations. Based on literature findings, a positive correlation between CS
and DTS has been observed 2, 3, 4, 5. Furthermore, the fracture resistance (FS) test has
frequently been used for the evaluation of flexural strength and a flexural modulus of RBC
materials by performing a three-point bending test, included in the ISO 4049:2009
standardization testing procedures 6. Although DTS test is not a part of the ISO 4049:2009
standards, it was recognized as technically reliable and a straightforward test for the
assessment of RBC mechanical properties. The DTS test was defined by the ADA/ANSI
Specification No.27 for the RBCs evaluation. The DTS has been designed to test brittle
materials by utilizing a cylinder-type specimen that is subject to compressive load across its
diameter. It has been observed that materials which exhibit plastic deformation would
produce false DTS values. Moreover, they would be anticipated to reveal strain rate
sensitivity 7.
There are some controversies associated with the DTS test, as per experimental
observation that resultant shear forces acting at the apex of the diametral plane have been
suggested to complicate the interpretation of strength, with the possibility that reported data
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from different studies would not be challenging to interpret 7, 9. However, DTS test can
provide useful information about the effectiveness of RBC polymerization and in conjunction
with other measuring methods can provide meaningful data on the mechanical properties of
given materials 8-16.
The study aimed to assess specific mechanical properties of two RBC brands,
polymerized by LED LPUs in the participating private dental practices, by conducting a DTS
test. The study objectives were to test the following research hypotheses. The first hypothesis
was that the average DTS values of Tetric EvoCeram and TPH Spectra polymerized using
the participating LED LPUs would not show any difference. The second hypothesis was that
LED LPUs with the capacity to produce a 20s radiant exposure of 16 J/cm2, would be
positively correlated with the DTS values of the materials tested. The third hypothesis was
that DTS values of the two RBC brands, polymerized by multiwave LPUs would not be
different from the DTS values polymerized by monowave LPUs.
Materials and methods
Specimens for the DTS test were made in the participating practices, by a single
administrator. However, before the specimens’ preparation, the assessment of LPUs in the
participating dental practices was completed by using a commercial, laboratory-grade light
measuring device (CheckMARC®, BlueLight Analytics, Halifax, NS). A 10s radiant
exposure obtained by the CheckMARC® device was multiplied by 2 to represent a 20s
radiant exposure. The study criterion required all LPUs to generate a minimum of 16 J/cm2 of
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radiant exposure in a 20s cycle. The specimens were fabricated using an opalescent,
cylindrical, and plastic mold (4 mm thick and 6 mm in diameter) (Figure 6.1). Furthermore,
five specimens per BC brand were prepared from the A2 shade of each of two nanohybrid
RBC materials: TPH Spectra (high viscosity) HV (Dentsply, Milford DE) and Tetric
EvoCeram (Ivoclar-Vivadent, Amherst NY). TPH Spectra exhibits a camphorquinone (CQ)
photoinitiator system, while Tetric EvoCeram consists of CQ and Lucirin TPO
photoinitiators 20. The RBC material was inserted into the mold and overlaid with a polyester
strip and a 1 mm microscopic glass-slide on both sides, and then light polymerized by the
LED LPU from the participating dental practice, for the 20s at the top and 20s at the bottom
surface. The polymerized specimens were extracted from the mold and stored in dark
containers at 370C and tested after 24h storage. Specimens were measured with a digital
caliper (Mitutoyo Canada, Mississauga, ON) before testing into a universal testing machine
(Model 8501, Instron Corp, Canton, MA). Each specimen was tested by placing the specimen
on its longitudinal side on the platens of the universal testing machine. The load was applied
vertically on the lateral portion of the cylinder at a crosshead speed of 0.5 mm/min,
producing tensile stress perpendicular to the vertical plane passing through the center of the
specimen (Figure 6.2). The specimens were loaded to failure in compression, and the
fracture load was recorded in Newton (N). Following the compressive test, each specimen
was visually inspected to determine whether the specimen fractured into two halves. Those
specimens fractured into unequal sizes were excluded from the analysis. The DTS was
calculated based on fracture load (F) expressed in Newton (N), diameter, and height of the
specimen, calculated using the formula from the Phillips’ Science of Dental Materials 325:
σt = 2𝐹
𝑑ℎ𝜋 (1)
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where are: d: diameter (6 mm); h: height (4 mm) of specimens; π: 3.14
To obtain average DTS values of the positive control group (n=5 per each RBC brand), a
reference, multiwave LPU (Bluephase Style, Ivoclar-Vivadent) was used.
• Statistical analyses
Considering that the sample used in this study consisted of 113 light-units, normality
was assumed. Therefore, parametric statistical tests were employed. However, to investigate
the potential impact of the violation of the normality assumption on statistical tests results,
nonparametric tests were also conducted. Whether the results of the parametric and non-
parametric tests deviated, then the results of the non-parametric tests would be reported.
However, it was discovered that the results of non-parametric tests and parametric tests were
identical. Thus, parametric tests were reported.
A paired-samples t-test was used to investigate whether average DTS values were
different between Tetric EvoCeram and TPS Spectra specimens. To further examine whether
LED LPUs with the capacity to deliver at least 16 J/cm2 of radiant exposure had higher levels
of DTS on average, a mixed ANOVA was conducted. The DTS levels of the two RBC
brands were used as a within factor in this analysis, and an indicator of LED LPUs capable of
delivering a minimum radiant exposure level, (yes/no) was used as a between factor. The
analyses were conducted using a pre-set alpha of 0.05.
The 10s radiant exposure data obtained by the CheckMARC® device were multiplied
by 2 to produce the 20s radiant exposure values, which were correlated to the study-required
minimum criterion of 16 J/cm2. It was determined that (n=87) LPUs could satisfy the
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criterion. Thus, to evaluate whether the association between the DTS values of two RBC
tested and LED LPU radiant exposure values, concerning the study-required energy of 16
J/cm2 exists, the Pearson correlation analysis was used, significant at the 0.001 level. The
SPSS software for Windows, version 22, (SPSS Inc., IBM, Somers, NY) was used for all
statistical analyses.
Results
The results of Shapiro-Wilk tests for normality of the average DTS values were not
significant for Tetric EvoCeram (p=0.081) and TPH Spectra (p=0.097), confirming that data
were normally distributed. Furthermore, the Q-Q plots were also used for normality
assessment, and they are presented in Figures 6.4 and 6.5.
The Levene’s test was used for the assessment of equality of variances across the
comparison groups. The results of Levene’s tests were not significant for all tests (p>0.05),
indicating that assumption of equal variances was satisfied. However, the postulate of equal
variances was violated once, in the independent-samples t-test analysis, where the
appropriate action was taken (by adjusting the degree of freedom). A power analysis using
the G Power program for the paired t-tests revealed that the sample size of 113 was capable
of small effect size with the paired-samples t-tests (Cohen's d=.23). A reference from the
SPSS manual (Green et al., 2010)21 was used to calculate a power analysis for mixed
ANOVA. According to the reference, each cell in the design should have at least 15 cases to
be able to express the minimum power of .80. Thus, the criterion was met in the current
study.
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A paired-samples t-test showed no significant differences in the average DTS values
(MPa) between two RBC brands (t (112) = -0.72, p=0.475, 95% CI: -0.92; 0.43). The mean
BTS values (Table 6.1) were 39.7 (SD=5) for Tetric EvoCeram and 39.9 (SD=4) for TPH
Spectra. It was observed that DTS levels were highly correlated between the two RBC brands
(r=0.71, p<.001), indicating that LED LPUs which produced higher DTS values on Tetric
EvoCeram specimens, also produced higher DTS values on TPH Spectra specimens.
A mixed ANOVA analysis showed that regardless of RBC brand, the LED LPUs that
were capable of delivering at least 16 J/cm2 of irradiant exposure, also produced higher DTS
levels, F (1, 111) = 18.00, p<.001. A partial ɳ2=0.14 was indicating a medium-to-large effect
size (95% CI: -5.66; -2.06). Specifically, the average DTS values for LED LPUs that did not
meet the criterion was 36.8 MPa (SD=0.8) and for LED LPUs that met the criterion was 40.7
MPa (SD=0.4).
The pool of LED LPUs tested consisted of 32 multiwave and 81 monowave LPUs.
The majority of LED LPUs (n=87, 77.0%) could deliver at least of 16 J/cm2 radiant exposure
in 20s exposure. The average DTS values of the control group were 42.9 MPa for Tetric
EvoCeram, and 43.9 MPa for TPH Spectra.
The Pearson correlation analyses (n=113) showed moderately strong correlations
between the incident irradiance values of LED LPUs and DTS values of Tetric EvoCeram
(r=0.54, p<.001) and TPH Spectra (r=.053, p<.001) RBCs. Furthermore, small-to-moderate,
but significant, correlations were found between LED LPUs capable of delivering at least 16
J/cm2 of radiant exposure (n=87), and average DTS values of Tetric EvoCeram and TPH
Spectra (r=0.37, p<.001, and r=0.31, p<.001), respectively.
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An independent-samples t-test showed no significant differences in the average DTS
values of Tetric EvoCeram (t (112) = 0.07, p=0.943, 95% CI: -1.70; 1.83), and TPH Spectra
(t (112) = 1.07, p=0.289, 95% CI: -0.68; 2.25), polymerized with multiwave and monowave
LPUs (Table 6.1). The Levene’s tests were significant (p<0.05), therefore, the assumption of
equal variances was violated, however, the tests with adjusted degrees of freedom were used.
Discussion
Adequate photopolymerization is crucial for the expected longevity of RBC
restorations. For a quick and straightforward assessment of polymerization efficiency, the
BTS test is frequently used to quantify the tensile strength of brittle dental restorative
materials 3-5. The appropriateness and reliability of this method to successfully analyze the
capacity of RBC materials to withstand strong masticatory forces in clinically appropriate
situations has been reported 7.
The first research hypothesis, that the average DTS values of Tetric EvoCeram and
TPH Spectra, polymerized LED LPUs in the participating dental practices would not show
any significant difference, was accepted. The results of a paired-samples t-test were not
significant, (t (112 =-0.72, p=0.475), indicating no difference in the average level of BTS
between the two materials. The 95% confidence interval of the difference in the average DTS
level was in the range from -0.92 to 0.43. The mean DTS value observed in the sample for
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Tetric EvoCeram was 39.7 MPa (SD=5), (95% CI for MD: 38.7; 40.6), and for TPH Spectra
was 39.9 MPa (SD=4), (95% CI for MD: 49.1; 40.7).
Although TPH Spectra and Tetric EvoCeram are Bis-GMA based nanohybrid RBCs,
which exhibit the presence of pre-polymerized filler particles in their compositions, they
differ in the total filler load values, being 60-62 volume percentage for TPH Spectra and 53–
55 volume percentage for Tetric EvoCeram. Furthermore, TPH Spectra exhibits a
camphorquinone (CQ) photoinitiator system, while Tetric EvoCeram has a mix of CQ and
Lucirin TPO photoinitiators 20. Thus, an alternative, Lucirin TPO photoinitiator system
exhibits its maximum peak absorption spectra below 420 nm, in the violet spectral range. It
would be expected that an extended light spectrum of multiwave LPUs yield the higher
average BTS values for Tetric EvoCeram, due to the presence of Lucirin TPO, and its
affinity for the violet range of multiwave LPUs spectral irradiance. However, experimental
data could not support that postulate, as the average DTS values of Tetric EvoCeram were
39.7 MPa, while the average DTS values of TPH Spectra were 39.9 MPa. Although Tetric
EvoCeram exhibits a blend of photoinitiators, which included the Lucerin TPO, and a lower
filler load percentage than of TPH Spectra, the occurrence of the Rayleigh scattering could
be associated with the lower average DTS values of Tetric EvoCeram.
The second hypothesis, that LED LPUs capable of producing at least of 16 J/cm2
radiant exposure in a 20s exposure cycle, would be positively correlated with the average
DTS values of Tetric EvoCeram and TPH Spectra, was accepted. A positive, significant, but
a small-to-moderate correlation between LED LPUs (n=87) capable of producing at least of
16 J/cm2 radiant exposure and the average DTS values of Tetric EvoCeram (r=0.37, p<.001),
and TPH Spectra (r=0.31, p<.001), was observed. Furthermore, the moderately strong
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correlations between a 40s radiant exposure delivered by LED LPUs (n=113), and the
average DTS values of Tetric EvoCeram (r=0.54, p<.001), and TPH Spectra (r=0.53,
p<.001), were noted. A further analysis unveiled that the average DTS levels were highly
correlated between the two RBC materials (r=0.71, p<.001), indicating that higher or lower
DTS values were observed on both materials depending on the radiant exposure generated by
the participating office’s LPU. Currently, this was the first study that correlated a minimum
level of radiant exposure of LED LPUs in the participating dental practices to the average
DTS level of two RBC materials. Many of published studies have tested a number selected
RBCs with a selected number of LPUs 8-16.
The third research hypothesis, that the average DTS values of two RBC brands,
polymerized by multiwave LPUs would not be different from the DTS values polymerized
by monowave LPUs, was accepted. An independent-samples t-test showed no significant
differences in the average DTS values of Tetric EvoCeram (p=0.943), and TPH Spectra
(p=0.289), polymerized with multiwave and monowave LPUs, as presented in Figure 6.3.
Although it would be expected that multiwave LPUs produce better results with Tetric
EvoCeram, that assumption was not observed. The average DTS values produced by
multiwave LPUs were 39.6 MPa for Tetric EvoCeram, and 39.4 MPa for TPH Spectra, while
the average DTS values produced by monowave LPUs were 39.7 MPa, and 40.1 MPa for
TPH Spectra, respectively.
The current project was the first study that explored DTS test values on a large
sample size of LED LPUs. This study design was dissimilar to all previous studies 8-13. The
decision to use 6mm x 4mm cylindrical specimens was based on a methodology that was
borrowed from previous studies, which, however, was not in full accordance with the
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ADA/ANSI Specification No.27. Specimens, following the polymerization were stored in
dry containers, instead of the distilled water, as advocated in the ADA specification. The
RBC materials tested complied with the Water Sorption (ISO 4049:2009) standards 6.
According to a pilot project data, no statistically significant difference was found between
two RBC material specimens during the 24h storage in either distilled water or dry
containers, followed by the DTS testing. Furthermore, the compressive load that was applied
in a universal testing machine, with a crosshead speed of 0.5 mm/min. There is no precise
agreement on the employment of different crosshead speed to the specimens for DTS
evaluation. However, the ADA/ANSI Specification No.27 recommends a crosshead speed of
0.75 mm/min. The test selection for 0.5 mm/min of crosshead speed, was based on the
premise that most of reported studies used the same parameter, and the study results showed
that the crosshead speed variations between 0.5 and 1 mm/min were not statistically
significant 19.
When compared with the average DTS values of the control group (42.9 MPa for
Tetric EvoCeram, and 43.9 MPa for TPH Spectra), and the average DTS values produced by
the LPUs tested, it was found that 77.9% LPUs produced lower average DTS values with
Tetric EvoCeram. Similarly, for TPH Spectra, 87.6% LPUs had the average DTS values
below the value of the reference LPU.
The broad spectrum of values recorded in this study could be attributed to a more
substantial number of LED LPUs that were applied for specimens` polymerization.
Furthermore, because travel distance between the participating dental practices was
sometimes reasonably long, the RBC materials could have been exposed to the ambient
temperature during the cold winter season. Thus, the RBCs temperature fluctuation could
203
have some influence on test results. Very shallow concentric grooves on the working faces of
the platens (universal testing machine) were used to prevent specimen’s slipping during the
compressive loading, which eliminated a need for paper slips. Although, paper-points were
not used to prevent contact points due to imperfection of cylindrical shape of the specimens,
however, any specimen fractured into unequal sizes was excluded from the analysis.
Limitations of the current project were associated with the current set-up where the
polymerization light was passing through glass slides, and Mylar strips, which could affect
the recorded DTS values to a certain degree. It would be better if the current set-up supported
the ADA/ANSI Specification No.27 recommendations for specimen's dimension and storage.
However, since the nature of this study was largely comparison of two restorative materials
and a current slight variation in specimen dimensions has little or no effect on the outcome.
Future research should include bulk-fill RBC restoratives evaluation, using similar
approach used in this project. Furthermore, to create a clinically relevant scenario, using the
same methodology, the radiant exposure should follow manufacturers' recommendation
values for the RBC, in respect to the incident irradiance of LPUs tested.
Conclusions
Within the limitations imposed by this study, it may be concluded that:
1. The average DTS values of Tetric EvoCeram and TPH Spectra, polymerized
with LED LPUs in participating dental practices, did not show a statistically
significant difference. The mean DTS values observed in the sample for Tetric
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EvoCeram was 39.7 MPa (SD=5), and for TPH Spectra, it was 39.9 MPa
(SD=4).
2. A positive, significant, but small-to-moderate correlation between LED LPUs
(n=87) capable of producing at least of 16 J/cm2 radiant exposure in the 20s
and the average DTS values of Tetric EvoCeram (r=0.37, p<.001), and TPH
Spectra (r=0.31, p<.001), was observed.
3. Moderately strong correlations between the 40s radiant exposure delivered by
LED LPUs (n=113), and the average DTS values of Tetric EvoCeram (r=0.54,
p<.001), and TPH Spectra (r=0.53, p<.001) were discovered.
4. The average DTS levels were highly correlated between the two RBC
materials (r=0.71, p<.001), indicating that higher or lower DTS values were
observed with both materials, depending on the radiant exposure applied by
the LPUs in the participating dental practices.
5. No significant differences were observed in the average DTS values of Tetric
EvoCeram (p=0.943) and TPH Spectra (p=0.289) polymerized with
multiwave and monowave LPU.
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Acknowledgements
The current study is part of a Ph.D. thesis submitted to the Faculty of Dentistry,
University of Toronto. It was supported by grants from the Faculty of Dentistry Research
Institute of the University of Toronto, and an American Academy of Esthetic Dentistry. The
CheckMARC® device was loaned and technically supported by BlueLight Analytics, while
material donations were made by Ivoclar-Vivadent and Dentsply for which the authors are
very grateful. Further, many thanks to Ivoclar-Vivadent for loaning the Bluephase Style as a
study reference LPU. Special thanks to our research assistants Ms. Ana Burilo and Ms. Ivana
Orlovic for their participation in this project.
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Figure 6.1 A DTS Specimen 4mm thick and 6mm in diameter
Figure 6.2 DTS testing in a universal testing machine.
207
Figure 6.3. The average DTS values (Mean ± SD) of Tetric EvoCeram and TPH
Spectra RBCs polymerized by multiwave (n=32) and monowave (n=81) LPUs,
following the 40s radiant exposure. No significant difference among the groups was
found.
.
Table 6.1 Average BTS values of Tetric EvoCeram and TPH Spectra, polymerized by
monowave (type 0) and multiwave (type 1) LPUs. *Levene’s tests for homogeneity
of group variances were significant for both tests (p<0.05), indicating that assumption
of equal variances was not assumed.
RBC LPU
Type
Mean SD Levene's
test
t p-
value
95% CI for the mean
difference
Lower Upper
Tetric
EvoCer
am
Mono 39.7 5 4.10* 0.07 0.943 -1.70 1.83
Multi 39.6 4
TPH
Spectra
Mono 40.1 5 8.68* 1.07 0.289 -0.68 2.25
Multi 39.4 3
208
Figure 6.4 Q-Q plot showing the average BTS values of TPH Spectra against
normally distributed data (represented by the line).
Figure 6.5 Q-Q plot showing the average BTS values of Tetric EvoCeram against
normally distributed data (represented by the line).
209
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Versluis A, Soares CJ. Mechanical properties, shrinkage stress, cuspal strain and
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exposure time on the degree of conversion and mechanical properties of a
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15. Garapati SN, Priyadarshini, Raturi P, Shetty D, Srikanth KV. An in vitro evaluation
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17. Neumann MG, Miranda WG Jr, Schmitt CC, Rueggeberg FA, Correa IC. Molar
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18. Son SA, Roh HM, Hur B, Kwon YH, Park JK. The effect of resin thickness on
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Prentice Hall Press Upper Saddle River, NJ, USA 2010
212
Chapter 7
Findings, Analyses, Discussion, Limitations, and Future
Directions, and Conclusions
213
7.1 Findings
This thesis analyzed the efficacy of 113 LED LPUs used by general dental
practitioners in 100 private practices in the Greater Toronto Area, by conducting a
quantitative assessment of LPUs and polymerization skills of dental professionals, as
well as the characterization of the surface and bulk-properties of two RBCs
polymerized with same LED LPUs. Within the limitations of each of the four studies
that this project comprised of, the following conclusions were made:
1. The study characterized the LED LPUs in 100 participating dental practices
concerning their ability to generate at least 16 J/cm2 of radiant exposure in the 20s
computed exposure time, measured by the Check MARC®.
• The 20s calculated radiant exposure ranged between 3.8 and 59.6 J/cm2 (M=21.7,
SD=8.6).
• The pool of tested LED LPUs consisted of 32 multiwave and 81 monowave LPUs. In
the multiwave group, the Valo (Ultradent, South Jordan, UT) was the most common
(57.6%), while Demi Plus (Kerr, Orange, CA) was the most common (26.3%) in the
monowave group.
• The minimum criterion for radiant exposure of 16 J/cm2 in 20s was met for the
majority of LED LPUs (n=87, 77.0%), where 29 were multiwave LPUs and 58 were
monowave LPUs.
• There were 26 underperforming LPUs (3 multiwave and 23 monowave)
214
• By calculating the ratio of substandard LPUs to a total number of LPUs tested,
(multiwave LPUs (3/32) versus monowave LPUs (23/81)), monowave LPUs
exhibited three times higher possibility to fail the study-required standard of 16 J/cm2
per 20s than multiwave LPUs from the same sample.
• In comparison with a computed 20s, radiant exposure of the reference LED LPU
(Bluephase Style, Ivoclar-Vivadent), which generated 21 J/cm2, 47.8% of LED LPUs
tested, delivered lower radiant exposure values.
• The LED LPUs in the sample were between 1.0 and 5.5 years of age (M=2.6,
SD=1.2) and had damage levels ranging from 0 to 4.0 (M=1.6, SD=1.2).
• The average age of LED LPUs that did not pass the criterion was 3.6 years (SD=1),
while the average age of LED LPUs that passed the criterion was 2.3 years (SD=1).
• The average damage level of LED LPUs that did not pass the criterion was 2.8
(SD=1), compared with 1.3 (SD=1) for LED LPUs that passed the criterion.
• The average age of monowave LPUs was 2.7 years (SD = 1), and the average age of
multiwave LPUs was 2.6 years (SD = 1).
• Similarly, the two types of LPUs were not significantly different on their level of
damage. Specifically, 17.7% of monowave LPUs and 17.6% of multiwave LPUs had
no damage, and 6.3% of monowave and 5.9% of multiwave LPUs had the highest
level of damage (level 4).
• A strong, r=-0.67 (p<.001) negative correlation between the LPU radiant exposure
and its age and a level of damage, was observed.
215
2. The study evaluated the ability of dental practitioners to deliver at least eight J/cm2 of
radiant exposure, in a 10s exposure cycle to the MARC®PS simulated restorations
(before/after instructions), using their LED LPUs.
• Before instructions, 73.6% of practitioners delivered required-energy to anterior
restoration, and 32.2% of practitioners delivered required-energy to posterior
restoration.
• Following instructions, 94.3% of practitioners delivered required-energy to anterior
restoration, and 73.6% of practitioners delivered required-energy to posterior
restoration.
• Dental practitioners, on average, increased delivered energy to the anterior restoration
by 22.6% and the posterior restoration by 30.4% because of instructions.
• Following specific instructions, dental practitioners improved the consistency of
delivered energy to simulated restorations on average by 25.9%.
• The positive control group (graduate students) delivered the following quantities of
energy to the MARC® PS: 9.8 J/cm2 to the anterior restoration, 9.9 J/cm2 to the
posterior restoration before instructions, and 12.6 J/cm2 to the anterior restoration,
and 11.1 J/cm2 to the posterior restoration, post-instructional.
3. The study evaluated relative hardness ratios (RH) of Tetric EvoCeram and TPH
Spectra, polymerized at 1 mm and 4 mm tip-to-target distance with the participating
LPUs.
216
• The relative hardness ratios of the two RBC brands demonstrated a weak-to-moderate
correlation (average r=0.36, p<.001) between the light probe tip-to-target distance,
and a 20s radiant exposure.
• The majority of LED LPUs reached the 0.80 RH level of two RBC brands at 1 mm
tip-to-target distance.
• The majority of LED LPUs did not obtain the 0.80 RH level of two RBC materials at
4 mm tip-to-target distance.
• The percentage of LPUs that exhibited insufficient polymerization at 4 mm tip-to-
target distance was observed to be higher than 60%.
• The average RH ratio reduction over a 3 mm tip-to-target distance, was 59% for
Tetric EvoCeram, and 54.8% for TPH Spectra, respectively.
• In comparison with RH ratio for Tetric EvoCeram obtained by the reference LPU
(0.85 at 1 mm and 0.80 at 4 mm material-to-probe distance), 108 participating LPUs
(95.6%) were below that RH level at 1 mm material-to-probe distance, and 81 LPUs
(71.7%) did not produce that RH level at 4 mm material-to-probe distance
• Similarly, in comparison with RH ratios for Spectra TPH obtained by a reference
LPU (0.84 at 1 mm and 0.83 at 4 mm material-to-probe distance), 97 LPUs (85.8%)
were below that RH level at 1 mm material-to-probe distance, while 101 LPUs
(89.4%) did not yield that RH level at 4 mm material-to-probe distance.
4. The study examined the Diametral Tensile Strength (DTS) of Tetric EvoCeram, and
TPH Spectra RBCs polymerized with the participating LPUs.
217
• The average DTS values of Tetric EvoCeram and TPH Spectra, polymerized with
LED LPUs in the participating dental practices did not show any significant
difference.
• The mean DTS value observed in the sample of Tetric EvoCeram was 39.7 MPa
(SD=5), and for TPH Spectra was 39.9 MPa (SD=4).
• A positive, significant, but small-to-moderate correlation between LED LPUs (n=87)
capable of producing at least of 16 J/cm2 radiant exposure in 20s exposure, and the
average DTS values, produced during a 40s radiant exposure of Tetric EvoCeram
(r=0.37, p<.001), and TPH Spectra (r=0.31, p<.001), were observed.
• A moderately strong correlation between a 40s radiant exposure delivered by LED
LPUs (n=113), and the average DTS values of Tetric EvoCeram (r=0.54, p<.001),
and TPH Spectra (r=0.53, p<.001), were noted.
• The average DTS levels were highly correlated between the two RBC materials
(r=0.71, p<.001), indicating that higher or lower DTS values were observed on both
materials depending on the radiant exposure applied by the participating offices’
LPU.
• No significant differences were observed in the average DTS values of Tetric
EvoCeram (p=0.943), and TPH Spectra (p=0.289), polymerized with multiwave
(n=32) and monowave (n=81) LPUs.
• The average DTS values obtained with a control LED LPU were 42.9 MPa for Tetric
EvoCeram, and 43.9 MPa for TPH Spectra.
• 77.9% LPUs tested produced lower average DTS values with Tetric EvoCeram, in
comparison to the average DTS values of the control LPU.
218
• 87.6% LPUs tested had lower average DTS values with TPH Spectra, in comparison
to average DTS values of the control LPU.
7.2 Analysis
Numerous studies reported that LPUs used in private practices exhibited a substantial
number of clinically unacceptable outputs. The percentage of clinically underperforming
LPUs reported in the literature ranged from 30.3% to 55.5%, depending on the study specific
testing criteria 308-316. Although all of these studies indicated that LPUs employed in dental
practices included a relatively large number of LPUs that produce insufficient irradiance
levels, it was evident that LED LPUs have shown much better results than QTH LPUs.
Although geographic variations, different methodologies, and differences in study
designs presented challenges in making meaningful comparisons, the analysis of a similar
2005 Toronto study would shed some new light on the effectiveness of LPUs in the same
geographic region. Many changes happened in private dental practice, since then the
principal change was the disappearance of QTH LPUs and their replacement with LED
LPUs. The following are the highlights:
• The sample size in the 2005 study involved a total of 214 QTH LPUs. The present
study consisted of only 113 LED LPUs.
219
• There was a tendency of single dental offices becoming aggregated into a group
dental practice. Thus, the current study included 100 dental practices as the 2005
study did.
• The mean reported irradiance in the 2005 study was 526 mW/cm2. The current study
reported 1,082 mW/cm2, which indicated that the irradiance values almost doubled in
the past ten years. The 2005 study reported that LPUs' mean age was 5.6 years, while
the current study reported just being 2.6 years.
• In the 2005 study, an analog radiometer with a range of 0 to 1,000 mW/cm2 (Optilux,
model 100, Kerr, Orange, CA) was used. The current study used a more sophisticated
the CheckMARC®, a laboratory-grade spectroradiometer with a range of 0 to 10,000
mW/cm2, which has a scientific-grade accuracy (an accuracy of +/- 5%).
• In the 2005 study, an incidence irradiance level of 400 mW/cm2 and above was
considered adequate, while the current study promotes that the incident irradiance
level of 800 mW/cm2 or above as appropriate.
• Based on the research criterion from the 2005 study, a total of 30.3% of the LPUs
generated less than the sufficient irradiance values, while the current study exhibited
23% of LPUs that were below the study-required criterion.
• The present study adequately evaluated the optical diameter of the light probe, by
measuring an outer tip diameter using Mitutoyo digital caliper (Mitutoyo Canada,
Mississauga ON) and inputting those values to the CheckMARC® software, which
automatically selected the appropriate active diameter values for the corresponding
incident irradiance calculations.
220
• Evaluation of the level of damage observed on the surface of the light-probe was
completed by using an evaluation scale (0-8), where 0 represents no damage, and 8
represents 100 percent of the damaged light probe.
• The 2005 study used an analog radiometer. However, there are a plethora of reports
associating dental radiometers with erroneous measurements and questionable results
281.
Comparison of raw data presented in both studies, ten years apart, showed that LED LPUs,
compared with QTH LPUs in the same demographics area, demonstrated, on average,
significantly better performance. This improvement could be credited to advances and
significant innovations with LED light sources.
7.3 Discussion
The study identified specific problems associated LPUs among the dental
practitioners in private dental practices in the Greater Toronto Area. The first issue was
associated with the ability of LED LPUs to generate the study-required energy minimum.
The age and level of light probe damage had a significant effect on the required quantities of
energy being produced. A regular test for damage and debris on the light probe should be
part of daily practice routine. Furthermore, a regular incident irradiance test should be
conducted using certified measuring equipment. LED LPUs with a service record of four
years or more should be tested more frequently, as they exhibited the highest potential for
failure.
221
The second issue that this study focused on was the ability of dental practitioners to
deliver adequate quantities of light energy to simulated restorations. To achieve this goal, the
MARC® PS was employed in the participating dental practices. The participants were
assessed for their clinical skills to deliver at least 8 J/cm2 radiant exposure in 10s to simulated
anterior and posterior restorations. The test was conducted in their regular office settings,
which included their choice of eye protection, if used, as a part of their typical daily routine.
Following customized instructions on proper polymerization, participants were retested using
provided blue-light protective glasses. The study revealed that a significant number of dental
practitioners exhibited inadequate polymerization skills, especially when they delivered light
energy to the posterior restoration. It was found that a substantial number of study
participants did not use appropriate eye protection. The post instructional results proved that
the use of protective glasses, along with individualized instructions had substantial effects on
improved photopolymerization technique among dental practitioners tested. It was observed
that a low level of awareness about the importance of photopolymerization among dental
practitioners had a critical effect on their ability to deliver sufficient energy to restorations.
7.4 Limitations and Future Directions
Bias can occur in any phase of a research project, from planning, data collection, and
analysis to the publication phase. The way in which subjects were identified, approached, and
respond to invitations to participate in research can be a significant source of bias, and,
therefore, it is essential for researchers to evaluate selection bias 323. Upon approval by the
University of Toronto Research Ethics Board, the participating private dental practices were
recruited. The 2012 listing of dentists published by the Royal College of Dental Surgeons of
222
Ontario was used to detect prospective dental practices in the Greater Toronto Area. Over
350 targeted practices were initially contacted by a single test administrator via telephone
using the same telephone script approved by the University of Toronto Research Ethics
Board, to determine their eligibility to join the study. However, a total of 250 suitable dental
practices were presented with the research protocol, along with a letter of consent. The
recruitment goal was to secure at least of 100 dental practices for the study. The recruitment
criteria for the study participants included: a general dental practice with proficiency in RBC
placement and physically located within the Greater Toronto Area. The study acceptance was
granted exclusively for participants who have never experienced MARC®PS before. To
proportionally include dental practices in the Greater Toronto Area, the target area of
Toronto was divided into four quadrants with a similar number of dental practices recruited
within each quadrant.
This study may had some inclusion issues to some extent because those dental
practices with possible lower quality of LPUs or questionable proficiency in RBC placements
may have refused to participate in order to avoid embarrassment. The main excuse for those
practices to decline participation was their inability to find time to accommodate the
research. The study exhibited standardized protocols for data collection and data entry. A
single administrator completed the entire study, which substantially reduced any possible
inter-observer variability and performance bias while, at the same time, enabling the test
administrator to evaluate internal validity during the entire research process.
Another potential limitation of this study was a possible conceptual problem with the
10s radiant exposure with the MARC® PS device. If the passing criterion of 16 J/cm2 with a
20s radiant exposure was performed, the results may have been slightly but not significantly
223
different. Furthermore, the 16 J/cm2 radiant exposure per 20s represented the typical value
that was more frequently cited in the literature 16, 94, 188.
The specimens (6 mm diameter and 4 mm height) for DTS test were prepared based
on preceding studies protocols 177, 181, 183, 187, 189, not the ADA/ANSI Specification No. 27 (6
mm diameter and 3 mm height). Although the study results were in line with that of
published findings, by using the ADA/ANSI Specification No. 27, the outcome may have
been more up-to-date, however, the comparative nature of this work places less value on this
aspect.
Future research directives should include the examination of bulk-fill materials
(packable and flowable) using the methodologies outlined in the current study, except for
DTS specimens’ specification, which would include ADA/ANSI Specification No. 27 (6 mm
diameter and 3 mm height). Furthermore, it would be useful to change research population,
thus to see how rural dental practitioners differ from the urban ones and possibly to include
assessment of Dental Public Health professionals in the future projects.
7.5 Conclusions
The findings from the study suggested that improvements in polymerization skills are
required and justified in a large number of participating dental practices. The results showed
that some dental professionals have been delivering insufficient quantities of light energy to
the simulated posterior restorations, which would have a detrimental effect on posterior RBC
polymerization and long-term longevity in clinical settings. Furthermore, clinicians should be
aware that their LPUs may not perform as expected, unless a rigorous maintenance protocol
is implemented, including close inspection of their LPU's light probe and efficient
224
measurement of their LPU's incident irradiance. LED LPUs with a service record of four
years or more, should be tested more frequently as they exhibit the highest potential for
failure.
225
Appendices
226
Appendix I
227
Table 1. Studies on survival of RBC restorations
Principal
author, year
Evaluation
period/study
design
Number of
restorations
/studies
Evaluated
restorations
Survival rate/
Annual Failure
Rate (AFR)
Factors
associated with
RBC failure
Manhart 298,
2004
2-14 years,
18 studies
Class I and II,
amalgams, RBCs,
GICs,
compomers
AFR: amalgams
3.0%
RBCs 2.2%,
GICs 7.2%,
copomers 1.1%
secondary caries,
fracture, marginal
deficiencies, wear
and postoperative
sensitivity
Demarco 300,
2012
5-22 years,
retrospective,
longitudinal
34 studies
Class I and Class II
posterior RBCs
AFRs of
vital teeth
1% to 3%.
AFRs of non-vital
teeth 2% to 12.4%.
tooth type and
location, operator,
socioeconomic,
demographic, and
behavioral
elements
Opdam 301,
2014
6-22 years,
prospective,
retrospective,
longitudinal
25 studies
2,816
restorations
Class I and Class II
RBCs
high/medium caries
risk, with 10-year
AFR of 4.6/4.1%
compared with
1.6% for low-risk
patient
Secondary caries
and tooth fracture
Lempel 302,
2014
10-year
retrospective
study
701
restorations
4 microhybrid RBCs
of Class II
10-year survival
rate of >97.8%.
Number of
surfaces, molars
vs premolars
Pallesen 303,
2015
27-year
prospective
study
99
restorations
60 premolars
39 molar Class II of
3 RBCs
27-year survival
rate
56.5%
Secondary caries
(54.1%) Wear
(21.6%) Fracture
(18.9%)
228
Table 1. Studies on survival of RBC restorations (continued)
Rasines
Alcaraz 304,
2014
5-7 years
retrospective
studies
7 RCT of
3,265 RBCs
and
1935
amalgams
Class II RBC and
amalgam
Restorations
Adults and children
Amalgam vs RBC:
Failure rate
RR 1.86
Secondary caries
RR 2.14
Fracture RR 0.87
Secondary caries,
Fracture and
Restoration loss
Beck 305,
2015
1-17 years,
Prospective
RCTs
88 studies
Class I/II RBCs
premolars/molars
AFR:
1.46% short-term
studies,
1.97% long-term
studies
Fractures,
secondary caries
and marginal gap
Borgia 306,
2017
11,5 years
Retrospective
Longitudinal
study
61 patients,
105 RBC
Restorations
(101 vital
teeth, 4
endo-
treatment)
Class I/II RBC
premolars/molars
103 (98%)
restorations were in
function,
98 (95.1%)
Clinically
successful
2% failed
Statistically higher
mean survival
time of premolars.
Caries-risk factor
shown to be
crucial.
Palotie 307,
2017
13-year
Retrospective
Longitudinal
study
5,542
restorations
RBC (93%)
and
amalgam
(7%)
Class II
premolars/molars
Median survival
time of all
restorations 9.9
years.
No statistical
difference between
RBC/amalgam
restorations
Toot location in
the mouth,
Premolars molars
restoration size
229
Appendix II
230
Table 2. Effectiveness of LPUs in dental practices
Principal
author,
year,
country
Type
of
LPUs
Number
of LPUs
assessed
Methodology Passing
criterion
Conclusion
Dunne 308,
1996,
UK
QTH
49
Depth of
polymerization
(DOP), analog
“LampChecker”
radiometer
5.0 to 7.0 on
analog scale
<50% of LPUs tested
were substandard
Burke 309,
1997,
UK
QTH
98
Assessment of DOP
(Heliotest and Z100)
DOP 2.6 mm
43.5% of LPUs tested
were substandard
Martin 310,
1998,
Australia
QTH
214
Analog radiometer
Demetron
400 mW/cm2 47.7% of LPUs tested
were substandard
Pilo 311,
1999,
Israel
QTH
130
Analog radiometer
Demetron, Knoop
hardness (KH)
300 mW/cm2
55% of LPUs tested
were substandard;
46% of LPUs failed
when KH data were
applied
Solomon 312,
1999,
South
Africa
QTH
35
Analog radiometer
300 mW/cm2
45.7% of LPUs tested
were substandard
El-Mowafy 313, 2005,
Canada
QTH
214
Analog radiometer
Knoop hardness
400 mW/cm2
30.3% of LPUs tested
were substandard
231
Table 2. Effectiveness of LPUs in dental practices (continued)
Santos 314,
2005,
Brazil
QTH
120
Analog radiometer
300 mW/cm2
56% of LPUs tested
were substandard
Hedge 315,
2009,
India
QTH
LED
200
Digital radiometer
400 mW/cm2
88% of LPUs tested
were substandard
AlShaafi 316,
2012,
Saudi
Arabia
QTH
LED
140
Laboratory-grade
spectroradiometer
QTH: 300
mW/cm2,
LED: 600
mW/cm2
59.5% of QTH LPUs
and 31.4% of LED LPUs
were substandard
Maghaireh 317, 2013,
Jordan
QTH
LED
295
Digital radiometer
300 mW/cm2
73.8% of QTH LPUs
and 20.8 %of LED LPUs
were substandard
Sadiku 318,
2010,
Switzerland
QTH
LED
220
Integrating sphere
Not available
40% of LPUs tested
were substandard
Kassim 319,
2013,
Kenya
QTH
LED
PAC
83
Digital radiometer,
DOP, hardness
300 mW/cm2
48.2% of LPUs tested
were substandard
Hao 320,
2015,
China
QTH
LED
196
Digital radiometer
300 mW/cm2
42.4% of QTH LPUs
and 9.4% of LED LPUs
were substandard
Kopperud 17, 2017
Norway
LED
713
Self-reporting
questionnaire
Reported
irradiance
1,000 to 2,000
mW/cm2
78.3% of dentist
unaware of their LPUs
intensity,
32% of dentist without
protective glasses
232
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