Fracture Strength of Implant-Supported Full-Contoured Titanium and Zirconia Single Crowns Connected to

Titanium Cores

By

Hooman Mohandesan

A thesis submitted in conformity with the requirements for the degree of Masters of Science in Prosthodontics

School of Graduate Studies and Discipline of Prosthodontics Faculty of Dentistry

University of Toronto

© Copyright by Hooman Mohandesan (2016)

ii

Fracture strength of implant supported full-contoured titanium and zirconia single crowns connected to titanium cores

Hooman Mohandesan, DMD

Masters of Science in Prosthodontics

School of Graduate Studies and Discipline of Prosthodontics

Faculty of Dentistry

University of Toronto

2016

Abstract

Objectives: To investigate the fracture strength and failure mode of implant-supported

screw-retained customized 2-piece zirconia and 1-piece titanium restorations, and to

evaluate the effect of aging on the mechanical performance of zirconia restorations.

Materials and Methods: Thirty identical specimens simulating maxillary first premolar

replacements were divided in 3 groups. Groups ZrA and ZrNA consisted of zirconia and

Group Ti consisted of titanium restorations, anchored to implants embedded in PMMA.

Specimens in Group ZrA underwent chewing simulation. Static load was applied until

failure.

Results: Group Ti showed the highest fracture strength. The difference was significant

between Groups ZrA and Ti. Failures included partial or complete fracture of titanium

insert or deformation of restoration with screw fracture.

Conclusion: Evidence was inadequate to reject similarity in fracture strength between

non-aged zirconia and titanium restorations, or between zirconia restorations with

different aging conditions. Aging affected the failure mode of zirconia restorations.

iii

Acknowledgements

I would like to sincerely thank Dr. Grace De Souza for kindly accepting to be my

Supervisor on the research topic of my interest and for all her patience,

constructive guidance and practical assistance throughout the course of the

research project.

I would also like to express my gratitude to Dr. Laura Tam (Co-supervisor and

Committee Member) and Dr. Babak Shokati (Committee Member) for their kind

contribution and support. Their knowledge and expertise was instrumental in the

fulfillment of this project.

I would like to acknowledge the generous provision of the implant fixtures and

prosthetic components by Straumann North America, with special thanks to Ms.

Jade Lee Choon, Mr. Matthew Reynolds and Mr. Dino Vlahavas (Straumann

Canada) for their endless efforts in this regard.

I would further like to thank LHM Dental Studios, Toronto, especially Mr. Slawek

Bilko (President), Mr. Jim Agoritsas (Manager), Mr. Evan Katz and Ms. Sarah

Leandro for letting me use their facility and for their kind assistance in designing

and preparing the specimens.

Lastly, I want to thank my life-long mentor, my devoted mother, Shirin

Bozorgmehr, whose wisdom, unconditional support and life-long guidance have

played an important role in the achievements through my life journey and also

my responsible and caring father, Mehrdad Mohandesan, for his endless support

that has made my educational goals come true.

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Table of Contents Abstract ii Acknowledgements ii List of Tables v List of Figures vi Chapter 1. Introduction 1 Chapter 2. Literature Review 2

Type of fixation of restoration to the implant 2 Abutment material 4 Types of commercially available zirconia restorations 7 Configuration of implant-abutment connection 8 One-piece zirconia restorations 9 Two-piece zirconia restorations with intermediary titanium insert 11

Chapter 3. Rationale and Objectives 16 Rationale 16 Objectives 17 Hypotheses 17

Chapter 4. Materials and Methods 18 Materials 18 Experimental groups 18 Fabrication of the CAD/CAM restorations 22 Preparation of the restorations and attachment to fixtures 22 Artificial aging 24 Fracture strength test 25 Visual evaluation of the specimens 26 Scanning Electron Microscopy assessment 26 Statistical analysis 26

Chapter 5. Results 27 Outcome of chewing simulation 27 Fracture strength 27 Mode of failure 28

Chapter 6. Discussion 32 Chapter 7. Conclusions and Clinical Significance 43

Conclusion 43 Clinical Significance 43

Chapter 8. Future Research Directions 44 References 45 Appendices 54

Appendix A: Fracture strength values and failure mode of the specimens 54 Appendix B: Representative stress-strain curves for each test group 57

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List of Tables

Table 1. Characteristics of materials used for the study 19

Table 2. Mean fracture strength and SD for each experimental group 27

Table 3. One-way ANOVA for fracture strength 28

Table 4. Tukey HSD post-hoc test for fracture strength 28

Table 5. Chi-Square tests for modes of failure of zirconia test groups 29

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List of Figures

Figure 1. Components used for preparation of the specimens 19

Figure 2. Preparation of acrylic resin block and embedding of

the implant fixture 21

Figure 3. Preparation of 2-piece zirconia restoration component

and attachment to fixture 23

Figure 4. Artificial aging of 2-piece zirconia restorations in the

chewing simulator 24

Figure 5. Fracture strength test under static load 25

Figure 6. Mean fracture strength for experimental groups 27

Figure 7. Complete fracture of titanium insert observed in aged

zirconia restorations 30

Figure 8. Partial fracture of titanium insert observed in aged and

non-aged zirconia restorations 31

Figure 9. One-piece titanium restoration after fracture strength test 31

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Chapter 1. Introduction

Use of implants for single tooth restorations was first reported by Jemt in 1986

and has currently become a standard treatment modality with high survival rates

and predictability (Jung et al. 2008, 2012; Pjetursson et al. 2004, 2007, 2012).

Besides various biological and health outcomes such as patient comfort,

enhanced masticatory function, stable physiologic occlusion, sustained osseo-

integration, level of alveolar bone support and health of peri-implant soft tissues,

the success of implant-based treatments also depends on surrogate clinical

outcomes such as mechanical stability of the prosthetic components, rate of

technical complications and satisfactory esthetics (Bidra and Rungruanganunt

2013).

The association of mechanical stability and satisfactory esthetics is particularly

important for reconstructions in the areas of the mouth where esthetics becomes

a critical factor in overall success of the treatment and patient satisfaction.

Selection of the implant-supported restoration in the esthetic zone depends on

various factors such as patient’s expectations, position of the smile line, gingival

biotype, position and angulation of the fixture, available restorative space,

preferred type of retention of the restoration to the fixture and treatment

expenses (Bidra and Rungruanganunt 2013). The selection of the most suitable

implant-based replacement for a single missing tooth that can be satisfactory for

the esthetically demanding patient along with long-term mechanical resistance to

heavy occlusal forces is a controversial issue in the dental literature and among

the restorative dentists.

The aim of the present in vitro study is to evaluate and compare the mechanical

performance of 2 types of implant-supported screw-retained customized full-

contoured restorations, and to evaluate the effect of simulated oral conditions on

full-contoured 2-piece zirconia restorations.

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Chapter 2. Literature Review

Different types of implant-supported restorations for replacement of a single

missing tooth have been described in the literature. These restorations can be

categorized based on type of fixation to the supporting fixture, material, method

of fabrication and the configuration of the connection with the restorative platform

of the implant.

Type of fixation of restoration to the implant For implant-supported single crowns, the connection of the final restoration to the

implant can be cement-retained or screw-retained (Hebel and Gajjar 1997). In

cement-retained prostheses, the abutment (mesostructure) is fabricated

separately and the restoration is cemented to the abutment (two-piece). This

technique resembles the clinical and laboratory procedures for tooth-supported

restorations. Multiple factors affect the strength of cement retention between the

abutment and the super-structure, including the abutment degree of taper,

surface area and roughness and type of cement (Hebel and Gajjar 1997).

With screw-retained prostheses, the restoration might be separate from the

abutment (two-piece) or combined as part of the fabrication procedure and

directly screwed to the implant (one-piece). An important stabilizing factor for

screw-retained systems is the “preload” or the tensile force developed on the

connecting screw thread by the tightening torque, which creates compressive

forces at the interface of the restoration and the implant and helps hold the

components together (Dittmer et al. 2012). Screw joint stability is maintained by

adequate preloading of the screw, precise fit between the implant platform and

the restoration, effective anti-rotational interface and adequate friction between

the screw threads and the implant (Dhingra et al. 2013).

Both retention methods are associated with a number of advantages and

limitations (Wittneben et al. 2014). The main advantage of screw-retained

implant reconstructions is predictable retrievability, which facilitates the removal

of the restoration with lower risk of damage to the restoration or the fixture.

3

Removal of the supra structures is sometimes necessary for periodontal

maintenance, for management of mechanical complications such as tightening of

loosened screws, repair or modification of the prosthesis and replacement of

prosthetic components or when surgical interventions are required for

management of biologic complications (Michalakis et al. 2003). Other important

advantages of screw-retention include applicability in areas with limited inter-arch

space and elimination of the cement from the restoration assembly (Sailer et al.

2012). However, screw-retained reconstructions require precise placement of the

fixture, due to the position of the screw access channel. Compromised structural

durability and difficulty in achieving proper occlusion are other concerns related

to screw-retained restorations. Screw channel access might occupy 50% - 75%

of the occlusal surface of the premolar and molar teeth, compromising the

integrity of the framework and the veneering layers, as well as interfering with

centric and excursive occlusal contacts (Hebel and Gajjar 1997; Wittneben et al.

2014). The conventional manufacturing process of screw-retained restorations is

more technique-sensitive than cemented reconstructions (Michalakis et al.

2003).

The advantages of cement-retained restorations are the possibility of

compensation for discrepancies in implant position and angulation, passivity of

fit, easier seating and chair-side adjustments, higher esthetics, and easier control

of occlusion (Hebel and Gajjar 1997; Michalakis et al. 2003). Major

disadvantages of cement-retained prostheses are difficulty in retrieving the

restoration, need for more inter-arch space and limitations in proper removal of

excess cement (Linkevicius et al. 2013). A prospective clinical study on the

relationship between remaining cement and peri-implant disease reported that

excessive dental cement was associated with the development of peri-implant

soft and hard tissue diseases and bone loss in 81% of the cases (Wilson 2009).

A recent comprehensive systematic review of 73 studies with 5858 different fixed

implant reconstructions including single crowns (1720), partial prostheses (979),

full arch prostheses (928) and cantilever partial prostheses (61), different

retention systems including one-piece screw-retained (59%) and cement-

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retained (41%), and different materials, with follow-up of at least 3 years reported

estimations of 5- and 10-year survival rates as 95.5% and 91.3% for screw-

retained and 96.0% and 92.2% for cement-retained restorations respectively.

None of the reconstruction groups showed statistically significant differences in

the estimated survival rates, based on the type of retention. The review reported

a significantly higher total event rate of biological and technical complications

associated to cement-retained implant-based fixed restorations. Due to easier

retrievability for management of technical and biologic complications, screw-

retained restorations were recommended (Wittneben et al. 2014).

Abutment material Commercially pure titanium has been traditionally used as the material of choice

for fabrication of the implant abutment or the restoration framework for

replacement of the single tooth, because of its well-proven biocompatibility (Adell

et al. 1981; Buser et al. 1997; Lindhe and Berglundh 1998; Linkevicius et al.

2010) and mechanical properties (Pjetursson et al. 2007). Historically, the

prosthetic components for single implant restorations were derived from the

previous designs for rehabilitation of completely edentulous patients. These

restorations were initially made of machined titanium components veneered with

acrylic resin (Jemt 1986). This design then evolved to a restoration consisting of

a prefabricated titanium abutment supporting a cemented metal-ceramic crown

(Bidra and Rungruanganunt 2013). This was followed by the introduction of

UCLA abutments, in which for the first time a customized cast metal restoration

could be directly screwed to the dental implant (Lewis et al. 1988). UCLA

abutments are still used for screw- and cement-retained implant-supported

prostheses.

A systematic review reported a few complications associated with metal

abutments supporting fixed implant-based prostheses; with the most frequently

occurring problem being the loosened abutment screw (Pjetursson et al. 2007).

A major complication of titanium-based restorations is the unnatural blue or gray

discoloration at the peri-implant soft tissues (Tan and Dunne 2004). This

5

phenomenon might compromise the clinical outcome of the implant-borne

reconstruction in the esthetic zone, particularly in patients with high smile line,

thin (less than 2mm) gingival biotype, when the fixture is placed in close

proximity to the labial cortical plate or superficially in the residual alveolar bone

and also following peri-implant bone loss and soft tissue recession (Sailer et al.

2007a, Sailer et al. 2007b; Jung et al. 2008; van Brakel et al. 2011).

Numerous strategies have been developed in order to overcome the esthetic

complications related to the dark-hued titanium supra-structures, including the

use of cast gold alloys and gold-colored titanium nitride-coated abutments (Sailer

et al. 2009b). These materials might improve the gingival discoloration to some

extent, but the overall translucency of the restoration may remain unsatisfactory

due to the opaque nature of the metal (Bressan et al. 2011).

In order to achieve optimal esthetics in the anterior and premolar regions, all-

ceramic abutments made of alumina or zirconia were introduced. The use of

alumina implant abutments was first reported by Prestipino and Ingber in 1993.

These abutments, made of densely sintered highly purified 99.5% aluminum-

oxide ceramic cores, provided improved esthetics but showed relatively low

fracture resistance of about 241N (Att et al. 2006). Fracture of alumina

abutments was also reported in a number of clinical studies (Andersson et al.

2003; Henriksson and Jemt 2003). The weaknesses of the alumina restorations

prompted the development of zirconia implant abutments.

Pure zirconia is a polymorphic crystal that can be found in 3 different phases:

monoclinic (room temperature – 1170°C), tetragonal (1170° – 2370°C) and cubic

(2370° – melting point) (Piconi and Maccauro 1999). “Stabilizers” such as yttrium

are added to the composition of zirconia to minimize the phase transformation

and to retain the crystals in the more stable tetragonal phase with enhanced

physical and mechanical properties, at room temperature (Zembic et al. 2013).

Wohlwend et al. introduced the first 3-yttria-stabilized tetragonal zirconia

polycrystalline (Y-TZP) as an all-zirconia abutment in 1997. Glauser et al. first

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described the densely sintered (Y-TZP) as an implant abutment, in 2004.

Zirconia is a biocompatible material with high mucosal attachment

(Abrahamsson et al. 1998) and lower rates of bacterial adhesion, compared to

titanium (Scarano et al. 2004). This material has shown high flexure strength of

900 to 1200 MPa, high fracture toughness (resistance to crack propagation or

the tensile stress that must be achieved in a crack tip before fracture is initiated)

of 9 to 10 MPa/m2 (although lower than 45 MPa/m2 reported for pure titanium),

low thermal conductivity and low corrosion potential (Kim et al. 1997, Andersson

et al. 2001, Yildirim et al. 2003, Adatia et al. 2009; Zembic et al. 2013, Delben et

al. 2014). Zirconia is also radiopaque and presents superior optical properties

over titanium (Brodbeck 2003; Tan and Dunne 2004). Depending on its

thickness, zirconia exhibits different degrees of translucency and therefore

enables the fabrication of an esthetic restoration. A systematic review of clinical

studies showed that based on spectrophotometric analyses, peri-implant

mucosal discoloration is significantly lower for zirconia abutments compared to

titanium or cast metal abutments (Bidra and Rungruanganunt 2013).

Several weaknesses have been mentioned for zirconia-based restorations. In

contrast to ductile metals with enhanced tolerance to both compressive and

tensile forces, zirconia is brittle and vulnerable to bending and subcritical crack

growth (Guazzato et al. 2004), especially when prepared in thin sections (less

than 0.5 mm) or with sharp edges (Aboushelib and Salameh 2009). The

configuration of the implant-restoration connection, abutment thickness at the

implant interface, and angulation of the loading forces might also have a

substantial influence on the durability and stability of the all-zirconia abutments

(Albosefi et al. 2014). Another shortcoming of zirconia is its inherent accelerated

aging (Zembic et al. 2013), in which a spontaneous slow transformation of the

tetragonal to the monoclinic phase occurs in the moist oral environment with

changing temperatures and constant load that may lead to decreased strength of

the material and increased risk of catastrophic failures over time (Kim et al.

2010). Aggressive adjustments or improper laboratory and clinical handling might

7

exacerbate this aging process (Luthardt et al. 2004).

Types of commercially available zirconia restorations Zirconia abutments are currently available as standard prefabricated or

individually customized components, using computer-aided design/computer

aided manufacturing (CAD/CAM) technology (Conrad et al. 2007). The

standardized shape and diameter of prefabricated abutments may be a limiting

factor in proper positioning of the future crown margin and also in establishing an

appropriate emergence profile. The need for substantial modifications of these

abutments prior to insertion might increase the risk of micro crack formation,

accelerated phase transformation and possible catastrophic mechanical failures

(Canullo et al. 2013). The CAD/CAM systems enable the fabrication of an

individualized component with the desired diameter, contour, emergence profile

and relationship with the adjacent soft and hard tissues that mimic the properties

of the lost natural dentition (Abbo et al. 2008). Customized zirconia restoration is

initially milled in its soft (green) state to the final shape and then sintered with

minimal need for further adjustments. The custom-made abutment can also be

colored before stabilization for enhanced esthetic results (Conrad et al. 2007).

Several clinical studies have demonstrated the longevity of zirconia abutments in

implant-supported single restorations for anterior and premolar regions. One of

these was a combined prospective and retrospective study investigating the

clinical outcomes for 185 implant-supported custom-made single-tooth zirconia

abutments for up to 5 years. Various biological, mechanical and esthetic

parameters as well as subjective patient factors were considered. The results of

this study demonstrated satisfactory performance of zirconia abutments over the

follow-up period with relatively low biological and technical complications and

high patient satisfaction rates. Biological complications were reported for 36% of

the implants. Regarding the mechanical complications, only 2 abutments (1%)

fractured and were replaced, 3 crowns exhibited non-significant chipping of the

veneering porcelain and 1 abutment experienced screw loosening. The authors

concluded that zirconia abutments for single implant crowns demonstrate good

8

short-term biological and mechanical outcomes (Ekfeldt et al. 2011). A

randomized controlled clinical trial assessed the survival rates of CAD/CAM

zirconia abutments (n=18) attached to external hexagon regular-platform

implants and compared it with titanium abutments (n=10) supporting cemented

all-ceramic and metal-ceramic crowns for 18 patients with single-tooth gaps in

canine and posterior areas of both arches, after minimum 5 years of clinical

service, using predetermined technical and biological parameters. The zirconia

abutments were used for implant-supported replacement of missing canines

(n=2), premolars (n=11) and molars (n=5). Based on the results of this study, the

survival of restorations for both groups was 100% with no fracture of abutment or

the reconstruction, no loosening of the abutment screw or loss of retention of the

crown. Minor chipping of the veneering ceramic was reported for metal-ceramic

crowns on titanium abutments (n=3) that were repaired intra-orally. No major

biological complication was reported. More plaque was detected on titanium

reconstructions. A slight increase in peri-implant bone levels was observed for

zirconia group, whereas bone levels remained unchanged for the titanium

restorations. None of these differences were significant (Zembic et al. 2013).

Configuration of implant-abutment connection Different types and geometries of connection exist between the restoration and

the currently available implants. These connections can be divided into 2 major

groups, external connection in which the connective part of the abutment is

attached to an extension of the implant body and internal connection where the

abutment is attached into the implant body (Truninger et al. 2012).

For single implant restorations with external connections, the short index and

higher center of rotation may lead to lower resistance against rotational and

lateral movements (Coppede et al. 2009). This may cause loss of implant-

restoration joint stability and loosening or fracture of the screw due to micro

movement of the assembly, possibly leading to critical biological and mechanical

complications (Tsuge and Higawara 2009). For internal connection systems, the

restoration is closely connected and stabilized to the implant via a preloaded

9

screw and the friction locking effect at the longer junction of the abutment and

the restorative platform of the implant (Chun et al. 2015). In order to investigate

the effect of connection on mechanical performance of the implant-restoration

assembly an in vitro study evaluated the resistance to bending forces of metal

abutments attached to 7 implant systems (n=10) with various connections and

joint depths (0.6 – 6.0 mm), under static load at 90° angle. Systems with external

connection and shallow joints showed significantly less resistance compared to

internally connected systems with deep joining walls (Mollersten et al. 1997).

Internal connections provide better biological sealing, joint stability, stress

distribution and resistance to bending forces, because of deeper joining walls,

increased surface contact area, more intimate contact with the implant platform

and overall enhanced anti-rotational effect (Delben 2014).

Zirconia restorations are compatible with both external and internal connections.

Internal connection of zirconia abutments can be established by the uniform

zirconia component (one-piece system) or by a secondary element made of

titanium alloy that engages with the restorative platform of the implant on one

side and with the zirconia abutment/restoration on the other side (two-piece

system) (Sailer et al. 2007a).

One-piece zirconia restorations There are certain weaknesses associated with the 1-piece zirconia abutments.

Firstly, these abutments cannot be machined to the same degree of precision as

metal abutments and therefore their fit with the restorative platform of the implant

would be less intimate (Ebert et al. 2007). This matter was tested by an in vitro

study that measured the rotational misfit between the abutment and implant at

their junction for 5 abutment systems (2 zirconia, 2 alumina, 1 titanium, n=10)

connected to 2 different implant systems (Branemark Nobel Biocare and Biomet

3i), using scanning electron microscopy and a precision optical encoder. 1-piece

all-zirconia abutments showed significantly higher values of abutment-implant

linear gap and rotational misfit compared to 2-piece systems. The titanium

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abutment group showed the lowest values among all groups. Scanning electron

microscopy (SEM) assessment revealed well-defined corners for metal

abutments, but less definite angles and rounded or flattened corners for the

zirconia abutments (Garine et al. 2007).

Secondly, with 1-piece systems there is a higher risk of damage to the implant

platform as zirconia is a harder material compared to titanium and when in direct

contact, it may cause wear of the titanium implant interface during function and

micro movement. Comparing the wear of the implant interface with CAD/CAM 1-

piece zirconia and titanium abutments (n=3) after cyclic loading (1,200,000

cycles, 100 N), an in vitro study using SEM and computer tomography (CT)

indicated that the titanium interface of the implants experienced significantly

higher wear in the 1-piece zirconia group (10.2 ± 1.5 µm) than in the titanium

group (0.7 ± 0.3 µm). For the 1-piece group, SEM revealed various modes of

deformation such as scratches, furrows and vertical undercuts on the interface of

titanium implant, due to micro rotational movement of the zirconia abutment.

Only minimal wear was observed for the titanium abutments (Stimmelmayr et al.

2012). Extensive damage to the anti-rotational features of the interface might

compromise the restorability of a well-integrated implant (Brodbeck 2003).

Brittleness and higher risk of fracture of thin zirconia at the interface with the

platform of the implant might be another limitation of 1-piece abutments. With the

aim of further investigation of this assumption, a laboratory study compared the

fracture strength of CAD/CAM 1-piece zirconia abutments with 2-piece

abutments, connected to 32 implants with 2 different diameters for restoration of

the missing maxillary right first (3.75 mm) and second premolar (5.5 mm). The

samples were divided in 4 groups based on the implant diameter and type of

abutment (n=8). All the samples were artificially aged under simultaneous

thermal cycling (1000 cycles, 5° – 55°C, 30s dwelling time) and chewing

simulation (100,000 cycles, 120 N) with 30° off-axis angle, followed by the

application of static fracture loads. No failure was recorded during the aging

process. The median fracture resistance of the 2-piece abutments (1241 ± 269 N

11

for 3.75 mm and 2225 ± 63 N for 5.5 mm) was significantly higher than 1-piece

abutments (526 ± 32 N for 3.75 mm and 1894 ± 137 N for 5.5 mm), with

significantly higher fracture loads reported for larger diameter abutments. The

mode of failure for the 1-piece system was the fracture of the zirconia abutment

at the connecting internal hexagonal walls (area of the lowest thickness of

zirconia), whereas the mode of failure for the 2-piece system was the abutment

screw fracture (Stimmelmayr et al. 2013).

In order to investigate the influence of abutment material on the joint surface a

laboratory study utilized SEM to compare the surface changes of the implant-

abutment joint for CAD/CAM 1-piece zirconia and titanium abutments (Atlantis,

Astra Tech) with internal hexagonal implant connection (Biohorizons, 3.8 mm)

supporting a bonded restoration of a single missing maxillary central incisor,

following cyclic loading (2 – 200 N, 1,000,000 cycles). Both groups showed

degrees of abutment screw torque loss after the cyclic loading with significantly

greater loss for the zirconia group. This group also experienced pronounced

changes at the abutment-implant interface after the loading procedure compared

to the titanium group, including rounded hexes as a result of excessive abrasion,

scratched and gouged contact surfaces, significant accumulation of debris

(consisting of titanium, vanadium, aluminum), embedded metal alloy particles in

zirconia, and presence of cracks on the abutment surface. The authors

concluded that fitting inaccuracies might cause a greater flat-to-flat rotation at the

implant-restoration interface for 1-piece zirconia abutments, leading to screw

loosening and excessive wear at the joint (Dhingra et al. 2013).

Two-piece zirconia restorations with intermediary titanium insert In order to prevent the complications related to the 1-piece abutment systems,

the titanium insert (coping) was introduced as a connecting element between the

zirconia abutment and the titanium implant platform to establish a stable metal-

to-metal connection that is well maintained by the preloaded abutment screw

(Brodbeck 2003). The potential mechanical advantage of an internal 2-piece

implant-abutment connection for zirconia restorations has been demonstrated in

12

a number of studies. One of these studies measured the fracture load of 4

different types of CAD/CAM zirconia abutments with different connections

(external hexagon, 1-piece internal octagon, 2-piece internal octagon and 2-

piece internal tri-channel), following the application of 30° off-axis static load.

Significantly higher fracture load values were reported for 2-piece systems (tri-

channel: 724.9 ± 207.9 N, octagon: 594.5 ± 227.7 N), compared to external

(480.9 ± 82.8 N) and 1-piece internal (292.0 ± 218.4 N) abutments. Analysis of

the samples showed that in the 1-piece connections, the abutment was the least

resistant component of the assembly, fracturing prior to deformation of the

abutment screw or the implant. The 2-piece groups showed various types of

failure including fracture of the titanium insert (20%), combined fracture of the

zirconia abutment and the insert (10%) and fracture of the zirconia abutment with

no metal deformation (70%) (Sailer et al. 2009a). A similar laboratory study

evaluated the fracture load of the same types of CAD/CAM zirconia abutments

(external hexagon, 1-piece internal octagon, 2-piece internal octagon and 2-

piece internal tri-channel) and internally connected titanium abutments,

associated with artificial aging by simultaneous thermocycling and chewing

simulation (1,200,000 cycles, 49N). None of the tested specimens experienced

any visually detectable fracture after the artificial aging process. Static load was

applied to the specimens with a 30° angle, until failure. The bending moment of

titanium abutments (714.1 ± 184.9 Ncm) was significantly higher than the

zirconia abutments (ranging between 285.8 ± 64.4 Ncm for external and 429.7 ±

62.8 Ncm for 2-piece tri-channel abutments). However, among the zirconia

restorations, 2-piece abutments showed significantly higher bending moments

compared to the other abutments. Different modes of failure were reported for

the tested specimens in this study. The 2-piece internal connection systems

showed failure of the zirconia abutment prior to fracture of their secondary

metallic insert (Truninger et al. 2012).

The type of internal connection and presence of a cemented superstructure over

the 2-piece zirconia abutment may affect the fracture load and mode of failure of

the restoration assembly. A recent in vitro study investigated the fracture load

13

and modes of failure of CAD/CAM zirconia abutments with 5 different implant

connections (external hexagon, 1-piece octagon, 1-piece long conical, 2-piece

tri-channel and 2-piece octagon) supporting an all-ceramic crown for

replacement of a missing maxillary central incisor, and compared it with the

performance of titanium abutments (1-piece octagon). All the specimens were

exposed to thermocycling and chewing simulation (1,200,000 cycles, 49 N),

followed by application of static load at 30° angle from the implant axis. The

titanium abutments demonstrated the highest bending moment (1042.0 ± 86.8

Ncm), followed by 2-piece octagon (605.4 ± 54.7 Ncm), 2-piece tri-channel

(581.8 ±172.8 Ncm), external (556.7 ± 128.4 Ncm), 1-piece octagon (464.9 ±

106.6 Ncm) and 1- piece long conical (216.4 ± 90.0 Ncm) abutment systems.

The differences were found to be significant for the titanium (highest) and 1-

piece long conical abutments (lowest). All the abutments survived the artificial

aging. However, after the fracture strength test, fracture of 1-piece abutments

was observed on the internal part of the cone at the thinnest portion of the

zirconia and for the 2-piece octagon group, at the apical part of the junction

between the zirconia abutment and the titanium insert. The authors concluded

that 2-piece connections demonstrated a more favorable mechanical

performance compared to the 1-piece systems and also the type of 2-piece

internal connection had a minor effect on the performance of zirconia abutments

(Muhlemann et al. 2014).

The attachment of zirconia restoration to the titanium insert in 2-piece internal

connection systems could be achieved mainly through friction fit or bonding by

resin cement. In bonded assemblies, the adhesion of components is optimized

under proper laboratory conditions to enhance the long-term stability of the

bonding and health of the peri-implant soft tissues (Rosentritt et al. 2015). In

order to assess the effect of implant-abutment interface and also method of

attachment of zirconia to titanium insert on fracture resistance of the assembly, a

laboratory study compared the maximum static load capacity and mode of failure

for 3 different types of CAD/CAM zirconia abutments (1-piece, 2-piece with

friction-fitted titanium insert and 2-piece with resin bonded titanium insert),

14

attached to regular platform implants through a tri-channel internal connection

(Nobel Replace Tapered Groovy RP), for replacement of a single missing

maxillary central incisor. All samples were exposed to artificial aging by means of

thermocycling (20000 cycles, 5° – 55°C, 20s dwelling time). The result of this

study showed significantly higher fracture loads for the 2-piece abutment with

bonded titanium insert (729.2 ± 35.9 N) compared to 2-piece with friction-fitted

titanium insert (484.6 ± 56.6 N) and 1-piece system (503.09 ± 46.3 N).

Furthermore, different modes of failure were detected among the samples: 1-

piece samples fractured at the thinnest segment of zirconia, located between the

walls of the tri-channel connection. 2-piece friction-fitted system showed fracture

of zirconia at the internal aspect of the contact area with the titanium insert,

whereas 2-piece bonded systems showed separation between the zirconia

abutment and titanium insert and fracture of the titanium insert (Kim et al. 2013).

Two-piece zirconia abutment systems are advocated for minimizing the

shortcomings of 1-piece abutments by eliminating the direct contact of the thin

and weak point of zirconia abutment with the implant surface, and therefore

maintaining the biological and biomechanical advantages of the internal

connection along with the esthetic properties of zirconia (Brodbeck 2003). To

clinically test this hypothesis, a prospective clinical study investigated the

survival rate of 30 single implant-supported 2-piece CAD/CAM customized

zirconia abutments bonded to titanium inserts, supporting all-ceramic crowns for

replacement of maxillary and mandibular incisors (n=6), canines (n=2),

premolars (n=8 and 2) and molars (n=2) for 25 patients. The survival rate of the

restorations was 100% over a period of 36 to 44 months with no biologic or

technical complication during the follow-up period. Favorable soft tissue reaction

to the abutment assembly was reported (Canullo 2007). Additionally, an in vitro

study evaluated the effect of the titanium insert on the fracture strength of the

zirconia abutments by comparing 3 different abutments (titanium, 1-piece and 2-

piece zirconia; n=5) with a specific internal hexagonal implant connection, for

replacement of a missing maxillary single anterior tooth. This study showed that

under 30° off-axis compressive loading, titanium abutments had the highest

15

fracture load (1404.7 ± 19.5 N), followed by 2-piece (1216.8 ± 41.2 N) and 1-

piece (1119.5 ± 4.8 N) zirconia abutments, with statistically significant

differences among groups. The failure mode for titanium abutments was found to

be deformation while the failure mode for both zirconia groups was fracture of

zirconia with or without screw fracture. In titanium and 1-piece zirconia abutment

groups, the failure occurred at the implant-abutment junction, while in the 2-piece

group, it occurred at the junction of zirconia and the titanium insert, above the

level of the implant platform. This failure mode might be more desirable in a

clinical situation due to higher retrievability of the components without further

damage to the implant (Chun et al. 2015).

Aging of the zirconia has been considered to influence the mechanical

performance of 2-piece zirconia restoration assemblies. To test this matter, a

recent in vitro study compared the fracture load of 36 CAD/CAM 2-piece

customized zirconia abutments with different marginal preparation depths on the

abutments (0.5 mm, 0.7 mm and 0.9 mm) supporting a cemented all-ceramic

crown for replacement of a missing maxillary right central incisor. Half of the

samples (n=18) were exposed to 5-year artificial aging under chewing simulation

(1,200,000 cycles, 49N, 45° angle) and thermocycling (5° - 55°), prior to fracture

test under static load. Preparation depth and artificial aging significantly affected

the fracture load. Marginal preparation depth of 0.5 mm had a significantly higher

fracture load than the other groups for both aged and non-aged samples.

Artificially aged samples experienced significant decrease in fracture load for all

groups of different preparation depths. However, the effect of preparation depth

on the fracture load of 2-piece zirconia abutments found to be more critical than

the artificial aging. For all groups, the mode of failure was fracture of the titanium

insert at the internal connection with the implant (Joo et al. 2015).

16

Chapter 3. Rationale and Objectives

Rationale As previously reported, several in vitro studies have investigated the load

capacity of 1- and 2-piece zirconia abutments attached to different implant

systems, with or without the overlying full-coverage restoration. Limited

information is available on the fracture strength of implant-supported full-

contoured 2-piece zirconia restorations as well as the effect of artificial aging on

the fracture load and mode of failure of these restorations. Furthermore, the

majority of the existing in vitro investigations have evaluated the fracture strength

of implant-supported replacements for single missing anterior maxillary teeth.

There is not a significant amount of data available regarding the performance of

posterior restorations. When compared with the incisors, occlusal forces are

significantly higher in the premolar regions. This matter becomes more critical in

patients with strong musculature or when lateral parafunctional forces are

applied to the restorations (Ferrario et al. 2004, de Zee et al. 2007).

For implant-supported replacement of a single missing maxillary first premolar

tooth, metal restorations with veneering porcelain have been traditionally used

with relatively high biological and mechanical predictability. However, there is a

growing demand among patients for esthetic replacements for the missing

posterior teeth. To address this demand and with the recent advancements in

materials and manufacturing technologies, highly esthetic high precision

customized milled 2-piece zirconia restorations with a prefabricated titanium

insert have been introduced by the manufacturers at a relatively low cost. It is

claimed that these restorations associate the mechanical resistance and esthetic

properties of zirconia, and they also assert increased stability of the connection

between the titanium insert and the restorative platform of the implant. It is

critical to verify these claims by well-designed in vitro and in vivo studies prior to

recommending these relatively recent restorations to the patients. Although in

vitro studies have inevitable integral limitations, they can provide valuable

information regarding the mechanical performance of these restorations by

17

measuring the fracture strength and defining the mode of failure under simulated

extreme conditions using precise and sophisticated methodologies.

Objectives The aims of the present in vitro study are:

- To investigate the fracture strength of implant-supported screw-retained

CAD/CAM customized full-contoured 2-piece zirconia and 1-piece titanium

single restorations with internal octagon connection;

- To evaluate the effect of artificial aging on the fracture strength of the

implant-supported full-contoured 2-piece zirconia restorations;

- To evaluate the mode of failure for the tested implant-restoration

assemblies.

Hypotheses The null hypotheses are:

- There is no difference between the fracture strength of implant-supported

screw-retained CAD/CAM full-contoured 2-piece zirconia and 1-piece

titanium restorations for replacement of single missing maxillary first

premolars;

- Artificial aging has no effect on fracture strength of 2-piece zirconia single

restorations.

18

Chapter 4. Materials and Methods

Materials Identical titanium endosseous dental implants with internal conical octagon

connection configuration (Bone Level, diameter: 4.1mm, length: 10 mm, Regular

Cross fit, SLActive, Straumann, Basel, Switzerland) were used for this in vitro

study to support the restorations. The tested restorations were 2-piece zirconia

restoration consisting of a customized high translucency zirconia component

(CARES, X-Stream, Zerion HT, Straumann, Basel, Switzerland) bonded to a

prefabricated titanium insert (CARES RC Variobase TAN abutment, Diameter:

4.5mm, Height: 3.5mm, Straumann, Basel, Switzerland) with resin cement

(Panavia 21, Kuraray Dental, Tokyo, Japan), and 1-piece customized titanium

restoration (CARES RC abutment Ti/TAN, Straumann, Basel, Switzerland). All

the restorations were anchored to their supporting dental implants with identical

titanium screws (RC Basal Screw TAN, Straumann). The dimensions and

characteristics of the components used in the present study are described in

Table 1 and presented in Figure 1.

Experimental groups Thirty specimens were prepared for three experimental groups (n=10). The

sample size was determined based on sample size calculations using data from

previously performed similar studies (Sailer et al. 2009a, Apicella et al. 2011,

Canullo et al. 2013, Dhingra et al. 2013, Albosefi et al. 2014). Groups “Aged

Zirconia” (ZrA) and “Non-Aged Zirconia” (ZrNA) consisted of implant-supported

screw-retained CAD/CAM customized full-contoured high translucency 2-piece

zirconia restorations. Group “Titanium” (Ti) included implant-supported screw-

retained CAD/CAM customized full-contoured 1-piece titanium restorations.

Zirconia restorations in Group ZrA were exposed to artificial aging under chewing

simulation. The tested samples represented implant-supported replacements for

a single missing left maxillary first premolar tooth 24 and were identical in shape

and diameter.

19

A

B

C

D

Figure 1. Components used for preparation of the specimens. A) Implant fixture (BL, RC,

SLActive, Straumann) attached to the transfer piece. B) Titanium insert (Variobase, RC,

Straumann). C) Screw-retained customized CAD/CAM full-contoured high translucency

zirconia component (CARES, Zerion HT, Straumann). D) Screw-retained full-contoured milled

titanium restoration (CARES, RC, TAN, Straumann).

Table 1. Characteristics of materials used for the present study

Preparation of the implant fixtures Each implant fixture was embedded in auto-polymerizing polymethyl

methacrylate resin (Shade Clear, Jet Tooth Shade Powder and Jet Liquid, Lang

Dental Manufacturing Co, Inc, Wheeling, IL.) inside a hard plastic container with

Components Characteristics Composition N

Implant Fixture Straumann® Bone Level, Ø4.1mm X 10mm, Regular CrossFit®, SLActive® Titanium 30

Restorations 30 Full-contoured customized titanium Straumann® CARES RC abutment Ti/TAN. Titanium-aluminum-niobium 10

Full-contoured customized zirconia CARES® X-Stream™ Zerion® HT crown High translucency

zirconium-dioxide ceramic 20

Titanium insert Straumann® CARES® RC Variobase™ TAN abutment, Diameter: 4.5mm, Height: 3.5mm Titanium-aluminum-niobium 20

20

a 60° angle relative to the horizontal plane in order to ensure that vertical

chewing and fracture loads were applied to the samples with 30° angle relative to

the long axis of the implant-restoration assembly (Canullo et al. 2013, Wang et

al. 2013, Delben et al. 2014, Muhlemann et al. 2014, Chun et al. 2015). The

platform of the fixture was flush with the surface of the acrylic resin, simulating a

clinical situation in which the implant was placed at the residual alveolar bone

crest level (Dhingra et al. 2013, Foong et al. 2013, Stimmelmayr et al. 2013, Joo

et al. 2015) (Figure 2).

To standardize the position and angulation of the fixtures in the resin block, a

laboratory milling and drilling unit (Cendres + Metaux S.A, Bienne, Switzerland)

was utilized as the positioning device (Foong et al. 2013) (Figure 2A). The

implant fixture was attached to the device while the plastic container was

stabilized on the model holder of the unit. The model holder was set and locked

at 30° angle relative to the vertical arm in a position that would enable central

placement of the fixture inside the plastic container (Figure 2B). Baseplate dental

wax (Regular Pink, Henry Schein Inc. Melville, NY) was cut, and adapted over

the container in order to control the flow of the PMMA resin and to maintain the

resin at the level of the implant platform while setting (Figure 2C). The resin

powder and liquid were mixed following the manufacturer instructions and

carefully poured inside the mounted container until the whole length of the

implant fixture was embedded in the material (Figure 2D). For each specimen,

the container was left in the holder after placement of the fixture until initial

setting of the acrylic resin was achieved (Figure 2E). The acrylic resin block was

then allowed to completely polymerize for 24 hours at room temperature (Figure

2F) (Foong et al. 2013). All the prepared resin blocks were gathered in a box.

One block was randomly selected at a time out of the box and assigned to one of

the test groups in a sequence for attachment of the restorative component.

21

A

B

C

D

E

F

Figure 2. Preparation of acrylic resin block and embedding of the implant fixture. A) Positioning

device and mounted container on the model holder. B) Attachment of the implant fixture to the

device, model holder is set and locked at 30° angle relative to the long axis of the implant. C) Implant fixture centrally positioned inside the container with the adapted base plate wax for

controlling the flow of the acrylic resin. D) Acrylic resin powder and liquid mixed and poured in the

container. E) Container left on the model holder till initial setting of the acrylic resin achieved. F) Acrylic resin block with the embedded implant 24 hours after initial setting.

22

Fabrication of the CAD/CAM restorations A customized implant-supported full-contoured screw-retained single restoration

for a missing maxillary left first premolar tooth 24 was virtually designed using

the manufacturer’s specific computer software (Straumann CARES Visual CAD).

The digital file was then electronically submitted to the manufacturer’s milling

centre (Straumann, Arlington, TX) for fabrication of computer-aided

design/computer-aided manufacture (CAD/CAM) restorations with identical

shapes and dimensions, using high precision, high-speed cutting (HSC) milling

technology. To ensure proper and standardized engagement of the indenters of

chewing simulation and load devices with the specimens, all the restorations had

a mild depression (3 mm diameter) on the occlusal table (lingual incline of the

buccal cusp), 2 mm away from the buccal cusp tip, as part of the original design.

Preparation of the restorations and attachment to fixtures For each of the 2-piece zirconia specimens, the zirconia component was bonded

to the titanium insert, according to the manufacturer’s recommendations. Each

component was first cleaned with pressurized steam and alcohol (Figure 3B).

The titanium insert was then fixed to the implant with a laboratory screw (Figure

3C). Polytetrafluoroethylene (Teflon) tape was placed over the screw to block the

screw channel from penetration of the adhesive cement. After the application of

a thin layer of the mixed anaerobic-curing adhesive resin cement (Panavia 21,

Kuraray Dental, Tokyo, Japan) to the internal surface of the zirconia component

(Figures 3D and 3E), it was seated on the titanium insert and stabilized with

finger pressure for 2 minutes as per the manufacturer instructions (Figure 3F).

Oxygen barrier agent (Oxyguard II) was applied for 5 minutes to ensure

complete setting of the cement (Figure 3G). The restoration was then detached

from the implant and excess cement was gently removed with a periodontal

scaler under x10 magnification. Complete setting of the cement was allowed for

24 hours at room temperature. The assembly was then anchored to the implant

in the resin block with the definitive titanium screw (Figure 3I), tightened with a

calibrated torque control device (Straumann®) to 35 N/cm, according to the

23

manufacturer’s recommendation. The preload was checked and the screw

retightened if required, after 5 minutes (Delben et al. 2014).

For the 1-piece titanium specimens, each restoration was cleaned with

pressurized steam and alcohol and attached to the corresponding implant with a

similar abutment screw and preload force of 35 N/cm. All the specimens were

stored in room temperature prior to artificial aging process.

A

B

C

D

E

F

G

H

I

Figure 3. Preparation of 2-piece zirconia restoration and attachment to fixture. A) Components of

Straumann screw-retained restoration assembly (Variobase insert, full-contoured high

translucency Zerion crown and the RC Basal screw). B) Cleaning of the insert with pressurized

steam. C) Stabilization of the insert to the implant with a laboratory screw. D, E) Mixing of the

adhesive cement (Panavia) and application to the internal surface of the zirconia component. F) Seating and stabilization of the zirconia component on the insert. G) Application of the oxygen

barrier agent for initiation of setting. H) 2-piece zirconia restoration assembly after complete

setting of the adhesive cement. I) Attachment of the restoration to the implant fixture with the

definitive screw (RC, Basal)

24

Artificial aging The specimens from group ZrA were exposed to artificial aging (1.2 x 106 cycles,

2Hz, 80N) in a chewing simulator (CS-4.4, SD Mechatronik GMBH, Feldkirchen-

Westerham, Germany) under distilled water at room temperature (Figure 4A)

(Truninger et al. 2012, Muhlemann et al. 2014, Joo et al. 2015), which

theoretically corresponds to 5 years of service (Rosentritt et al. 2009, Joo et al.

2015, Rosentritt et al. 2015). A corrosion-free steel indenter with a rounded tip

(diameter: 2.8 mm) was used as the antagonist with vertical movement (Figure

4B). The angulation of the implant fixture in the resin block enabled the

application of simulated chewing load onto the predesigned concavity on the

occlusal table, with 30° off-axis angle (Figure 4C) (Truninger et al. 2012, Foong

et al. 2013, Stimmelmayr et al. 2013, Muhlemann et al. 2014). The specimens

from groups ZrNA and Ti were stored in distilled water at room temperature for

the same period of time. All the artificially aged specimens were assessed

visually for any mechanical complication after the aging process.

A

B

C

Figure 4. Artificial aging of 2-piece zirconia restorations in the chewing simulator. A) Chewing

simulator machine with the samples stabilized in the chambers. B) Vertical indenter engaging

with the zirconia restoration in one of the chambers. C) Application of simulated chewing force to

the predesigned concavity on the lingual incline of the buccal cusp by the indenter with 30° angle

to the implant-restoration assembly.

25

Fracture strength test Fracture strength test was performed for all the specimens using a universal

testing machine (Model 5566, Instron, Norwood, MA) (Figure 5A). Each sample

was clamped and stabilized in the holder apparatus of the machine (Figure 5B).

A steel indenter similar in shape and diameter to the one used for the artificial

aging was attached to the loading piston of the testing machine. The static

compressive load was applied to the concave area on the occlusal table of the

restoration with 30° off-axis angle, at a crosshead speed of 0.5 mm/min and a

progressive increase in load from 0 N until failure occurred (Truninger et al.

2012) (Figure 5C). Failure was determined as an audible crack sound or a

sudden fall of the load force applied to the specimen (Foong et al. 2013).

Computer software was used to control the testing machine and to record the

maximum load at failure for each specimen individually (Bluehill 2 Software,

Norwood, MA).

A

B

C

Figure 5. Fracture strength test under static load. A) Instron universal testing machine. B) A

specimen clamped and stabilized on the loading table of the machine. C) Application of static

load by the steel indenter to the titanium and zirconia restorations with 30° off-axis angle through

the concavity on the occlusal surface.

26

Visual evaluation of the specimens After aging and fracture load testing, all of the specimens were checked for

presence of mobility and visually analyzed to determine the mode of failure.

Failed specimens were examined under light microscope with 20x magnification.

Digital images of the fractured surfaces were recorded.

Scanning Electron Microscopy assessment One sample was randomly selected from each zirconia restoration test group

(groups ZrA and ZrNA), and then observed with Scanning Electron Microscopy

(SEM) for closer evaluation of the fractured surface and characteristics of the

failure mode (Foong et al. 2013, Kim et al. 2013, Muhlemann et al. 2014).

Representative specimens were cleaned with acetone in ultrasonic device for 15

minutes and then mounted on aluminum blocks with double sided carbon tape

and sputtered with gold in an argon gas environment (Polaran Range Sputter

Coater SC7620; Quorum Technologies Ltd, East Sussex, England) prior to

examination with SEM (JEOL JSM-6610LV, JEOL Ltd, Tokyo, Japan) with

secondary electron imaging. Digital images of these specimens were obtained at

various magnifications.

Statistical analysis Statistical Software (SPSS Version 21.0, SPSS Inc. Chicago, IL) was used for

statistical assessment. Descriptive statistics including means, standard

deviations, minimum and maximum values with 95% confidence interval were

reported for each group. The homogeneity of variance among groups was

checked by Levene’s test. The statistical significance of differences between

fracture load of artificially aged and non-aged 2-piece zirconia groups was

verified by independent-samples t-test. Under the equal variance assumption,

one-way analysis of variance (ANOVA) and Tukey HSD post-hoc tests were

performed to analyze the differences of fracture loads among all the test groups.

The significance of differences in the modes of failure between the aged and

non-aged zirconia restorations (groups ZrA and ZrNA) was assessed by Chi-

Square test. The level of statistical significance (P) was set at 5%.

27

Chapter 5. Results

Outcome of chewing simulation None of the tested specimens exhibited any visible morphological change or

mechanical complication after the 1,200,000 artificial chewing cycles with 80 N

force in the chewing simulator. All the restorations were found to be intact and

stable to their supporting fixtures in the acrylic resin block.

Fracture strength The mean fracture strength values for aged and non-aged 2-piece full-contoured

zirconia restorations were 731.01 ± 59.31 N and 762.91 ± 37.03 N respectively.

For the titanium control group the mean fracture strength was 808.22 ± 48.96 N

(Table 2, Figure 6).

Table 2. Mean fracture strength and SD for each experimental group

Sample Groups N Mean Fracture Strength (N) Std. Deviation

Zirconia-Aged (ZrA) 10 731.01 59.31

Zirconia-Non Aged (ZrNA) 10 762.91 37.03

Titanium (Ti) 10 808.22 48.96

Figure 6. Mean fracture strength for Aged zirconia (ZrA), Non-aged zirconia (ZrNA) and Titanium (Ti) groups

731.01

762.91

808.22

600

650

700

750

800

850

ZrA ZrNA Ti

Mea

n Fr

actu

re S

treng

th (N

)

Sample Groups

28

Independent-samples t-test did not identify any significant effect of artificial aging

on fracture strength of 2-piece zirconia restorations at a significance level of 5%.

Under the assumption of equal variance among groups, confirmed by Levene’s

test, one-way analysis of variance (ANOVA) detected significant differences in

fracture strength among the test groups (p=0.006) (Table 3). Tukey HSD post

hoc test indicated that the fracture strength values only differed significantly

between 1-piece titanium restorations and aged 2-piece zirconia restorations

(p=0.04) (Table 4). This test also confirmed the earlier observation that the

difference in fracture strength between aged and non-aged zirconia groups was

not statistically significant.

Table 3. One-way ANOVA exhibiting significant differences in fracture strength among test groups Sum of Squares df Mean Square F Sig. Between Groups 30108.91 2 15054.45 6.20 .006 Within Groups 65573.23 27 2428.64

Total 95682.13 29

Table 4. Tukey HSD post-hoc test. Mean values for groups in homogenous subsets are displayed

Sample Groups N Subset for alpha = 0.05 1 2

Zirconia-Aged (ZrA) 10 731.01

Zirconia-Non Aged (ZrNA) 10 762.91 762.91 Titanium 10 808.22 Sig. .332 .118

Mode of failure All of the restoration assemblies failed mechanically during the static load test. In

the aged zirconia group (ZrA), 4 of the samples (40%) showed complete fracture

and separation of the titanium insert at the level of the horizontal platform just

above the abutment screw head (Figure 6). For these specimens, the segment of

the titanium insert that remained attached to the implant as well as the

connecting screw could be easily retrieved. The remaining 6 restorations in this

29

group (60%) showed partial fracture of the titanium insert at the same location

without separation. All of the non-aged zirconia restorations (ZrNA) experienced

partial fracture (crack) at the horizontal platform of the titanium insert above the

joint surface of the implant, without complete detachment from the fixture (Figure

7). For all of the specimens with partial fracture, the abutment screw could be

engaged and loosened with the torque driver and the fractured restoration

assembly could be easily detached from the fixture. However, the abutment

screw could not be retrieved due to substantial alteration of path of screw

channel due to the fracture. Chi-Squared tests revealed that there was a

statistically significant difference between the modes of failure for aged and non-

aged zirconia groups (Table 6). Analysis of the specimens after the load test

revealed that in both groups the titanium insert was the weakest segment of the

implant-restoration assembly, as the zirconia component, abutment screw and

fixture remained intact (Figure 6). None of the restorations showed debonding of

the cement between the zirconia component and the titanium insert.

The mode of failure for 1-piece titanium restorations was different from the

zirconia restorations. All of these specimens (100%) presented progressive

plastic deformation (bending) of the full-contoured titanium restoration under the

static load force till fracture of the abutment screw occurred just below the screw

head (Figure 8). Retrieval of the restorations in this group was not possible.

None of the samples in this group showed fracture of the restoration.

Table 5. Chi-Square tests exhibiting significant difference for mode of failure between zirconia test groups

Value df Asymp. Sig. (2-sided)

Exact Sig. (2-sided)

Exact Sig. (1-sided)

Pearson Chi-Square 5.00 1 .025

Continuity Correction 2.81 1 .094

Likelihood Ratio 6.56 1 .010

Fisher's Exact Test .087 .043 Linear-by-Linear Association 4.75 1 .029

N of Valid Cases 20

30

A

B

C

D

Figure 7. Complete fracture of titanium insert observed in 40% of aged zirconia restorations.

A,B) Side and top view of the fractured insert attached to the fixture with the fracture line at

the horizontal platform just above the abutment screw head. The screw was intact, and

enabled easy removal of the insert. C) Light microscope image of the separated restoration

assembly showing the fractured titanium insert with surface irregularities, intact zirconia

surfaces and the intermediary adhesive cement. D) Light microscope image of the

embedded fixture after removal of the screw and titanium insert, showing unaffected intact

implant platform.

31

A

B

Figure 8. Partial fracture of titanium insert in 60% of aged and 100% of the non-aged zirconia

restorations. A) Partial fracture of the insert. The restoration still attached to the implant with

significant change in orientation. The restoration assembly could be completely disconnected

from the implant by loosening of the abutment screw. B) Same restoration assembly viewed by

Scanning Electron Microscopy shows the fractured secondary insert with intact zirconia

component.

A

B

Figure 9. One-piece titanium restoration after fracture strength test. A) Side view showing plastic

deformation of the restoration. B) Top view showing the fractured abutment screw head

32

Chapter 6. Discussion

In the present in vitro study, the fracture strength of implant-supported screw-

retained CAD/CAM customized full-contoured 2-piece zirconia and 1-piece

titanium restorations for replacement of a single missing maxillary first premolar

tooth was investigated. The effect of artificial aging by means of chewing

simulation on fracture strength of zirconia restorations was also assessed.

Previous in vitro studies generally focused on the performance of implant

abutments with or without cemented restorations for simulated missing maxillary

incisor teeth. There is no research in the literature investigating the performance

of full-contoured restorations for premolar teeth. Only one study simulated a

mandibular premolar replacement to evaluate the fracture strength of 1-piece

zirconia and titanium abutments but not full-contoured restorations (Apicella et al.

2011). The maxillary first premolar was selected for the experiments of the

present study, as there is a growing demand for esthetic implant restorations in

posterior regions, where high functional occlusal forces would require

restorations with reliable mechanical performance.

Materials and testing conditions in the present study were chosen in a manner to

simulate clinical conditions as best as possible. This applies to implant selection,

embedding of fixture in the holding material, design and fabrication of the

prostheses and also the artificial aging protocol. Bone-level implants with internal

conical octagon connection were utilized to support the restorations. This type of

implant is regularly placed in esthetically critical areas of the mouth, enabling

proper control of the dimensions, contour and emergence profile of the

corresponding restoration. The diameter of the selected implant (RC, 4.1mm)

was also in accordance with the surgical and prosthodontic treatment protocols

for replacement of a missing premolar under normal clinical conditions, based on

the dimensions of the tooth and local anatomical considerations. Some studies

tested the abutments that were supported by implant analogs (Aramouni et al.

2008, Adatia et al. 2009, Kim et al. 2013, Albosefi et al. 2014). Most analogs are

33

made of stainless steel and are used solely to replicate the dimensions and

configurations of the restorative platform of the corresponding endosseous

titanium implant inside the master cast without having the same mechanical

performance (Foong et al. 2013). Therefore, use of implant replicas instead of

titanium fixtures may affect the behavior of the implant restoration assembly

during the mechanical tests and fail to relate the laboratory tests to clinical

conditions.

The selected implant fixtures for the current investigation were embedded in

chemically cured PMMA resin. According to the manufacturer claim and previous

studies, the resin has a modulus of elasticity of approximately 12 GPa, which is

relatively close to that of human cancellous bone (Att et al. 2006, Delben et al.

2014). The acrylic resin holder was at the level of the implant shoulder, in

accordance with previous studies (Yildirim et al. 2003, Apicella et al. 2011,

Foong et al. 2013, Joo et al. 2015, Rosentritt et al. 2015), imitating absence of

clinical peri-implant horizontal bone loss. The angle of the fixture in the acrylic

resin enabled vertical chewing and fracture loads to be applied with 30° angle

relative to the long axis of the implant-restoration assembly, simulating a clinical

situation where the implant is angulated relative to the restoration axis or an

exaggerated scenario of parafunctional lateral movements, in accordance with

several previous studies (Truninger et al. 2012, Dittmer et al. 2012, Foong et al.

2013, Albosefi et al. 2014, Delben et al. 2014, Chun et al. 2015).

Full-contoured zirconia restorations without veneering porcelain layer were

tested in the present study. Developments in materials and manufacturing

technologies have lead to fabrication of strong monolithic zirconia restorations

with significantly improved esthetic properties for application in less esthetically

critical areas of the mouth. In conjunction with the CAD/CAM process, staining is

performed to achieve the desired shade of the milled zirconia restoration in its

green or soft state, prior to the sintering stage. Elimination of the veneering

porcelain layer can prevent a number of mechanical complications related to

these restorations such as cohesive failure within the veneering porcelain and

34

adhesive failure at abutment-veneer interface (Ekfeldt et al. 2011), which are

mostly related to the restoration geometry, mechanical limitations of the layering

porcelain and weak bonds between the veneering layer and underlying zirconia

structure (Delben et al. 2014). Moreover, in the absence of this layer a thicker

and mechanically stronger monolithic restoration can be fabricated in areas

where limited amount of restorative space is available.

The titanium specimens were also designed and fabricated as full-contoured with

no veneering porcelain. Although this type of restoration is rarely applied in a

clinical setting, it was decided to utilize it in the present study in order to maintain

homogeneity with the machined zirconia samples with regards to dimension and

contour, area of loading force application and length of the lever arm and to

eliminate the human factor related to the sintering process. Another reason was

to exclude any possible effect of the layering porcelain on the performance of the

titanium restoration under loading force.

All the components used in the present study were machine made using highly

advanced designing and milling technologies, and fabricated by the same

manufacturer in an effort to control the possible undesirable deficiencies caused

by human error and misfit of components from different manufacturing sources.

The present study showed that under the same non-aged conditions, implant-

supported screw-retained CAD/CAM customized full-contoured 2-piece zirconia

and 1-piece titanium restorations did not present a significant difference in

fracture strength; therefore there is not enough evidence available to reject the

first null hypothesis. However, the fracture strength of the artificially aged 2-piece

zirconia replacements was significantly lower than that of the 1-piece titanium

restorations.

Several methods have been introduced for assessing the resistance of implant-

restoration assemblies to fracture among which, fatigue testing is considered as

the most reliable method (Canullo et al. 2013). Although the fracture strength test

under compressive static load performed in the present investigation may not be

35

the best simulation of in vivo conditions, it offers the possibility to evaluate and

compare the mechanical performance of the restorations, as well as identifying

the modes of failure and weaker segments of the assembly (Dittmer et al. 2012).

This test may also model situations where an individual occludes into a hard

object or receives trauma (Kim et al. 2013). Several other studies have followed

a similar loading protocol (Sailer et al. 2009a, Dittmer et al. 2011, Dittmer et al.

2012, Kim et al. 2013, Stimmelmayr et al. 2013, Albosefi et al. 2014) but

relatively few have compared the fracture strength of titanium and 2-piece

zirconia abutments. Among these, an in vitro study exhibited significantly higher

fracture strength for titanium abutments compared to 1- and 2-piece zirconia

abutments under no artificial aging (Chun et al. 2015), while others observed the

same difference between titanium and various types of zirconia abutments

following artificial aging by thermocycling and chewing simulation (Truninger et

al. 2012, Muhlemann et al. 2014).

The mean fracture load for the sample groups in the present study ranged

between 731 N and 808 N. The physiologic occlusal forces at maximum

intercuspation range between 31 N and 145 N in the first premolar region,

depending on the length of the dental arch and the occlusal scheme (Hattori et

al. 2003). Higher occlusal forces are expected in individuals with parafunctional

habits such as clenching and bruxism. Clinical studies have recorded load peaks

of up to 255 N for first premolar teeth during maximal clenching (Ferrario et al.

2004) and 440 N during unilateral parafunctional movements (de Zee et al. 2007)

in healthy male young adults. Fracture load values for all groups in the present

study exceeded the normal occlusal forces. Moreover, the application of forces

to the specimens with 30° off-axis angle represents exaggerated lateral forces

that might be observed during parafunctional movements. Based on our findings,

it may be speculated that the implant-restoration assemblies evaluated in this

study would have a satisfactory performance under physiologic chewing forces in

a clinical scenario. However, due to the inevitable limitations of in vitro studies,

any clinical interpretation should be expressed with caution.

36

Identifying the failure mode is important for predictability of the treatment

outcome and retrievability of the components in case a mechanical failure

occurs. In the present investigation, the type of failure for full-contoured aged

zirconia restorations was complete fracture with separation of the titanium insert

(40%) or partial fracture without separation (60%), whereas 100% of the non-

aged zirconia restorations experienced partial fracture of the insert, with the

fracture line located on the axial wall just above the titanium screw head for all

the specimens. In both groups, the zirconia component, the implant platform and

the clamping screw remained intact. For specimens with complete fracture, it

was possible to detach the titanium insert from the implant and retrieve the

screw. The partially fractured assemblies could be disengaged from their

corresponding implants but recovery of the screw was not possible due to

substantial change in the path of the screw access. Some of the previous

investigations have reported similar failure modes (Sailer et al. 2009a,

Muhlemann et al. 2014), while other modes of failure for internally connected 2-

piece zirconia abutments with or without the overlying cemented restoration have

been observed in different studies. These failures include fracture of the titanium

insert at the implant-abutment joint (Joo et al. 2015), fracture of the screw

(Stimmelmayr et al. 2013, Rosentritt et al. 2015), separation of the zirconia

component from the titanium insert (Kim et al. 2013, Rosentritt et al. 2015),

fracture of zirconia component and adhesive failure between zirconia and

titanium insert by itself (Att et al. 2006, Sailer et al. 2009a, Truninger et al. 2012,

Canullo et al. 2013, Kim et al. 2013, Delben et al. 2014) or in conjunction with

screw fracture (Chun et al. 2015, Rosentritt et al. 2015) or plastic deformation of

the metallic components of the assembly (insert, screw and implant) (Sailer et al.

2009a, Truninger et al. 2012, Kim et al. 2013, Muhlemann et al. 2014).

Partial or complete fracture of the titanium insert under load without damage to

the fixture, the zirconia component and the screw might be due to the long and

thin wall of the insert in the tested implant-restoration system and the relatively

long lever arm. The stress that is generated from the compressive load reaches

the fracture strength of the titanium insert before affecting the implant-insert

37

interface or the zirconia component. Increasing the thickness of the walls of the

titanium insert may increase the resistance against fracturing loads. However,

thickening of the insert wall may result in thinness of the zirconia component due

to the limited available space between the implant platform and the surrounding

soft tissues and the subsequent possible risk of fracture of this component.

Moreover, the cross sectional square shape of the abutment screw head and the

intimate contact between the head of the preloaded screw and the wall of the

titanium insert may create areas of high stress concentration at the interface with

the insert, causing fracture right above the head, as observed in the present

study. Beveling of the angles of the screw head in order to relieve the areas of

stress concentration may increase the resistance of the assembly to the

unfavorable fracture loads.

An interesting finding of the present study was the preserved integrity of the

zirconia component after the mechanical failure of the assembly, which was

observed visually and confirmed by Scanning Electron Microscopy (SEM). This

phenomenon may be due to the enhanced mechanical properties of the tested

yttrium-stabilized polycrystalline zirconia component, including flexure strength

and fracture toughness (Vagkopoulou et al. 2009), exceeding the ones for the

titanium insert, under static fracture load conditions in the present study. These

properties may also be crucial for the satisfactory performance and survival of

these components under chewing simulation conditions.

Clinically, the mode of failure for the 2-piece zirconia restorations in the present

study may act as a protective mechanism for the implant fixture and the

anchoring screw, since the fracture of the insert will occur first, away from the

implant-restoration joint and above the screw head. Moreover, with this type of

failure all the components can be easily detached from the implant fixture. The

zirconia component can be separated from the titanium insert following the

disintegration of the adhesive resin cement by application of heat in a dental

porcelain furnace at temperatures ranging between 306° and 363°C, based on

the type of cement (Linkevicius et al. 2012) for approximately 5 minutes

38

(Saponaro et al. 2015). The temperature and duration of heat application

required for debonding of the cement is below the values critical for structural

stability and mechanical properties of zirconia (Swab 1991). Following the

retrieval process, the failed restoration can be easily replaced by bonding the

retrieved zirconia component to a new prefabricated titanium insert, anchored to

the fixture with a new abutment screw. Therefore the entire process of retrieval

and replacement of the failed restorations can be completed in a relatively short

period of time and at a low cost.

In the present study the failure mode for full-contoured 1-piece titanium

restorations was progressive deformation of the restoration and fracture of the

screw at the head as well as deformation of the implant platform. This finding

was in accordance with other investigations that evaluated titanium abutments

supported by the same (Dittmer et al. 2011, Truninger et al. 2012, Muhlemann et

al. 2014) or other implant systems (Apicella et al. 2011, Foong et al. 2013, Chun

et al. 2015). The screw is generally regarded as the most vulnerable component

in the implant-restoration assembly due to its structural and dimensional

limitations as well as high stress concentration at the head (Sailer et al. 2009b).

It may be hard to manage a mechanical failure of a titanium restoration similar to

the one observed in the present study in a clinical setting, as the retrieval of the

restoration and fractured screw would be very complicated. Furthermore,

deformation of the implant platform may result in non-restorability and the

inevitable need for removal of the osseointegrated fixture. According to the

findings of the present study it may be claimed that from a clinical standpoint the

mode of failure of implant-supported single 2-piece zirconia restorations is more

favorable than single 1-piece titanium restorations based on ease of retrievability

of the failed restoration, the time and the cost involved in replacement and also

preservation of the endosseous implant. However, plastic deformation of metal

components to the extreme level observed for the titanium restorations in the

present investigation may be a very rare clinical incidence. Even slight degrees

of deformation would affect the occlusal and proximal relationship of the

39

restoration with the neighboring teeth, resulting in altered patient perception and

the need to seek prosthodontic care. Moreover, higher fracture strength recorded

for titanium restorations may result in a lower rate of this type of failures that

would require removal or replacement of the restorations.

This study showed that artificial aging by means of chewing simulation

(1,200,000 cycles, 80 N) had no significant effect on fracture strength of implant-

supported customized full-contoured 2-piece zirconia restorations. Thus, not

enough evidence was available to reject the second null hypothesis. However,

this aging process significantly influenced the mode of failure of zirconia

restorations under static compressive fracturing loads.

Artificial aging of the specimens with a predetermined number of cycles and

force value is performed in laboratory studies to assess 2 major effects; one is

the direct effect of simulated chewing function on the mechanical performance

and stability of different components in the implant-restoration assembly and also

the adhesive bond between zirconia component and titanium insert; the other

one is the indirect effect caused by degradation and phase transformation of the

zirconia component in an aqueous environment under constant masticatory

loads. Several factors may influence the mechanical performance of the

restoration assembly after aging by chewing simulation such as the number of

chewing cycles and the frequency, amount and angulation of the force, the

features of the aging environment as well as the characteristics of zirconia

including the forming constituents and their percentage, and manufacturer

specific processing technique. The number of chewing cycles for this study was

determined based on previous investigations (Truninger et al. 2012, Muhlemann

et al. 2014, Joo et al. 2015), which is theoretically claimed to be equal to 5 years

of clinical service (Rosentritt et al. 2009, Joo et al. 2015, Rosentritt et al. 2015).

The chewing load force of 80 N was set, based on the average physiologic

forces recorded for the first premolars during physiologic masticatory functions

(Hattori et al. 2003). Several investigations have tested the fracture load of

different types of implant-supported restorations after exposing all of the test

40

samples to various aging protocols (Truninger et al. 2012, Kim et al. 2013,

Stimmelmayr et al. 2013, Muhlemann et al. 2014, Rosentritt et al. 2015).

Conversely, very few studies have been designed to examine the effect of aging

on the fracture strength of the 2-piece zirconia restorations. An in vitro study

claimed that the fracture load of 2-piece zirconia abutments was reduced after

1,200,000 chewing cycles combined with thermocycling (temperature 5° - 55°C,

dwelling time 120S) (Joo et al. 2015). For the specific implant-restoration

assembly tested in the present investigation, the aged zirconia restorations had

slightly lower, although not significant, fracture strength than the non-aged group

and also a significantly different mode of failure (complete fracture) was

observed for the specimens in this group. Meanwhile, Scanning Electron

Microscopy (SEM) analysis of the completely fractured components from the

aged restorations did not indicate any structural phase transformation of zirconia

at the fracture margin. Thus, the slight difference in fracture strength between the

aged and non-aged zirconia groups may be attributed to the mechanical effect of

the simulated off-axis chewing forces on the preload of the abutment screw and

its clamping action rather than phase transformation in the polycrystalline

structure of the zirconia component. Preload is the tensile force that is generated

in the screw threads by torque forces applied during screw tightening, which

firmly connects and retains the restoration assembly to the fixture by friction

resistance between the screw threads and implant platform (Hebel and Gajjar

1997). Lateral forces have shown the capacity for detrimental effects on the

preload of the abutment screw, leading to its loosening, stretching or fracture

over time (Hebel and Gajjar 1997). Chewing simulation in the present study may

have had a similar effect on the clamping action of the screw, compromising the

stability of the implant-restoration joint and proper distribution of occlusal forces

through the restoration into the implant body. This process may have ultimately

resulted in the lower fracture strength as well as the more catastrophic mode of

failure, observed among the aged specimens.

Based on the results of the present in vitro study, it is possible to estimate that

the application of a more aggressive chewing simulation with higher number of

41

cycles and/or load value under fluctuating temperatures and also use of an aging

medium with more similarity to the human saliva might have a more prominent

effect on fracture strength of full-contoured zirconia restorations and therefore

result in significant differences. Moreover, including the titanium restorations in

the process of artificial aging would be helpful for evaluation of the effect of

simulated chewing function on the stability of the components and the

performance of the assembly under fracturing loads and to compare these

findings with the ones for the zirconia restorations. Therefore, the findings of the

present study indicate the importance of replicating clinical conditions for a more

meaningful estimation of the performance of different restoration assemblies.

Clear comparison of the observations of the present study with those of previous

investigations seems complicated, due to the differences in design and sample

size of in vitro studies, characteristics of the selected implant system, and

implant-restoration connection configurations as well as variations in design,

dimensions, material compositions and methods of fabrication of restorations.

Furthermore, diversities in the angulation and position of the fixture platform

relative to the resin holder may impact the results. In some investigations a

vertical distance between the implant shoulder and the holding resin has been

left exposed in an attempt to mimic different degrees of horizontal peri-implant

bone loss (Sailer et al. 2009a, Dittmer et al. 2012, Albosefi et al. 2014, Joo et al.

2015), which may affect the location of the fulcrum and stress concentration

areas, the length of the lever arm, amount of stress as well as the pattern of

force distribution through the restoration assembly and implant body. Efforts

should be made to conduct standardized in vitro studies to allow better

comparison of the findings.

The performance of 2-piece zirconia restorations that were designed for a

specific implant system and diameter for replacement of simulated single missing

maxillary first premolars was tested in the present study, therefore the findings

might not be applicable to other implant systems. Additional studies are

necessary to achieve more comprehensive results and to provide guidelines for

42

fabrication and use of full-contoured 2-piece zirconia restorations supported by

other implant systems for replacement of different teeth in the dentition.

43

Chapter 7. Conclusions and Clinical Significance

Conclusion Implant-supported screw-retained CAD/CAM customized full-contoured 2-piece

zirconia and 1-piece titanium single restorations did not present a significant

difference in fracture strength, under the same non-aging conditions.

Two-piece zirconia assemblies that were exposed to artificial aging showed

lower fracture strength than 1-piece non-aged titanium restorations.

Evidence was inadequate to reject the assumption that artificial aging has no

effect on the fracture strength of full-contoured 2-piece zirconia restorations.

Artificial aging impacted the mode of failure of full-contoured 2-piece zirconia

restorations.

Clinical Significance The specific implant-supported screw-retained CAD/CAM customized 2-piece

zirconia restorations tested in the present investigation demonstrated high

reliability during artificial aging and withstood loads that were higher than the

average functional load forces reported for the premolar region. In addition, they

showed a relatively favorable mode of failure with regards to retrievability and

replacement of the mechanically failed restoration and preservation of the

structural integrity of the supporting implant. Therefore, these restorations may

be considered as mechanically and esthetically suitable options for implant-

based replacement of single missing maxillary first premolar teeth in patients

with high esthetic demands. However, the effect of long-term exposure to the

potentially detrimental intra oral conditions such as persistent functional loads,

fluctuating temperatures and the aqueous environment in presence of saliva on

the mechanical performance of these restorations is still unclear.

44

Chapter 8. Future Research Directions

Further in vitro studies are recommended on the following subjects:

- To evaluate the effect of artificial aging on the mechanical performance of

implant-supported single titanium restorations in comparison with zirconia

restorations.

- To investigate the effect of more aggressive artificial aging protocols on

the structural stability and mechanical performance of implant-supported

2-piece full-contoured zirconia restorations.

- To assess the fracture strength and mode of failure of full-contoured 2-

piece zirconia restorations with various heights and thicknesses of the

secondary titanium insert.

- To examine the effect of various screw head designs and dimensions on

the technical performance of the 2-piece zirconia restoration assembly.

- To compare the mechanical performance of various commercially

available 2-piece zirconia restorations for the simulated replacement of a

similar tooth under standard artificial aging and loading protocols.

Due to the various limitations of laboratory studies, high quality clinical trials with

large sample sizes and long follow-up periods are necessary to support the

laboratory findings and to help the patients make informed clinical decisions

based on valid and reliable scientific evidence.

45

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Appendices

Appendix A: Fracture strength values and failure mode of the specimens

Group ZrA. Implant-supported customized 2-piece aged zirconia restorations

Aged Zirconia Sample #

Fracture Force (N) Mode of Failure

Zr-A-1 658.53 Partial fracture of insert above the screw head

Zr-A-2 732.7 Complete fracture of insert above screw head

Zr-A-3 788.67 Partial fracture of insert above the screw head

Zr-A-4 725.18 Complete fracture of insert above the screw head

Zr-A-5 745.68 Complete fracture of insert above the screw head

Zr-A-6 757.73 Partial Fracture of insert above the screw head

Zr-A-7 599.77 Complete fracture of insert above the screw head

Zr-A-8 772.71 Partial fracture of insert above the screw head

Zr-A-9 785.74 Partial fracture of insert above the screw head

Zr-A-10 743.4 Partial fracture of insert above the screw head

55

Group ZrNA. Implant-supported customized 2-piece non-aged zirconia restorations

Non-Aged Zirconia Sample #

Fracture Force (N) Mode of failure

Zr-NA-1 799.09 Partial fracture of insert above the screw head

Zr-NA-2 804.71 Partial fracture of insert above the screw head

Zr-NA-3 698.86 Partial fracture of insert above the screw head

Zr-NA-4 736.31 Partial fracture of insert above the screw head

Zr-NA-5 735.99 Partial fracture of insert above the screw head

Zr-NA-6 761.07 Partial fracture of insert above the screw head

Zr-NA-7 765.62 Partial fracture of insert above the screw head

Zr-NA-8 822.29 Partial fracture of insert above the screw head

Zr-NA-9 752.27 Partial fracture of insert above the screw head

Zr-NA-10 752.92 Partial fracture of insert above the screw head

56

Group Ti. Implant-supported customized 1-piece titanium restorations

Titanium Sample # Fracture Force (N) Mode of Failure

Ti-1 768.3 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-2 829.75 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-3 780.91 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-4 807.93 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-5 768.52 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-6 825.19 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-7 740.51 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-8 790.34 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-9 892.23 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

Ti-10 878.56 Plastic deformation of restoration, Fracture of the screw head, Deformation of implant

57

Appendix B: Representative stress-strain curves for each test group

Group ZrA. Customized 2-piece aged zirconia restoration (Zr-5-A)

Group ZrNA. Customized 2-piece non-aged zirconia restoration (Zr-8-NA)

0

100

200

300

400

500

600

700

800

900

0 0.5 1 1.5 2

Stre

ss (N

)

0

100

200

300

400

500

600

700

800

900

0 0.5 1 1.5 2

Stre

ss (N

)

58

Group Ti. Customized 1-piece titanium restoration (Ti-9)

0

100

200

300

400

500

600

700

800

900

1000

0 0.5 1 1.5 2 2.5 3 3.5 4