Zirconia in Dentistry :Overview

Mohamed Mahmoud Abdul-MonemAssistant lecturer

Dental Biomaterials Department

Contents•Introduction•What is Zirconia ?•History of Zirconia as a biomaterial•Zirconia phase transformation•Zirconia (Ceramic steel)•Properties of Zirconia•Uses of Zirconia •Uses of Zirconia in dentistry•Bonding to Zirconia•References

Introduction

• Ceramics are very important in the science of dental biomaterials.

•Among all dental ceramics, Zirconia is the dental biomaterial of choice in contemporary restorative dentistry.

What is Zirconia ?

• Zirconium oxide (ZrO2), or zirconia, is a metal oxide that was identified as a reaction product of heating the gem( zircon) by the German chemist Martin Heinrich Klaproth in 1789 .

•The name of the metal, zirconium, comes from the Arabic Zargon (golden in colour) which in turn comes from the two Persian words Zar (Gold) and Gun (Colour).

•Zirconia occurs as a natural mineral called baddeleyite.

•Baddeleyite was first found in Sri Lanka in

1892. It was named for Joseph Baddeley. 

•This mineral contains 80–90% zirconium oxide(zirconia). The major impurities are usually TiO2, SiO2 and Fe2O3.

Baddeleyite

• It shouldn’t be confused with Zirconium (Zr ) which is found in the form of (zircon) or zirconium silicate(ZrSiO4 ) .

• It is lusturous gray-white transition metal .

History of zirconia as a biomaterial•Zirconia has been used as a biomaterial

since the 1970s.•It has been widely used as a hip

replacement material.• Its use in dentistry began in 2004.

Zirconia hip implants

Zirconia phase transformation• Zirconium oxide exists in three different crystal

structures: monoclinic at room temperature, tetragonal at ~1200°C and cubic at 2370°C.

• Zirconium oxide is transformed from one crystalline state to another during firing.

• At the firing temperature, zirconia is tetragonal and at room temperature, it is monoclinic, with a unit cell of monoclinic occupying about 4.4% more volume than when tetragonal.

Zirconia phase transformation

•The volume expansion caused by the cubic to

tetragonal to monoclinic transformation induces large stresses, and these stresses cause ZrO2 to crack upon cooling from high temperatures.

•When the zirconia is blended with some other oxides, the tetragonal and/or cubic phases are stabilized.

•Effective stabilizers include magnesium oxide (MgO), yttrium oxide (Y2O3, yttria), calcium oxide (CaO), and cerium(III) oxide (Ce2O3).

•Zirconia is often more useful in its phase 'stabilized' state. Upon heating, zirconia undergoes disruptive phase changes.

• By adding small percentages of yttria, these phase changes are eliminated, and the resulting material has superior thermal, mechanical, and electrical properties.

•In some cases, the tetragonal phase can be metastable.

• If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion(4.4%).

•This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness.

•This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia.

Zirconia transformation toughening

Zirconia = Ceramic steel•It may be noted that zirconia has many

features in common with systems based on iron (stainless steel )as :

1. Three allotropes2. Martesnitic transformation3. Metastable phases

Thus it is known as ceramic steel (Garvie et al , 1975).

Properties of ZirconiaPhysical properties :•Low thermal conductivity (20% that of

alumina)•OpaqueChemical properties:•Chemically inert and corrosion resistantBiological properties:•Biocompatible

Mechanical properties:•Flexural strength 900 Mpa•Compresive strength 2000MPa•Fracture toughness 8-10 MPa · m1/2

•High fracture resistance•Wear of opposing dentition (Monolithic

Zirconia)•Difficulty in adjusting occlusion

Hydrothermal degradation of zirconia•Hydrothermal degradation of zirconia

occurs between 200-400 ˚C .

•Longer exposure times at oral temperature may also degrade zirconia leading to increased surface roughness , fragmanted grains and microcracks .

Uses of Zirconia •Protective coating on particles of TiO2

pigments.•Refractory material .• In Oxygen sensors.•Electroceramics.•Thermal barrier coating (low thermal

conductivity) in jet and diesel engines.•Ceramic knives.•Diamond simulant (cubic zirconia).

Uses of Zirconia in dentistry•Crowns and bridges•Inlays and Onlays•Veneers•Endodontic post and cores•Fillers in dental composite•Implants•Implant abutments•Orthodontic brackets•Pedodontic crowns

Zirconia as fillers in dental composite• Surface-modified

zirconia/silica with a median particle size of approximately 3 microns or less

Zirconia inlays ,onlays and crowns

Zirconia bridges

Multilayered monolithic zirconia

Zirconia CAD/CAM BlocksSoft machining Hard machining

• Mostly, they are fabricated from a porous block, milled oversized by about 25%, and sintered to full density in a 4 - 6 hours cycle.

• Alternatively, fully dense blocks are milled.

• However, this approach requires approximately 2 hours of milling time per unit whereas milling of the porous block necessitates only 30 to 45 minutes for a three-unit bridge.

Zirconia sintering Temperature•Currently available zirconia for soft

machining of dental restorations utilize final sintering temperatures varying between 1350 and 1550 ◦C depending on the manufacturer.

•The microstructure of 3Y-TZP ceramics for dental applications consists of small equiaxed grains (0.2–0.5 µm) in diameter, depending on the sintering temperature

Steps of fabrication of zirconia restorations

Digital impression

Virtual model on software

Milling of zirconia blocks

Zirconia cores

Zirconia toughened alumina ZTA• 70-90% alumina• 10-20% zironia• Toughened by a stress-

induced transformation mechanism of zirconia leading to compressive stresses within alumina.

• The strength of alumina is doubled and toughness is increase 2-4 times .

Zirconia veneers

Zirconia endodontic post and cores

Zirconia implants and abutments

Surface treatment of zirconia implants•Changing the surface chemistry using

bioactive coating with different materials (calcium phosphates, bisphosphonate, collagen, etc.)

•Optimizing surface architecture and microroughness using different techniques.

Scanning electron microscopy (SEM) observation of fibroblasts cultured on zirconia: cells grow on the whole zirconia surface, covering it with a cellular layer.

Zirconia individualized CAD/CAM implants• Austrian Surgeons have

designed a system to produce via CAD/CAM individualized Zirconium Dental Implants for use in case of immediate implant placement.

• In short, after a tooth is extracted it is scanned and a zirconium copy is milled.

• The milled copy has macro-retentive features which help secure the new implant in the extraction socket. They report a 90% success rate.

Zirconia orthodontic brackets

Pedodontic zirconia crowns

Bonding to Zirconia • One problem of zirconia application is its adhesion to

different substrates.

• Surface treatment of zirconia produces an activated surface in different applications.

• Since zirconia is resistant to aggressive chemical treatment.

• Very aggressive mechanical abrasion methods must be used to provide sufficient surface roughness.

Conventional surface treatment techniques are:

(1)acid etching with hot acids • This procedure based on corrosion–

controlled process and metallic nature of pure zirconium.

• It selectively etches the zirconia and creates micro-retentions on the surface by modifying the grain boundaries through removal of the less arranged atoms .

• Hot acid etching with combinations of highly corrosive acids (HNO3, H2SO4, and HF) improves both initial bond strengths and durability .

• Sulfuric acid in solution with hydrogen peroxide (H2O2) (Piranha solution) appeared to have a positive effect on the bonding of zirconia with resin cements .

(2)Abrasion with diamond (or other) rotary instruments (can cause microcracks)

(3)Coatings of low melting temperature porcelain micro pearls.

(4)Selective infiltration etching (SIE)•SIE uses a heat-induced maturation

process to pre-stress surface grain boundaries in ZrO2 to allow infiltration of boundaries(1) with molten glass.

•The glass is then etched out (2)using HF, creating a network of inter-granular porosity that allows nano-mechanical interlocking of resin cement .

SIE of Zirconia

(5)Fusion sputtering : • Fusion sputtering is a new technique used to

create rough zirconia surface by spraying an air-water jet carrying microscopic zirconia particles onto unsintered zirconia frameworks.

• Upon impact with the surface, the particles achieve good contact and adherence.

• After sintering, these particles become structurally fused with the underlying framework and create undercuts suitable for establishing mechanical retention with resin adhesives.

(6) Air abrasion with alumina (Mechnical)

low pressure particle abrasion (LPPA)(7)Application of different laser types(8) Plasma oxyfluoride This process presents a method to

chemically modify zirconia by creating thin oxyfluoride conversion layer on its surface that is receptive to organosilane attachment .

(9) Tribiochemical silica coating

(1o) Plasma spray (siloxane coating)

Conclusion in bonding to zirconia•The use of phosphate monomer luting

cements on freshly air-abraded zirconia is the simplest and most effective way for zirconia cementation procedure.

•These resin cements have shown good mechanical retention.

MDP resin cements• MDP-containing resin

cement continues to be a popular choice for luting ZrO2 prosthetics in clinical applications due to its low failure rate and loss of retention.

• The hydroxyl groups of the passive zirconia surface bond to the phosphate ester group of the MDP.

• 10-Methacryloyloxydecyl dihydrogen phosphate

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review.DJH.2012;4:1-153. MN Aboushelib et al.Bonding to Zirconia(A systematic

review).Open Access Journal of Dental Sciences.20164. K Nakamura et al.Zirconia as a Dental Implant

AbutmentMaterial: A Systematic Review.Int. J Prosth.2010

5. R Pandero et al .Zirconia in fixed prosthesis. A literature review . Journal Clin Exp Dent.2014;6:66-73

6. Philips’ science of dental materials.12th edition.