Ceramics

Question 1

Examples of application of ceramics in dentistry

Answer 1

• Indirect restorations
• Crowns
• Denture teeth
• Inlays
• Onlays
• Veneers
• Implant components
• Fixed partial prostheses

Question 2

Definition of a ceramic

Answer 2

• Inorganic, non-metallic materials which are compounds formed between metallic elements and non-metallic elements
• For example, silicon-oxygen in silicate glasses
• Can be crystalline or non-crystalline (glassy or amorphous)

Question 3

Bonding that occurs in ceramics

Answer 3

-Generally ionic and covalently bonded materials

Question 4

Porcelain v Ceramic

Answer 4

• Porcelain is a subset of ceramics
• Porcelain is a ceramic but not all ceramics are porcelains
• Porcelain refers to a specific composition of kaolin (hydrated aluminosilicate), quartz and feldspar (calcium, sodium and potassium aluminosilicates) exposed to high temperatures during formation

Question 5

Types of ceramics used in dentistry and the common ones nowadays

Answer 5

-Silicate glasses
Entirely non-crystalline
Used as glazes only

-Porcelains
Predominantly non-crystalline
Used for veneers
Can also use glass ceramics

-Glass ceramics (mixture of crystalline and non-crystalline phases)
Used for inlays only as crowns and anterior bridges

-Highly crystalline materials (polycrystalline- used for crowns, bridges and implant materials)

Glass ceramics and highly crystalline materials most common nowadays

Question 6

Properties of Dental Ceramics

Answer 6

• Do not readily react with most liquids, gases, alkalis and weak acids
• Remain stable over long periods of time
• Can exhibit strength but their fracture toughness is much lower than metals
• High hardness (can causes wear in opposing dentition)
• Optically favourable for cosmetic applications

Question 7

Why are ceramics getting increasingly used nowadays rather than all metal/metal ceramics

Answer 7

• Aesthetics

- Cost and NHS (gold, palladium and platinum are very expensive)

Question 8

Describe the structure of the glassy phase of dental ceramic

Components of structure and names

Answer 8

• Glass= disordered structure
• Network of silicon and oxygen is drawn in a fairly random way
• Silicon oxygen (silicate) characterized by a tetrahedra (SiO4)
• Silicone known as the network former
• Alkali cations such as potassium or sodium can be added to disrupt the silicate chains to modify sintering temperatures or properties such as thermal expansion coefficient
• Known as network modifiers
• Atoms such as tituanium, aluminium, zirconium can act as an intermediate by both network forming and modifying actions
• Both ionic and covalent bonds

Question 9

Difference in structure and energy required between glassy and crystalline phases using silicon and oxygen as an example.

Answer 9

Disordered structure is glassy, amorphous state
Crystalline structure known as quartz is highly ordered with repeating unit cells

Greater energy is required to disrupt ordered systems- so increasing the crystallinity of a dental ceramic enhances the mechanical properties

Question 10

What happens to a ceramic when you increase the crystallinity of it

Answer 10

-As you increase the crystallinity of a material,
You increase the energy required to disrupt the system
Increase the fracture toughness
Increase the strength
Increase the opacity
Increase the shrinkage after sintering

Question 11

Explain the brittle nature of ceramics

Answer 11

-Theoretically ceramics real strength is massive

But

• Inheritently weakened by any defects within it
• The bigger the defect, the greater strength reduction

Brittle- when you load a material and it fails with very little deformation up to that point

• Very little plastic deformation occurs at failure
• Failure occurs rapidly

Question 12

Clinical significance of fracture toughness of ceramics

Answer 12

-If a ceramic crown does not fit, so you use a bur to alter the shape or size, you are adding defects into the structure which greatly reduces its strength

Question 13

Fracture Toughness definition and ceramics ft

Answer 13

• Measure of the resistance to crack growth under a state of tensile stress
• Ceramics have a low to moderate fracture toughness
• Brittle material
• Less time for cracks to propagate

Question 14

Tensile Strength of ceramics

Answer 14

• Theoretical tensile strength (amount of energy to pull atoms apart) of ceramics is extremely high
• But ceramics are never defect free, and these defects determine the actual strength of the material
• Strength varies with specimen size, shape, loading rate, surface prep and environment
• Dental ceramics restorations fracture under tensile loading

Question 15

Feldspathic Ceramics (Porcelain) definition

Answer 15

• Mainly glassy with a minor crystalline component
• Optically the best
• Feldpar KAlSi3O8 is the main component
• When feldspar melts it forms leucite and molten glass
• Upon cooling, the separate structure of leucite and glass remains
• Flexural strength of 60-80MPa
• Closely matches shade translucency and luster of natural dentition
• But low strength and prone to mechanical failure

-Addition of other crystal phases can increase the mechanical performance

Question 16

How glassy ceramics are made and examples

Answer 16

• Crystalline reinforcement of porcelains
• Leading to mechanical toughening
• Distribute higher strength crystals throughout the glassy matrix results in the crack having to go around the crystals in order to propagate
• Crystal phase must have compatibility with the glass to be effective (If not, then the crystal will serve as a defect itself)

-Crystal phases used to reinforce the ceramic

• Leucite Reinforced Ceramics
• Lithium disilicates

Question 17

Issue with increasing crystallinity in ceramics

Answer 17

• Increase in shrinkage during sintering, which affects dimensional accuracy during fabrication
• Decrease transparency

Question 18

What properties of the crystal will determine the mechanical and optical properties

Answer 18

• Crystal size, type and amount

- Important to match refractive index of the crystal and amorphous phase for translucency

Question 19

Different manufacturing methods of ceramics

Answer 19

• Sintering
• Heat-pressing
• Slip-casting (archaic so rarely used)
• CADCAM

Question 20

Sintered All-Ceramic Restorations explanation of procedure and common examples

Answer 20

• Most aesthetic and oldest ceramics were sintered
• Final object is formed by first pressing the constituents in powdered form under pressure into a mold, and then firing the pressed object at high temperatures
• Often shaped on a die
• Final ceramics are typically veneered in a translucent porcelain and glazed (heat treated) to improve aesthetic

-Examples of crystalline reinforced sintered dental ceramics include:
Alumina-based (up to 40% crystalllinity)
Leucite Reinforced Felspathic

Question 21

Sintered Alumina-Based Materials properties, requirements and problems

Answer 21

• Alumina is aluminium oxide, Al2O3
• Added in 40-50% by weight
• Excellent bond with the glass phase
• Coefficient of thermal expansion between glass and alumina phases should be closely matched
• Alumina ceramic is formed by dry-pressing and then sintering
• Shrinkage of 10-20% can occur during sintering
• Flexural strength of 600mPa

Shrinkage during sintering leads to dimensional inaccuracies
Opacity of the core requires veneering with glassy materials to improve aesthetics
Rarely used system nowadays
Competitor is leucite reinforced ceramics

Question 22

Sintered Leucite-Reinforced Materials structure and properties

Answer 22

• Potassium, aluminium, silicone and oxygen K(AlSi2O6)
• Increased leucite content over traditional feldspathic porcelain which increases the flexural strength to over 100MPa
• Leucite crystals have up to a 3-fold larger thermal coefficient of expansion compared to the glassy phase
• During cooling, the leucite contracts more than the glass resulting in residual stresses. These resuldual stresses act as a crack deflecting mechanism and increase the strength

-Highly aesthetic- has been used extensively for anterior crowns and inlays

Question 23

Heat pressing of all-ceramic restorations

Answer 23

• Simultaneous application of both heat and pressure to form and sinter the ceramic
• Employs a lost wax process
• Contrast with sintering where constituent powders are first pressed and then fired
• Pressures in range of 0.3-0.4mPa for 10-20 minutes
• Temperature depends on the ceramics used

Question 24

Why is heat pressing used over classical sintering. Examples of ceramics and how can you optimise properties

Answer 24

• Improves crystal dispersion
• Can obtain higher crystallinity
• Decreases crystal size
• Control dimensions of the material if it is pre-sintered as a lump and then heat pressed into a desired shape
• Final heat-pressed ceramic is veneered with translucent porcelain
• Leucite-Reinforced ceramics
• Lithium disilicated-based ceramics

Question 25

Heat-Pressed All Ceramic Leucite-Reinforced Materials and advantages of sintered

Answer 25

- Ceramics reinforced with K[AlSi2O6] in amounts between 35-55% by volume - In contrast with 45% achievable using sintering - 120MPa is the flexural strength - Increase over the strength of sintered leucite-reinforced ceramics - Reduced operator induced variability compared with sintered leucite ceramics - Improved strength as a result of increased crystallinity and improved dispersion - Similiar to sintered leuicte-reinforced, the coefficient of thermal expansion mismatch between crystal and glass phases creates a crack-deflecting mechanism

Question 26

Heat Pressed Lithium Disilicate Materials definition, uses and properties Alternative name at Guys

Answer 26

-Can only be used anteriorly but not posteriorly -Lithium disilicate, Li2Si2O5, is the major crystal phase up to 65% by volume -Flexural strength 300-400MPa -Similiar to leucite reinforced, crystal phase has a coefficient of thermal expansion mismatch with the glassy phase, creating a crack-deflection mechanism -Crack deflection is further improved by the interlocking of elongated crystals IPS e.max

Question 27

Slip-Cast All Ceramic Restorations examples

Answer 27

- Interpenetrating phase composite - Ceramic frit layer is first condensed on a refractory die or a layer is machines from a preformed block - Glass phase is then drawn into the porous ceramic at high temperatures after the crystal phase has been sintered - Alumina-based - Spinel-based - Zirconia-toughened alumina - Much less common now

Question 28

CAD-CAM All-Ceramic Restorations definitions, classification disadvantage

Answer 28

- Desired restoration (tooth or teeth) is scanned to obtain a digital recostruction (CAD) - Then imported to a milling device for machining of a bulk ceramic material (CAM) - Disadvantage can manifest through the use of machining with a brittle material - Bulk ceramic materials are either sintered or partially sintered in a block format, which are then machines to the final shape - Hard Machining: when ceramic is fully sintered before machining (either fully crystallized or partially crystallized) - Soft Machining: machining is performed on a partially sintered ceramic (Full sintering occurs after machining)

Question 29

Examples of fully sintered materials being used in CAD-CAM restorations and properties of these Advantages of fully sintered

Answer 29

-Feldspar Concentration on 30% by volume Flexural strength of 120MPa -Leucite-Reinforced 35-55% leucite by volume Very similiar form to heat-pressed leucite-reinforced ceramics 120MPa flexural strength -Lithium Disilicate Can be machined either fully or partially crystallized If partially crystallized, the structure contains both lithium metasilicate (Li2SiO3) and disilicate (Li2Si2O5) Crystallization procedure post machining will depend on concentration of lithium disilicates 360MPa typical flexural strength Hard-Machining CAD-CAM restoration production allows for rapid production of restorations in-clinic (hours) with more practical equipment as compared to other manufacturing methods

Question 30

CAD-CAM partially sintered materials examples and properties

Answer 30

-Alumina High purity aluminium oxide used (99%) Designed and manufactured in an oversized form to accommodate for shrinkage during sintering Flexural strengths (500-650MPa) Opaque to requires a veneering layer (feldspathic or glass ceramic) -Zirconia Tetragonal zirconia polycrystals are partially stabilized with yttrium Highest flexural strength of all dental ceramics, randing from 900-1500MPa Straying from perscribed surface/heat treatments of yttrium can alter structure and properites

Question 31

Mechanical toughening of Zirconia

Answer 31

- Phase Transformation Reinforcement - Specific to ZrO2 ceramics (zirconia) - Tetragonal phase zirconia is only stable between 1170-2370 Degrees C - Yttria oxide (Y2O3) or cerium oxide can stabilize zirconia in its tetragonal form - When stabilized tetragonal phase zirconia reaches a threshold stress, it undergoes a transformation to monoclinic phase zirconia which includes a volume increase - The volume increase results in compressive stresses around the crystal, which stabilizes the crack

Question 32

Ceramic-Metal Restorations definition and structure of layers and process

Answer 32

-Restoration formed by at least 2 layers of ceramic on a metal framework -Layer 1: Ceramic with opacifying oxides Critical for bonding to the metal structure and aesthetically hiding the metal -Additional Layers: Application of more translucent porcelain for size and colour matching After additional layers are added, the restoration is vibrated and slowly dried to condense the ceramic - The restoration is then vibrated and slowly dried to condense the ceramic - Restoration is then sintered under vacuum at 0.1atm

Question 33

Importance of vacuum when sintering

Answer 33

-Decreases porosite of ceramic

Question 34

Requirements of materials involved in a | ceramic-metal restoration

Answer 34

- Metal must have a melting temperature significantly greater than the firing temperature used for the ceramic. Metal cannot melt during application of the ceramic to metal core - Veneering porcelain should have a low temperature of fusion to eliminate distortion during sintering - Base porcelain must be able to wet the alloy adequately for proper ceramic-metal adhesion - Strong adhesion between the ceramic and metal- achieved primarily through oxides on the metal surface (chemical adhesion) and increased metal surface roughness (mechanical adhesion) - Coefficients of TE must be compatible. Metal typically designed to have a slightly higher coefficient of TE to place the ceramic in a state of compression upon cooling - High elastic modulus desirable to mitigate stresses and strains within the weaker porcelain - Metal shape should not change during the firing cycle of the cycle. Prolonged exposure of metals to heightened temperatures can cause changes in shape which can reduce the fit with ceramic - Design of shape is critical to reduce regions of stress concentrations

Question 35

Uses of Ceramics in indirect restorations and the possible ceramic options for each and INDICATIONS

Answer 35

-Porcelain Laminate Veneers Dental porcelain or glass ceramics Thin facings to correct colour, shape and position -Ceramic Inlays and Onlays Direct composite often used nowadays as less invasive Onlays replacing cusps Glass Ceramics or Dental porcelain -Glass Ceramic crowns and bridges Anterior crowns extending to pre molars Anterior bridges Glass ceramics (leucite or lithium disilicate) ``` -High strength core veneer crowns and bridges Anterior and posterior crowns Posterior bridges 3-unit Alumina Core or Zirconia Core with veneered dental porcelain ``` -Monolithic high strength crowns and bridges All crowns 3-4 posterior bridges -Porcelain fused to metal crowns and bridges All crowns Anterior and posterior bridges Precious and non-precious alloys with dental porcelain

Question 36

Indications and classification of dental porcelains

Answer 36

- Porcelain laminate veneers - Inlays - Onlays - Aesthetic veneering layer on high strength ceramic or metal cores - Fabrication of porcelain denture teeth Classified according to the temperature they are sintered/fired at: - High fusing: 1300-1370 for denture teeth and aluminous core porcelain - Medium fusing: 1093-1260 for inlays and porcelain jacket crowns - Low fusing: as a veneering material for porcelain fused to metal crowns The reason you want a low fusing porcelain is so you don't damage the material that you're veneering to

Question 37

Dental Porcelain Application steps

Answer 37

-Restorations are made using a compaction/condensation process: 1)Porcelain powder is mixed with water to form a paste 2) Paste is applied to a die (either a refractory material or platinum foil) 3) Fabricated from multiple powder to recreate the aesthetic features 4) Opaque powders are used to mask discoloured tooth cores 5) Dental shade are used to mimic the optical properties of dentine 6) More translucent enamel layers used to mimic enamel 7) As each layer is added the paste is condensed (vibrating and water removal) Condensation aims to maximise particle density to minimise firing shrinkage Optimisation of particle shape and size help the condensation process 8) At the end of condensation, the condensed powder is known as being a 'Green State' Then the porcelain is fired

Question 38

Firing of Porcelain Steps following compaction/condensation stage

Answer 38

1) Initially, heating is slow to drive off excess water (if you hear too quick, steam causes cracks) 2) Starch binder which may have been used is burnt out leaving no residue 3) Initial porcelain fusion occurs at point contacts between particles 4) Once the initial porcelain fusion has occured the object will hold its shape and is known as being in a 'bisque state' 5) As temperature increases, more fusion takes place and molten glass flows drawing particles closer together and closing down voids to result in a non-porous material 6) Most shrinkage occurs at this stgae 7) Excessive heating leads to undesirable pyroplastic flow and the restoration will lose its form. Surface will also look highly glazed 8) Following sintering, slow cooling is required to minimise the generation of thermal residual stresses which may cause fracture 9) Vacuum furnaces are used during the whole firing process to reduce porosity 10) Porosity reduces strength and increases opacity Porcelain is then glazed

Question 39

Glazing of porcelain steps after furnace and significance

Answer 39

- Required to generate a smooth surface by filling in surface pores - Reduces biofilm formartion - Use glasses that fuse at low temperatures, either applies as a spray or liquid paint - Final firing of the crown under carefully controlled conditions allows for the glaze to adapt to the surface, fill any surface porosity without associated changes to the dimensions of the restorations

Question 40

Metal-ceramic restorations definition and failure

Answer 40

- AKA porcelain fused to metal crowns - Overcomes the susceptibility of relatively weak ceramics failing due to fracture - Require application of a dental porcelain or glass ceramic to a metal core (known as coping) - Coping is made to closely adapt to the prepared tooth and the porcelain is applied to create a desired cosmetic outcome - Metal cropping covered by an opaque layer, covered by a gingival shade apically and a body of dentine coronally, and then a more translucent enamel shade superficially Most likely failure is breakdown of the interface leading to chipping and delamination

Question 41

Strength of Metal-Ceramic Interface depend on

Answer 41

- Mechanical retention (surface of the metal roughened using burs or airborne particle abrasion) - Chemical bonding during ceramic sintering (fusion between metal oxides at the metal surface and oxides in the ceramic) the bonding alloy must contain oxide forming elements for this to happen - Coefficient of thermal expansion of ceramic and the underlying material

Question 42

Importance of the correct coefficient of thermal expansion between the ceramic and metal and what happens with too big or too little differences

Answer 42

- Correct coefficient of thermal expansion between the ceramic and a metal - Metal and porcelain chosen to have a slight mismatch is coefficient of thermal expansion - If mismatch is too great, then the ceramic will crack - If too little, then the ceramic will not be reinforced - Metal must have a higher coefficient of TE than the ceramic - On cooling after sintering, the metal cools at a quicker rate leading to generation of compressive stresses in the ceramic at the interface - If ceramic fails under tensile loading, the ceramic being placed in compressive stress means that a greater amount of energy is required for a crackt o propagate, thereby strengthening the ceramic itself

Question 43

Difference between precious and non-precious copings in MCCs and which do we not use more

Answer 43

-Precious: Rare, naturally occuring metallic elements that are often high of economic value High palladium, high gold alloys Nowadays we use non-precious

Question 44

Choosing the correct alloy for MCCs and different options

Answer 44

Melting temperature of the metal must be sufficiently higher than the sintering temperature of the chosen porcelain 1) High gold alloys for MCC restorations Contains Pt or Pd to raise the melting temperature Small fractions of oxide forming metals eg. Sn are added to increase chemical bonding potential to the ceramic High cost of gold has reduced their usage considerably 2) Gold-Palladium alloys Contain typically 50% gold with the balance Pd and Ag Have similiar casting accuracy and common resistance to gold ``` 3) Palladium-Silver alloys Stiff (high elastic modulus) Tendency to sag on porcelain firing 30% silver with 60% palladium Ga, Zn, Cu and In as the balance More difficult to cast than gold containing alloys ``` 4) Ni-Cr and Co-Cr V stiff Thin copings (prevents restorations from overcontouring) Stiffness good for long span metal ceramic bridges Very Difficult to cast Made via CADCAM nowadays

Question 45

First high strength cores used, why they were invented and why are they no longer used

Answer 45

- Increased fracture toughness and tensile strength and control excessive shrinkage associated with sintering compared to porcelains - Demonstrated that dispersion of high strength and elastic modulus alumina crystals within the glassy matrix of conventional feldpathic porcelain resulted in improved fracture toughness - But due to poor aesthetics and difficulties with varying shades associated with the increase in crystallinity, it was decided to add a layer of matched thermal expansion, low fusing feldspathic veneering porcelain to cover the cores appearance - Allowing for all ceramic posterior crowns - Major issue was matching the veneering ceramic with the core material to prevent chipping of the veneer - Initially maximum dispersion strengthening with alumina in the core was about 40%- beyond this there was excessive shrinkage on sintering and so the restoration had a poor fit

Question 46

2 types of modern high strength core-veneer restorations Explanation of the procedure of the less commonly used one

Answer 46

-High purity alumina cores Indicated for both anteriorly and posterior crowns Die was digitized and then a new oversized die is created to avoid issues with shrinkage (predictive shrinkage) Pure alumina is dry pressed to the oversized die Sintered and shrinkage results in desired shape and size Veneer with matched porcelain for aesthetics Nowadays, pure alumina blocks are milled with CADCAM and then sintered Provide strengths of around 600MPa, but less commonly used than zirconia -Zirconia

Question 47

Zirconia restorations structural phases and properties of each phase

Answer 47

- Zirconium oxide (Zirconia) is a crystalline material with 3 structural phases - Cubic, Tetragonal and Monoclinic - Strength of Zirconia is dependent on the dominant phase, which is dependent on the temperature -Monoclinic (lower strength, occurpies a larger volume) -Tetragonal (highest strength, reduction in volume) -Cubic (moderate strength, lowest volume) with increasing temperature

Question 48

Why do we have to modify Zirconia for use in dentistry and how do you do it

Answer 48

- Zirconia in a fully crystallized form is too hard to machine effectively - Needs to be machined while partially sintered (green state) and then sinter it - During cooling from sintering a transformation occurs between the tetragonal structural phase to the monoclinic phase between 670 and 1070 degrees - Associated with an expansion of 3-4%, which would generate stresses which would crack the object - So you cannot use pure zirconia - Overcome by using a stabilising agent (Yttrium oxide) - Allows maintenance of the tetragonal form at oral temperature - Yttrium stabilised zirconia exhibits excellent properties- high strength and fracture toughness, high hardness and excellent chemical resistance

Question 49

How much should you stabilize ziroconia and effect of too much/too little

Answer 49

- If you fully stabilise you end up in a cubic form, but you want a tetragonal form - Partial stabilisation with 2mol%to3mol% keeps it in a tetragonal form

Question 50

What properties of yttrium are important in formation of yttrium stabilsied zirconium

Answer 50

-Formation of structure is highly dependent on temperature, yttrium content and grain size (must be less than 0.8mm)

Question 51

Phase transitioning toughening of Y-TZP

Answer 51

-Zirconia is able to exhibit phase transformation toughening - Fracture toughness is the resistance of rapid crack propagation - Under stress such as crack propagation within the restoration, the tetragonal grains can be transformed to monoclinic - Associated with a 3-5% increase in volume, leading to compressive stresses generated which close the crack and prevent its extension

Question 52

Problems with Y-TZP in terms of structure and how do we overcome them

Answer 52

- Properties are dependent on being able to maintain the tetragonal phase - Susceptible to an ageing process which diminishes fracture toughness and fracture resistance - Degradation of mechanical properties in presence of water - Slow surface transformation to a monoclinic stable phase - Begins at the surface and nucleates and cascades leading to micro cracks Can reduce LTD by: - Reducing the particle size - Reducing the amount of stabilizing oxide - Excellent surface finishing reducing the initial defect population - Forming composite materials with alumina

Question 53

Problem with zirconia-core restorations and how is it overcome

Answer 53

-Chipping of the veneer layer due to: mismatch in elasticity of core and veneer ceramics Difficulties in effectively matching coefficients of TE Limited chemical bonding (compared with bonding to metal oxides) Poor framework design Monolithic zirconia (made from a single zirconia substrate has been introduced)

Question 54

Monolithic zirconia explanation and how produced compared to normal zirconia

Answer 54

- Single zirconia substrate - no veneering of porcelain Reducing the % crystallinity and altering the grain size which leads to improved translucency at a cost of reduced toughness and strength) Application of surface staining to add characterisation (incorporated into glazing)

Question 55

All the different types of indirect restorations used by ceramics and how they are cemented onto the tooth structure

Answer 55

-Porcelain Laminate Veneers Adhesive Cementation HF acid etch, silane coating and resin cementation -Ceramic Inlays and Onlays Adhesive cementation possible and provides better clinical outcomes GIC cements -Glass Ceramic crowns and bridges Adhesive cementation possible and provides better clinical outcomes GIC cements Sometimes luting cements eg Zinc phosphate but clinical outcome poorer -High strength core veneer crowns and bridges Adhesive cementation challenging but there are methods Alternatives are GIC cements and luting materials including zinc phosphate -Monolithic high strength crowns and bridges Adhesive cementation challenging but there are methods Alternatives are GIC cements and luting materials including zinc phosphate -Porcelain fused to metal crowns and bridges GIC, RMGIC, zinc phosphate, zinc polycarboxylate cements

Question 56

Why is adhesive bonding to ceramics useful

Answer 56

- Increases retention - For porcelain and glass ceramics it can increase the fracture resistance of a restoations - Often need to repair minor chips on veneering porcelain where the core is intact

Question 57

Bonding to porcelains and glass ceramics

Answer 57

- Generate micro-roughness through acid (HF) etching which provides micro mechanical interlocking for the resin cement - SIlane coupling agent (3-MPS) links inorganic silica to the organic polymer of the resin-based material - Providing covalent bonding

Question 58

Bonding to zirconia

Answer 58

- Bonding to zirconia and aluminate is challenging - Resistant to HF acid etching/no silica network for silane bonding 1) Microroughness produced using air abrasion or surface grinding 2) Chemical bonding either by using tribochemical coating (form of air abrasion using silica particles or silica coated alumina particles which through contact forced bond silica to the YTZP surface. Silane coupling agent is then used to bond to this layer or Following particle air abrasion use a phosphoric acidic monomer metal primer Has a phosphate group which binds to aluminium oxides and zirconia oxides at one end and a methacryl group which co-polymerises with monomer from the resin cement -Despite these strategies the bonding potential of zirconia and high purity alumina cores is considerably inferior to etched glass-ceramics

Question 59

Wear associated with dental ceramics

Answer 59

-Compared with other dental restorative- the wear resistance of all classes of dental ceramic used today is very high -However due to the relative hardness of the ceramic compared with enamel and dentine, teeth opposing ceramic restorations can be subject to excessive wear -This is worse if: The glaze is lost over time High hardness ceramics Following adjustments to the surface leaving a roughened ceramic surface (occlusal adjustments must be highly polished following machining and ideally re-glazed if the crown has not been fitted)