Dental Ceramics / Dental Porcelain

25-mark university answer

Contents

1. Definition
2. Ideal requirements
3. Composition of dental porcelain
4. Classification
5. Types of dental ceramics
6. Fabrication methods
7. Properties
8. Advantages
9. Disadvantages
10. Uses / indications
11. Failure and prevention
12. Conclusion

1. Definition

2. Ideal requirements of a dental ceramic

• Natural translucency, fluorescence, and shade matching teeth
• High compressive strength and adequate flexural strength
• High hardness and wear resistance
• Good fracture toughness
• Resistance to dissolution and chemical degradation in saliva
• Biocompatibility and tissue compatibility
• Low thermal conductivity
• Dimensional stability during firing
• Coefficient of thermal expansion compatible with tooth structure or metal/ceramic substructure
• Ability to bond to tooth structure or suitable luting cement
• Smooth, highly polishable surface
• Minimal abrasion of opposing natural teeth

3. Composition of conventional dental porcelain

Functions of main constituents

Feldspar

• Potassium and sodium aluminosilicate.
• On heating, it forms a glassy phase.
• Responsible for translucency and esthetics.

Quartz

• Remains largely unchanged during firing.
• Provides strength and reduces excessive shrinkage.

Kaolin

• Gives plasticity to the unfired porcelain mass.
• Present only in small quantity in modern dental porcelains.

Leucite

• Crystalline phase within a glass matrix.
• Improves strength by resisting crack propagation.
• Raises coefficient of thermal expansion, hence is useful in porcelain intended for metal-ceramic restorations.

4. Classification of dental ceramics

A. According to fusion temperature

• High-fusing porcelains: denture teeth
• Medium-fusing porcelains: some older ceramic restorations
• Low-fusing porcelains: metal-ceramic restorations
• Ultra-low-fusing porcelains: repair porcelains and low-temperature systems

B. According to microstructure and composition

1. Glass-matrix ceramics
2. Polycrystalline ceramics
3. Resin-matrix ceramic materials

1. Glass-matrix ceramics

• Feldspathic porcelain
• Leucite-reinforced glass ceramic
• Lithium disilicate glass ceramic
• Fluorapatite glass ceramic
• Zirconia-reinforced lithium silicate

2. Glass-infiltrated ceramics

• In-Ceram Alumina
• In-Ceram Spinell
• In-Ceram Zirconia

3. Polycrystalline ceramics

• Alumina
• Zirconia, especially yttria-stabilized tetragonal zirconia polycrystal, or Y-TZP

4. Resin-matrix ceramics

• Nanoceramic resin
• Polymer-infiltrated ceramic network materials

5. Types of dental ceramics

5.1 Feldspathic porcelain

• Excellent translucency and esthetics
• Low strength and low fracture toughness
• Brittle
• Etchable with hydrofluoric acid
• Can be bonded effectively to enamel with resin cement

• Porcelain veneers
• Veneering porcelain for metal-ceramic crowns
• Veneering porcelain for zirconia frameworks
• Characterization of ceramic restorations

5.2 Leucite-reinforced glass ceramic

• Stronger than feldspathic porcelain
• Good esthetics
• Suitable for adhesive bonding
• Moderate fracture resistance

• Veneers
• Inlays and onlays
• Anterior crowns
• Selected posterior single crowns

5.3 Lithium disilicate glass ceramic

• High esthetics and translucency
• Greater strength than feldspathic and leucite ceramics
• Can be etched with hydrofluoric acid and silanated
• Suitable for adhesive cementation
• Available as pressable and CAD/CAM forms

• Veneers
• Inlays and onlays
• Anterior crowns
• Posterior single crowns
• Short-span fixed partial dentures in selected cases

5.4 Alumina ceramic

• High hardness and strength
• More opaque than glass ceramics
• Limited ability to etch and bond
• Mostly replaced by zirconia in current practice

• Ceramic copings and frameworks
• Earlier all-ceramic crown systems

5.5 Zirconia ceramic

• A crack causes tetragonal zirconia crystals to transform to the monoclinic phase.
• This transformation causes slight expansion.
• The expansion produces compressive stress around the crack.
• Crack propagation is reduced.

• Very high flexural strength
• High fracture toughness
• Excellent wear resistance
• Biocompatible
• Radiopaque
• More opaque than glass ceramics, although newer translucent zirconias have improved esthetics
• Cannot be conventionally etched with hydrofluoric acid because it contains no silica glass phase

• Posterior crowns
• Monolithic crowns
• Fixed partial denture frameworks
• Implant-supported crowns and bridges
• Abutments
• Full-arch prostheses

6. Methods of fabrication

A. Powder-liquid condensation and firing

1. A refractory die or metal framework is prepared.
2. Porcelain powder is mixed with water or a special liquid.
3. Porcelain slurry is applied in increments.
4. Condensation is done to remove water and reduce porosity.
5. The porcelain is dried.
6. Firing is carried out in a porcelain furnace, usually under vacuum.
7. Additional layers are added and fired.
8. Glazing and finishing are done.

B. Sintering

• Reduces porosity
• Increases density
• Improves strength

C. Heat pressing

• Pressable leucite ceramics
• Pressable lithium disilicate ceramics

D. Slip-casting and glass infiltration

E. CAD/CAM milling

• Lithium disilicate blocks
• Zirconia discs
• Leucite-reinforced ceramic blocks

F. Additive manufacturing

7. Properties of dental ceramics

7.1 Mechanical properties

High compressive strength

Low tensile strength

Brittleness

High hardness

Fracture toughness

• Low in feldspathic porcelain
• Higher in lithium disilicate
• Highest in zirconia

7.2 Optical properties

• Translucency
• Opalescence
• Fluorescence
• Shade stability
• Ability to reproduce enamel and dentin appearance

7.3 Thermal properties

• Low thermal conductivity
• Low thermal diffusivity
• Relatively insulating to the pulp
• Thermal expansion must be compatible with a metal or ceramic framework in layered restorations

7.4 Chemical properties

• Excellent resistance to saliva and oral fluids
• Do not corrode
• Stable colour
• Acid resistance varies with ceramic type
• Glass ceramics can be etched by hydrofluoric acid
• Zirconia cannot be effectively etched with hydrofluoric acid

7.5 Biological properties

• Generally biocompatible
• Smooth glazed surfaces show low plaque accumulation
• Well-polished ceramic margins support periodontal health
• Rough, fractured, or poorly finished surfaces may retain plaque and abrade opposing teeth

8. Advantages of dental ceramics

1. Excellent esthetics and life-like translucency
2. Stable shade with no discoloration over time
3. High compressive strength
4. High hardness and wear resistance
5. Excellent biocompatibility
6. No corrosion or metallic taste
7. Low thermal conductivity
8. Smooth glazed surface and low plaque retention
9. Radiopacity in many modern systems
10. Can be bonded to tooth structure in silica-based ceramics
11. Suitable for metal-free restorations
12. Excellent resistance to oral fluids and chemical degradation

9. Disadvantages of dental ceramics

1. Brittle and susceptible to catastrophic fracture
2. Low tensile strength
3. Require adequate tooth reduction and material thickness
4. May wear opposing enamel if surface is rough
5. Difficult to repair intraorally
6. Technique-sensitive fabrication and cementation
7. Risk of chipping of veneering porcelain in layered restorations
8. Some ceramics, especially zirconia, have inferior translucency compared with feldspathic porcelain
9. Cost may be high
10. Ceramic restorations cannot be adjusted excessively after glazing without repolishing

10. Uses and indications

Feldspathic ceramic

• Laminate veneers
• Esthetic layering porcelain
• Porcelain-fused-to-metal crowns
• Characterization of restorations

Leucite-reinforced ceramic

• Veneers
• Inlays
• Onlays
• Anterior crowns

Lithium disilicate

• Veneers
• Inlays and onlays
• Anterior crowns
• Posterior single crowns
• Selected short-span bridges

Zirconia

• Posterior crowns
• High-load crowns
• Long-span fixed partial dentures
• Implant-supported crowns
• Implant abutments
• Full-mouth rehabilitation
• Monolithic crowns in patients with high occlusal load

Metal-ceramic restoration

• Anterior and posterior crowns
• Fixed partial dentures
• Situations requiring combined strength of metal and esthetics of porcelain

11. Failures of ceramic restorations and prevention

Common failures

1. Fracture of ceramic

• Inadequate thickness
• Sharp internal line angles
• Excessive occlusal load
• Parafunctional habits such as bruxism
• Poor bonding or cementation
• Defects and porosity within ceramic

• Adequate tooth preparation
• Rounded internal angles
• Correct material selection
• Adequate occlusal reduction
• Proper bonding or cementation
• Occlusal adjustment
• Night guard for bruxism patients

2. Chipping of veneering porcelain

• Improper framework support
• Mismatch in thermal expansion
• Poor cooling after firing
• Occlusal stress

• Anatomically supported framework
• Compatible veneering ceramic
• Controlled cooling
• Avoid excessive occlusal contact on veneering ceramic

3. Wear of opposing teeth

4. Debonding

• Poor surface treatment
• Contamination
• Inadequate adhesive procedure
• Incorrect cement selection

• For glass ceramics: hydrofluoric acid etching, silane application, and resin cement where indicated
• For zirconia: airborne-particle abrasion, zirconia-compatible primer containing phosphate monomer such as MDP, and appropriate resin cement when adhesive bonding is needed

12. Conclusion

Suggested diagram for the answer sheet

Dental ceramics
│
├── Glass-matrix ceramics
│   ├── Feldspathic porcelain
│   ├── Leucite-reinforced ceramic
│   └── Lithium disilicate
│
├── Glass-infiltrated ceramics
│   ├── Alumina
│   ├── Spinell
│   └── Zirconia-alumina
│
└── Polycrystalline ceramics
    ├── Alumina
    └── Zirconia