All-Ceramic Restorations

• All-ceramic restorations refer to advanced ceramic materials constructed with improved techniques to overcome the disadvantages of conventional porcelain.
• These restorations are indicated for inlays, onlays, laminates, crowns, and short-span bridges.

History of Ceramics

• The word "ceramic" comes from the Greek word "keramos," which means "burnt stuff".
• Fire was first used by man around 400,000 B.C.
• In 1728, Fauchard suggested the use of porcelain in dentistry.
• Duchateau was the first to make a porcelain denture in 1774.
• The first single-tooth porcelain restoration was introduced in 1884.

Ceramics

• Ceramics are compounds containing one or more metallic and non-metallic elements, commonly oxygen, carbon, or nitrogen.
• Strong atomic bonds (covalent and ionic) in ceramics offer great stability, hardness, and a high modulus of elasticity.
• The same bonding contributes to the brittleness of ceramic materials.

Conventional Dental Porcelain

• Conventional dental porcelain is a vitreous ceramic based on silica (SiO2) network and potash feldspar (K2O.Al2O3.6SiO2) or soda feldspar (Na2O.Al2O3.6SiO2).
• High fusing dental porcelain fuses at 1300°C
• Medium fusing dental porcelain fuses between 1101-1300°C
• Low fusing dental porcelain fuses between 850-1100°C
• Ultra-low fusing dental porcelain fuses at below 850°C
• Medium and high fusing porcelains are used for denture teeth, while low and ultra-low fusing porcelains are used for crowns and bridges.

Advantages of all-ceramic restorations

• Superior esthetics
• High tensile strength
• Bio-compatible

Disadvantages of all-ceramic restorations

• Possess low strength compared to PFM restorations.
• The color of luting cement affects the restoration's shade.

Contraindications for all-ceramic restorations

• Long span bridges
• Cases with increased occlusal load
• Situations where esthetic demands are not essential.

Main Cause of Failure in Ceramic Restorations

• Incomplete fusion of ceramic particles during sintering results in surface cracks.
• Moisture increases flaw size and crack initiation, leading to crack propagation.

Evolution of All-Ceramic Restorations

• Land introduced porcelain jacket crowns (PJC) in 1886.
• MacLaean & Hugh developed an inner core of alumina porcelain containing 40%-50% alumina crystal in 1965 to block crack propagation.
• Resulting in restorations approximately twice as strong as traditional PJCs.

Classification of All-Ceramic Systems

• All ceramic systems can be classified based on microstructural classification and fabrication methods.
• The concept of etching ceramic with hydrofluoric acid and then bonding it to enamel with a luting resin medium introduced in the early 1980s.

Etched or Bonded Ceramic Materials

• Glass-based systems (glass matrix + filler particles) are susceptible to hydrofluoric acid etching.
• Traditional hand-stacked feldspathic porcelain
• Leucite reinforced glass ceramic restoration
• Lithium-disilicate reinforced glass ceramic restoration can be etched or bonded.

Non-Etchable All-Ceramic Restorations

• Non-etchable all-ceramic restorations are made with high-strength core material and veneered with weaker, more translucent porcelain.
• Alumina reinforced ceramic
• Zirconia reinforced ceramic
• Combination of Alumina & Zirconia can be used.

Ceramics as Restorative Materials: Issues

• Brittle, leading to catastrophic fracture.
• Causes abrasive wear of opposing tooth structure.

Factors Affecting Strength of Dental Ceramics

• Brittleness is due to the covalent and ionic nature of atomic bonds.
• Limited capacity to distribute localized stresses at nominal temperatures.
• Inherently fragile in tension.
• Low critical strain, withstanding only about 0.1% deformation before fracture.
• Failure occurs with little or no plastic deformation.

Effect of Flaws on Ceramic Strength

• Decrease in strength is due to the presence of flaws (microcracks).
• Two populations of flaws: fabrication defects and inherent flaws.
• Fabrication defects occur during processing, such as grinding damage, polishing, or pores.
• Inherent flaws are microcracks around large grains with unmatched thermal expansion properties and pores developed during firing.
• The surface integrity of ceramic restorations significantly affects their longevity.
• High-strength ceramic with a badly flawed surface may perform worse than weaker ceramic with a flaw-free surface.
• Repetitive light loading results in fluctuating stresses and strains, enlarging flaws to a critical dimension and resulting in slow crack propagation.
• Moisture aggravates crack propagation, leading to static or delayed fatigue.
• The oral environment affects the silicone-oxygen bond due to moisture, water in saliva, and temperature.

Mechanisms of Strengthening Dental Ceramics

• Development of residual compressive stresses
• Interruption of crack propagation with dispersion of a crystalline phase
• Design of dental ceramic restorations

Development of Residual Compressive Stresses

• Chemical strengthening or ion exchange technique involves exchanging larger potassium ions with smaller sodium ions.
• The potassium ion is about 35% larger, creating large residual compressive stresses on the surface.
• This process is best used on the internal surface to protect it from grinding and acids.
• Thermal tempering involves rapidly cooling the surface, creating a rigid glass skin around a soft core.
• As the molten core solidifies and shrinks, it creates residual tensile stresses in the core and compressive stress within the outer surface.
• Glazing forms a low-expansion layer on the surface at high temperature.
• During cooling, the low-expansion glaze places the ceramic surface in compression, reducing the depth and width of surface flaws, which effectively reduces crack propagation.

Interruption of Crack Propagation

• Dispersion of a crystalline phase helps manage crack growth because glass is the weak component.
• During firing, the glass melts and flows around the crystals, forming an ionic bond between the glassy matrix and the crystals.
• Fracture lines must pass through both phases or go around the crystals.
• This restricts flaw size and increases toughness proportionally to the amount of dispersed phase.

Design of Dental Ceramic Restorations

• Ceramic restorations should be used with caution on posterior teeth due to large tensile stresses generated by occlusal forces and bruxism.
• Large tensile stresses can also occur on anterior teeth with great vertical overlap (overbite) and moderate horizontal overlap (overjet).
• Stress raisers are discontinuities in ceramic structures that cause stress concentration.
• Abrupt changes in shape or thickness make ceramic restorations prone to failure.
• Preparations need rounded internal line angles.
• Carefully adjusted occlusion is needed to produce contact points rather than contact areas.

Classification of All-Ceramic Systems Based on Processing Technique

• Conventional powder-slurry ceramics
• Castable glass ceramics
• Machinable ceramics
• Pressable ceramics
• Infiltrated (slip-casting) ceramics
• Thermal sprayed ceramics

Conventional Powder-Slurry Ceramics

• These are supplied as powders mixed with water to form a slurry, built up in layers on a die.
• Powders are available in various shades and translucencies with characterizing stains and glazes.
• Examples include Optec HSP, Vitadur-N, NBK 100, Hi-Ceram, Mirage II, and Duceram LFC.

Castable Glass Ceramics

• Polycrystalline solids obtained by controlled devitrification of glasses, including nucleating agents.
• Various crystalline phases can nucleate and grow within the glass, depending on its composition.
• Dental restorations are cast using the lost-wax technique.
• Examples include Dicor and Cerapearl.

Machinable Ceramics

• They are supplied as ceramic ingots used in CAD-CAM procedures.
• They do not require further high-temperature processing.
• Eliminating potential inaccuracies and minimizing cross-infection.
• Marginal fit is typically 100 microns or more despite high cost and extra chairside time.
• Examples include CEREC, CELAY, and PROCERA.

Pressable Ceramics

• Supplied as ceramic ingots, melted at high temperatures, and pressed into a mold using the lost-wax technique.
• The pressed form is either made to full contour or used as a substrate for feldspathic porcelain buildup.
• Examples include IPS Empress, Cerestore, and IPS Empress 2.

Infiltrated Ceramics

• Supplied as two components: a powder (aluminum oxide or spinel) fabricated into a porous substrate and a glass infiltrated at high temperatures.
• Veneered using the conventional feldspathic porcelain technique.
• Examples include InCeram, InCeram SPINELL, and InCeram ZIRCONIA.

Thermal Sprayed Ceramics

• A highly dense alumina core is produced by spraying fine alumina crystals onto a refractory die through an oxygen/acetylene flame.
• Alumina particles melt and "splat-form" on the die model.
• Each particle contracts before the next lands, resulting in cores that do not shrink during sintering at 1170°C.
• An example is TECHCERAM.

Conclusion

• Selecting the appropriate all-ceramic system for clinical use requires familiarity with the differences between systems.
• No currently available restorative system is an ideal replacement for natural tooth structure.