DM 18 - DENTAL CERAMICS

Dental ceramics

Dental materials made of silicate glasses, porcelains, glass-ceramics, or highly crystalline solids. They are used because they are esthetic, corrosion-resistant, biocompatible, temperature-resistant, hard, wear-resistant, and stable over long periods.

Main properties of dental ceramics

Dental ceramics resist corrosion, do not readily react with most liquids, gases, alkalis, and weak acids, remain stable over time, and may show good to excellent strength and fracture toughness depending on their composition and microstructure.

Zirconium dioxide / zirconia

The strongest and toughest dental ceramic listed in the PDF. It has flexural strength similar to steel, but its fracture toughness is still much lower than steel.

General characteristics of dental ceramics

Dental ceramics are biocompatible, temperature-resistant, esthetic, refractory, hard, wear-resistant, and chemically stable, but they have low to moderate fracture toughness and are susceptible to tensile fracture.

Ceramic susceptibility to tensile fracture

A major limitation of ceramics because ceramics are brittle and tend to fail when tensile stresses concentrate at flaws, sharp angles, thin areas, or unsupported regions.

Applications of ceramics in dentistry

Dental ceramics are used for inlays, onlays, veneers, crowns, and fixed dental prostheses or FDPs.

Parameters for classifying dental ceramics

Dental ceramics may be classified by use or indication, composition, principal crystal or matrix phase, processing method, firing temperature, microstructure, translucency, fracture resistance, and abrasiveness compared with enamel.

Classification by use

Dental ceramics may be indicated for anterior crowns, posterior crowns, veneers, post and core systems, fixed dental prostheses, ceramic stains, and glazes.

Classification by principal crystal or matrix phase

Ceramics may be classified as silica glass, leucite-based feldspathic porcelain, leucite-based glass-ceramic, lithium disilicate glass-ceramic, aluminous porcelain, alumina, glass-infiltrated alumina, glass-infiltrated spinel, glass-infiltrated alumina-zirconia, or zirconia.

Classification by processing method

Dental ceramics may be processed by casting, sintering, partial sintering and glass infiltration, slip casting and sintering, hot-isostatic pressing, CAD-CAM milling, copy milling, and machining or grinding of pressed powder blocks.

Classification by firing temperature

Dental ceramics may be ultralow fusing, low fusing, medium fusing, or high fusing, depending on their sintering temperature range.

Classification by microstructure

Ceramic microstructure may be amorphous glass, crystalline, or crystalline particles dispersed in a glass matrix.

Classification by translucency

Dental ceramics may be opaque, translucent, or transparent, which affects esthetic use and case selection.

Metal-ceramic system

A system where ceramic is used to veneer a metal framework. This is also called a porcelain-fused-to-metal or PFM prosthesis.

All-ceramic system

A ceramic prosthesis made without a metal framework. These systems rely on ceramic strength, bonding, thickness, and proper design to resist fracture.

Two main categories of dental ceramics

Dental ceramics are divided into ceramics used to veneer metal frameworks for metal-ceramic/PFM prostheses and ceramics used to produce all-ceramic prostheses.

PFM crown structure

A PFM crown contains a metal coping/framework with porcelain layers such as opaque porcelain, body porcelain, incisal porcelain, cervical porcelain, surface stain, and transparent porcelain, as shown in the schematic illustration on page 1.

Major advantage of PFM restorations

The greatest advantage is fracture resistance, because the metal framework strengthens and supports the porcelain.

PFM fracture rate

One clinical study cited in the PDF reported a fracture rate as low as 2.3% over 7.5 years for metal-ceramic crowns and bridges made from high noble alloy.

Metal occlusal surfaces in PFM

Using metal on occlusal surfaces in posterior PFM restorations can further reduce fracture rate and reduce wear of opposing enamel compared with ceramic occlusal surfaces.

PFM tooth reduction advantage

Less tooth structure may need to be removed if designs use thin noble foil metal, metal occlusal surfaces, lingual metal cusps, or porcelain butt-joint margins on facial or buccal surfaces.

PFM endodontic access advantage

Endodontic access is easier through PFM restorations compared with some all-ceramic restorations because the metal-ceramic structure is more predictable to access and repair.

PFM repair advantage

Temporary repairs can be made for ceramic fractures in metal-ceramic prostheses.

PFM disadvantages

Disadvantages include possible metal allergy, inferior esthetics for single maxillary anterior teeth, and possible dark facial margin if gingival recession exposes a metal collar or metal margin.

Metal allergy in PFM

Metal allergy is possible but rare, except it may be more relevant when nickel-containing alloys are used.

Dark line at PFM facial margin

A dark line may appear when gingival recession exposes a metal collar or metal margin. This is an esthetic concern, especially in anterior teeth.

Managing dark PFM margins

The dark margin can be reduced by using a ceramic margin or a very thin knife-edge metal margin about 0.1–0.5 mm veneered with opaque shoulder porcelain.

Ceramic margin in PFM

A ceramic margin improves esthetics but must be polished and/or glazed to avoid a rough surface at the margin.

Feldspathic porcelain

A dental porcelain that contains a glass matrix and one or more crystal phases. Its main components include silica glass, alumina, potash, and soda.

Feldspathic porcelain composition

By weight, feldspathic porcelain contains approximately 52–65% SiO₂ silica glass, 11–20% Al₂O₃ alumina, 10–15% K₂O potash, and 4–15% Na₂O soda, plus additives such as boric oxide, yttrium oxide, lithium oxide, cerium oxide, and titanium oxide.

Why feldspathic ceramics are called porcelains

They are called porcelains because they contain a glass matrix and one or more crystal phases, even though industrial porcelain traditionally refers to products made with kaolin as a major component.

Silicate glass

The matrix phase of feldspathic porcelain. Silica can exist as crystalline quartz, crystalline cristobalite, crystalline tridymite, or noncrystalline fused silica.

Fused silica

A high-melting noncrystalline form of silica. Its high melting temperature comes from a three-dimensional network of covalent bonds between silica tetrahedra.

Silica tetrahedra

The basic structural units of the silicate glass network. Their covalent bonding creates a strong three-dimensional glass structure.

Low coefficient of thermal contraction of fused silica

Fused silica has a low thermal contraction coefficient, so fluxes are added to reduce sintering temperature and make porcelain compatible with the metal framework.

Fluxes in dental porcelain

Low-fusing glasses added to reduce the temperature needed to sinter porcelain powder particles so the metal alloy does not melt, sag, or undergo flexural creep during firing.

Potassium and sodium feldspar

Naturally occurring minerals composed of potash or soda, alumina, and silica. They are important because they tend to form leucite when melted.

Leucite

A potassium-aluminum-silicate crystal, written as K₂O·Al₂O₃·4SiO₂, that forms when feldspar undergoes incongruent melting. Leucite increases thermal expansion of porcelain.

Leucite thermal expansion

Leucite has a high coefficient of thermal expansion, about 20–25 × 10⁻⁶/K, compared with feldspar glasses at about 8.6 × 10⁻⁶/K.

Incongruent melting

A process where one material melts to form a liquid plus a different crystalline material. Feldspar undergoes incongruent melting between 1150°C and 1530°C, forming leucite crystals in liquid glass.

Importance of leucite in metal-ceramic porcelain

Leucite helps control porcelain thermal expansion so it can be made compatible with the metal coping.

Soda, potash, and leucite in metal-ceramic porcelain

Specific amounts of soda, potash, and/or leucite are needed to reduce sintering temperature, increase thermal expansion to match the metal coping, and increase fluidity.

Glass modifiers

Additives that alter glass properties such as viscosity, softening temperature, and glass network structure.

Boric oxide / B₂O₃

A glass modifier that decreases viscosity, lowers softening temperature, acts as a flux, and can form its own glass network.

Aluminum oxide / Al₂O₃

A complex oxide in glass formation. It is not a true glass former, but it can alter the softening point and viscosity and may act as a stabilizer.

Pigmenting oxides

Metallic oxides added to porcelain to reproduce natural tooth shades. They are made by fusing metallic oxides with fine glass and feldspar and then regrinding the material into powder.

Iron or nickel oxide

Pigmenting oxides that contribute a brown color to porcelain.

Copper oxide

A pigmenting oxide that contributes a green color to porcelain.

Titanium oxide

A pigmenting oxide that contributes a yellowish-brown color to porcelain.

Manganese oxide

A pigmenting oxide that contributes a lavender color to porcelain.

Cobalt oxide

A pigmenting oxide that contributes a blue color to porcelain.

Opacity modifiers in porcelain

Cerium oxide, zirconium oxide, titanium oxide, and tin oxide may be added to increase opacity or whiten porcelain.

Veneering ceramics

Ceramics used to cover or veneer another structure, such as a metal coping or ceramic core, to improve esthetics.

Types of veneering ceramics

Four types are ultralow and low-fusing ceramics, low-fusing specialty ceramics, ceramic stains, and ceramic glazes.

Ultralow-fusing ceramics

Ceramics with sintering temperatures below 850°C. They are used for crown and bridge veneering and may be compatible with lower-fusing alloys.

Composition of ultralow-fusing ceramics

They contain less alumina and increased concentrations of CaO, K₂O, Li₂O, and Na₂O, with less leucite crystals.

Properties of ultralow-fusing ceramics

They have lower expansion and contraction coefficients and often contain a well-distributed dispersion of small crystals or few/no crystals.

High-fusing ceramics

Ceramics fired above 1300°C. They are used for denture teeth and fully sintered alumina and zirconia core ceramics.

Medium-fusing ceramics

Ceramics fired at 1101–1300°C, used for denture teeth and presintered zirconia.

Low-fusing ceramics

Ceramics fired at 850–1100°C, commonly used for crown and bridge veneer ceramics.

Ultralow-fusing ceramic firing temperature

Ceramics fired below 850°C, used for crown and bridge veneer ceramics.

Cast metal for metal-ceramic systems

The alloy used under porcelain must have a sufficiently low porcelain-sintering compatibility, matched thermal expansion/contraction, and resistance to sag, creep, or melting during firing.

High noble alloys for metal-ceramic systems

Examples include Au-Pt-Pd, Au-Pd-Ag, Au-Pd, Pd-Au, Pd-Au-Ag, Pd-Ag, Pd-Cu-Ga, and Pd-Ga-Ag.

Base metal alloys for metal-ceramic systems

Examples include Co-Cr, Ni-Cr, and Ni-Cr-Be.

Metal oxide requirement for porcelain bonding

A metal oxide layer is needed to promote chemical bonding between metal and porcelain.

High noble PFM alloy base metal additives

High noble gold alloys for porcelain bonding contain small amounts, about 1%, of base metals such as iron, indium, and tin to form bonding oxides.

Degassing

A commonly used but technically misleading term for the oxidation heat treatment of PFM alloys. Its main purpose is not gas removal but forming an adherent metal oxide layer for porcelain bonding.

Porcelain-metal bond

Primarily ionic and chemical in nature, with possible covalent contribution. It can form even when the metal surface is smooth and mechanical interlocking is minimal.

Factors controlling metal-ceramic bond durability

Durability depends on mechanical interlocking or interatomic bonding at the porcelain-metal oxide interface, interatomic bonding across the oxide-porcelain interface, and the type/magnitude of residual stress in the veneering ceramic.

Glazes and stains

Ceramic materials applied to enhance esthetics, improve color matching, create lifelike appearance, and smooth porcelain surfaces.

Ceramic stains

Pigmented ceramic materials used to adjust the color and shade of porcelain restorations.

Ceramic glaze

A glassy surface layer applied or created on ceramic to enhance smoothness, esthetics, and possibly reduce surface flaws.

Effect of glass modifiers on glazes

Glass modifiers, usually alkali oxides, reduce glaze fusing temperature but also reduce chemical durability.

Glazing feldspathic porcelain

Glazing is believed to eliminate or seal surface flaws and create a smoother surface.

Autoglaze

A glazed surface produced by raising the temperature of the ceramic enough to create surface flow.

Overglaze

A glass coating formed by fusing glaze particles onto the ceramic surface.

Autoglazed porcelain

Stronger than unglazed porcelain because the glaze seals surface flaws and reduces stress concentrations.

Glaze and opposing tooth wear

A smoother glazed surface reduces abrasion damage to opposing teeth or restorations.

Best surface refinement assumption

Fine polishing of a roughened surface followed by glazing likely produces smoother surfaces than polishing alone, sandblasting plus glazing, or diamond grinding plus glazing.

Ceramic processing methods

Major processing methods include condensation and sintering, hot pressing, slip casting, casting, CAD-CAM milling, copy milling, and machining or grinding of dry-pressed powder on an enlarged die.

Condensation and sintering

A ceramic processing method where powder-liquid ceramic is condensed, often by vibration, and sintered at high temperature. Used for some aluminous porcelains and pure alumina ceramic systems.

Hot-pressed ceramics

Ceramic ingots heated and injected under pressure into a mold, then stained, glazed, or veneered. Examples include IPS Empress, IPS Empress 2, Finesse All-Ceramic, and OPC-3G.

Slip-cast technique

A technique where a slurry of liquid and ceramic particles is placed on a dry refractory die, which draws water out of the slurry. Used for In-Ceram Alumina, In-Ceram Spinell, and In-Ceram Zirconia.

Methods of strengthening ceramics

Ceramics can be strengthened by minimizing stress concentration, selecting tougher ceramics, thermal tempering, creating residual compressive stress, using stiffer support, reducing firing cycles, increasing bulk and connector radius, and adhesive bonding.

Stress concentration in ceramics

Localized stress at flaws, sharp angles, abrupt thickness changes, or localized loading. It increases fracture risk because ceramics are weak under tensile stress.

Design principles for ceramic restorations

Provide sufficient bulk, minimize sharp angular changes, avoid abrupt changes in shape or thickness, and avoid large localized loading stresses.

Thermal tempering

A strengthening method that creates residual compressive stresses within the surface of a ceramic material.

Residual compressive stress

A beneficial stress state that helps resist crack opening and improves fracture resistance in ceramic surfaces or interfaces.

Stiffer supporting materials for ceramics

Materials with higher elastic modulus can reduce tensile stress in ceramic restorations by limiting flexure.

Firing cycles and feldspathic porcelain

Minimizing the number of firing cycles helps maintain ceramic strength because repeated firing can affect microstructure and increase risk of weakness.

Connector design in ceramic prostheses

Connectors should have greater bulk and broader radii of curvature in areas of potential tensile stress to reduce stress concentration and fracture risk.

Adhesive bonding of ceramic crowns

Bonding ceramic crowns to tooth structure can increase fracture resistance by improving support and reducing tensile stress concentration.

Aluminous porcelain

Alumina-reinforced porcelain used as core material for anterior crowns. It provides better esthetics than PFM for anterior teeth but lacks enough strength for posterior teeth.

Glass ceramic

A material first formed as glass and then heat-treated to induce partial devitrification, meaning crystallization of the glass.

Devitrification

Loss of glassy structure through crystallization during heat treatment.

Dicor

The first commercially available castable ceramic material for dental use, developed by Corning Glass Works and marketed by Dentsply International.

Dicor processing

Dicor was cast into inlays, veneers, or crowns by a lost-wax process, then sandblasted, separated from sprues, covered with embedment material, and heat-treated to grow tetrasilicic fluormica crystals.

Ceramming

The process of crystal nucleation and growth through heat treatment.

Tetrasilicic fluormica

Microscopic plate-like crystals grown within the Dicor glass matrix during ceramming.

Leucite-based glass-ceramic core restorations

Pressed glass-ceramic restorations containing about 35% leucite crystals by volume, used for anterior veneers, anterior crowns, and premolar inlays, onlays, and crowns.