Brief Overview Of Dental Ceramics
Derek W. Jones, PhD, FIM, C.Chem. FRSC(UK), FBSE
The labs are always wanting me to try their latest product. It has been a while since Ive taken a materials course and Im pretty rusty. Can you please tell me what is the difference between the various porcelain materials available for crowns?
© J Can Dent Assoc 1998; 64:648-50
[Dr. Jones's Answer | Some Properties Of Ceramics | Earlier Developments | Latest Developments | Glossary | Acknowledgements]
This is a very important question because there have been a number of interesting developments in the field of dental ceramics in recent years.
Ceramic materials have been used in dentistry for well over 200 years. They are the most biocompatible of dental restorative materials, because they are chemically very stable. Essentially, they are metallic oxides which are in the lowest energy state. This chemical stability is illustrated by the fact that the oldest known artifacts associated with human habitation are ceramic materials such as pottery fragments, which have not degraded with time.
A desirable feature of ceramics is that their appearance can be customized to simulate the colour, translucency and fluorescence of natural teeth. A major problem with the use of ceramics as tooth replacement materials is their very low fracture toughness (the energy required to propagate a crack), and the fact that fracture occurs at a very low strain of about 0.1%. In other words, the ceramic structure only exhibits a very low flexibility before fracture. Another problem is that if the ceramic structure is formed by the condensation and sintering of fine frit particles, fusion is accompanied by a relatively large firing shrinkage.
The earliest successful porcelain systems used conventional feldspathic porcelain, derived from the natural mineral feldspar. This material was used for producing all-ceramic jacket crowns, which were very esthetic. Dental feldspathic porcelain is predominantly a glass material with an amorphous (non-crystalline) structure. Glass mainly consists of a three-dimensional network structure of silica (silicon-oxygen) in which each silicon atom is bonded to four oxygen atoms in the form of a tetrahedron. These tetrahedra are linked together by sharing common oxygen atoms to form a continuous three dimensional network. Other oxides (such as aluminum), may be incorporated to a very limited extent as substitute network formers.
The introduction of oxides of alkali metals (e.g. potassium, sodium and calcium) into a silica glass composition results in disruption of the three-dimensional structure formed by the oxygen-silica bonds. When the alkali metals in the ionic form disrupt the oxygen-silicon bonds (resulting in non-bridging oxygen) the three-dimensional network structure breaks down to some extent, resulting in a lower fusion temperature and a more fluid behaviour during heating. While pure (100%) silica glass fuses at about 1700°C, the introduction of alkali ions disrupts some of the silicon-oxygen bonds causing a more open network structure. The more open network structure has less cross-linkages, causing a lowering of the fusion temperature, together with a reduction in strength and chemical inertness. Thus, fusion temperature, strength and chemical inertness depend on the amount of alkali (or non-bridging oxygen) present in the glass.
Porcelain fused to metal systems were introduced in the 1950s. A development in 1962 greatly improved these systems; that is, the incorporation of a high proportion of leucite crystals into the feldspathic porcelain composition which veneered the cast gold alloy substructure. The leucite crystals serve to increase the thermal expansion of the porcelain to bring it closer to that of the metal substrate. The leucite prevents stresses occurring, due to a thermal mismatch, which could lower the strength. Metals are between 10 and 100 times tougher than ceramics; the presence of a metal substrate can contribute to a very strong restoration.
The incorporation of a small trace of tin and/or iron into the gold alloy was necessary to allow formation of the necessary oxide on the surface to permit good wetting by the porcelain and subsequent bonding to the alloy surface.
Base metal alloys were introduced in competition to the gold alloys. These are more technique sensitive than gold, but are now well established. An opaque ceramic such as a titanium oxide glass frit has to be applied as the first layer of veneer, to mask the metallic hue in the porcelain fused to metal systems.
Although the porcelain fused to metal systems have high strength, the opacity of the metal substructure has encouraged the development of all-ceramic core materials containing crystalline components which are stronger than the traditional (predominantly glassy amorphous) feldspathic porcelain. This type of core material can then be veneered with a more translucent ceramic material. The core structure for the In-Ceram infiltration system is much stronger than the other all-ceramic core systems.
The all-ceramic systems described in Table I are used for producing crowns and inlays. It is important to note that all of the ceramic materials listed will fracture at approximately the same critical strain (i.e. with the same degree of flexing before fracture) of about 0.1%. The only way that we can increase the strength of such ceramic materials is to make them stiffer to increase their elastic modulus. This is why the much higher modulus of elasticity of the In-Ceram core material results in a significantly stronger ceramic. The first In-Ceram alumina material produced made use of alumina as the reinforcing phase, in fact, it is much stronger than the newer, more translucent spinel containing In-Ceram (Spinel).
Alumina: High strength crystalline oxides of aluminum.
CAD-CAM: Computer-aided design combined with computer-aided machining. In dentistry, laser mapping of a cavity preparation can be fed to a computer-controlled milling machine.
Dispersion Strengthening: A mechanism by which a crystalline phase of high strength and high modulus of elasticity is dispersed in an amorphous glassy matrix to produce a high strength composition.
Feldspar: A range of natural crystalline minerals of aluminum, silicon and oxygen combined with smaller amounts of sodium, potassium and calcium. The presence of the alkalis controls the softening point of feldspar which is lowered by increased sodium, but increases with potassium content.
Frit: A glass powder produced by rapid chilling from the molten state followed by grinding.
Glass-Ceramic: A composition which could be converted from an amorphous glass to a high strength polycrystalline structure by precipitation when heated within a specific temperature range.
Opacifier: A white or pale cream coloured oxide which is used to decrease translucency and act as a masking agent, often tin oxide or titanium dioxide.
Sintering: The process of fusion by point contact of particles resulting in densification by viscous flow of a ceramic or glass powder, produced by heating or heat and pressure.
Slip-cast slurry: A fine particle ceramic dispersed in an aqueous liquid medium is poured into a porous mould which rapidly extracts the liquid causing the formation of a close-packed but weak ceramic particle structure.
Spinel: A range of compounds with specific crystalline structures, the spinels of recent dental interest are of magnesium and aluminate composition.
Dr. Jones is professor of biomaterials, at Dalhousie University, Halifax, Nova Scotia.
The author has no declared financial interest in any company manufacturing the types of products mentioned in this article.
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