Bones and teeth, the hard tissues in the human body have an inorganic component. The inorganic component primarily consists of hydroxyapatite (Ca10(PO4)6(OH)2, HA). In most cases, they also an organic component which is most often collagen.
Enamel, the outer layer of teeth is the hardest material in the body, and thus it is not surprising that it consists of approximately 92% hydroxyapatite.
The Oral Environment and Requirements
Teeth function in one of the most inhospitable environments in the human body. They are subject to larger temperature variations than most other parts, coping with the cold of ice (0°C) through to hot coffee and soups. They also encounter pH changes in the range 0.5 to 8. Add to this the stresses associated with chewing where cyclic stresses may vary from 20 to about 100MPa.
Requirements of Dental Materials
While having to be stable in the environment described above and able to withstand the loads associated with chewing, dental materials need to satisfy another criteria, aesthetics. With society becoming increasingly self conscious, any dental materials that are used in visible locations must have colour and translucency as close to natural teeth as possible.
History of Dental Ceramics
Dental ceramics were first used about 225 years ago. The first application were porcelain dentures. Interestingly porcelain is still quite widely used in dentistry.
Ceramics in Modern Dentistry
Current applications of ceramics in dentistry include fillings, crowns, veneers, implants and dental brackets.
Traditionally filling have been made from silver/tin/mercury amalgams. However, resin based filling materials are rapidly gaining in popularity for health and aesthetic reasons.
These resins are usually filled with 35-85% ceramic fillers such as silicate glasses, colloidal silica or quartz. Despite the advantages of using ceramic filled resins, these materials may be susceptibe to wear when applied to chewing surfaces.
For this reason, all-ceramic inlays, onlays and crowns are also gaining in popularity. CAD-CAM systems are also becoming more popular allow dentists to machine dental ceramic blanks to suit individual patients and fir them in the same visit. Materials that are suitable for this application include:
• Leucite reinforced feldspathic porcelain
• Alumina with continuously interconnected porosity, infiltrated with lanthanum aluminosilicate glass, after machining to provide translucency
• Glass infiltrated porous spinel
• Glass infiltrated porous zirconia
Crowns and Veneers
Porcelain fused to metal (PFM) materials constitute about 75% all crowns in a market consuming about 30 million crowns in the USA alone. The porcelain used is a feldspathic porcelain. The porcelain contains varying amounts of crystallised leucite. The amount of crystallised leucite influences properties such as strength and thermal expansion. Thermal expansion is critical in the manufacture PFM implants to avoid cracking of the ceramic, which could lead to failure during manufacture or in operation.
PFM materials are also used for veneers to cover damaged front teeth and crowns, although ceramic cores are being used in some crowns. All ceramic crowns are normally coated with porcelain so that colour and translucency can be matched.
Dental implants are used as an alternative to bridges where a tooth has been lost or removed. These are also made of PFM, where a biocompatible metal post made out a material such as titanium is anchored into the jaw bone and the porcelain crown is affixed to the post.
The metal post may also be coated with hydroxyapatite to aid bone bonding and rapid osseointegration. Bioglass posts are sometimes used for this purpose for the same reason.
The most recent use for ceramics in dentistry is orthodontic brackets. The development and demand for these items has been driven solely by aesthetics. Polycrystalline alumina is the material of choice in this application.