Teeth function in a complex environment of mechanical, chemical and thermal stresses. In normal chewing, modest stresses of 20 MPa are encountered about 1000 to 3000 times a day. Hard biting may produce occasional stresses of ~ 100 MPa. This cyclic loading occurs in a water based fluid that can have a wide range of pH (0.5 to 8) and temperature excursions of 65°F (body temperature to ice). Most people grind their teeth occasionally, so abrasive wear is also an issue.
While any engineered material used in dental application must be capable of surviving in the above physical environment, the most critical property is a subjective one—aesthetics. An implant, bridge or crown should look like its neighboring teeth. Thus, in any visible use the ability to tailor color, translucency and fluorescence becomes the key acceptance criteria. And as with all biomaterials, the ceramic must be non-toxic and biocompatible.
The first dental application of ceramics was porcelain dentures about 225 years ago, and this is still a major use. More recent contributions of ceramic materials to dentistry include fillings, crowns, veneers, bridges, implants and orthodontic brackets.
To improve the wear and strength properties, there is a trend (greater in Europe than in the U.S.) to use all-ceramic restorations. These may be inlays, onlays or crowns. Facilitating this trend is the availability of “chair-side” CAD-CAM systems. Designed for use with dental ceramics, these systems image a patient’s prepared tooth, produce a digitized instruction file and machine a ceramic restoration blank in the dentist’s office. Using a CAD-CAM system, the dentist can fit an inlay or a crown in one visit versus two for the old technology of taking an impression.
Ceramic materials available as machinable blocks include leucite reinforced feldspathic porcelain; alumina with continuously interconnected porosity, which is infiltrated with lanthanum aluminosilicate glass after machining (to create translucency); porous spinel later infiltrated with glass; and porous zirconia later infiltrated with glass. These all-ceramic restorations represent a small fraction of crowns in the U.S., but the trend is growing.
About 75% of all crowns are still of porcelain fused to metal (PFM) construction. Dental porcelains, which are feldspathic porcelains with varying percentages of crystallized leucite phase to control strength and coefficient of thermal expansion (CTE), are fused to a metal substrate for strength. Matching the CTE of the porcelain to the metal supporting base is critical to avoid cracking during manufacture and use. Even all-ceramic crowns are usually coated with a porcelain layer to match color and translucency. PFM is also used for veneers, which cover damaged front teeth, and bridges. However, some bridges now have ceramic cores and an aesthetic (porcelain) outer coating.