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Advanced Ceramics: Dental Ceramics

November 21, 2000
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The enamel on the exposed surfaces of our teeth is the hardest material in our bodies. It is ~ 92 volume % hydroxyapatite {HA, Ca10(PO4)6(OH)2}—a ceramic. Thus, it is not surprising that a significant and growing area of dental materials R&D and application is focused on ceramics.

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.

Fillings, Crowns, Veneers and Bridges

Silver-tin-mercury amalgam fillings are rapidly giving way to ceramic filled resin. These fillings typically contain 35-85% ceramic filler, which most generally is a silicate glass, colloidal silica or quartz. They are more attractive than silver amalgams and raise no concerns about mercury in the body. However, they have been noted to be more susceptible to wear and fracture when used on the chewing surface of posterior teeth.

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.

Implants

Rather than replace a missing tooth using a bridge, many patients (about 400,000 per year) now choose to use a dental implant. A post of a biocompatible material is implanted into the jawbone and a ceramic or PFC crown is affixed. Most implants use either pure or HA coated titanium. HA facilitates biointegration (a tight iterface between the implant and bone). Other ceramics used in this application include single crystal sapphire and a calcium phosphate-MgAl2O4 spinel. Another application of ceramic implants is specially formulated Bioglass® inserts, which are placed into empty root sockets after extractions in order to maintain healthy bone for patients requiring dentures.

Orthodontic Brackets

The newest application for ceramics in dentistry is, not unexpectedly, aesthetics driven in the form of orthodontic brackets. The use of translucent polycrystalline alumina in this application has made brackets much less obvious, and consequently has increased the number of adults wearing orthodontics.

Markets

Over 30 million crowns are made in the U.S. per year. About 75% of these are PFM and 3-5% are all-ceramic. A reasonable estimate is that each crown requires $8-12 of ceramic material. Thus, crowns alone represent a U.S. market of $200-250 million for the ceramic industry. Add inlays, veneers, implants, bridges and brackets, and a market in excess of $300 million is likely.

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