Toothsome technology: scientists strive to improve dental materials - includes related information on use of surface sealants on microchannels

Science News, Oct 11, 1997 by Corinna Wu

In George Washington's time, a set of wooden dentures was the only fashionable remedy for a lifetime of poor dental hygiene. Nowadays, people needing dental work can opt for more durable materials, such as metal, not to mention more aesthetically pleasing ones, such as composites and ceramics.

The desire for a more natural-looking mouth has increasingly driven the search for new dental materials. People are no longer content to flash a metallic smile, but neither will they accept better-looking repairs that necessitate more visits to the dentist.

Researchers are trying to combine high strength with dental glamour. At a meeting of the American Chemical Society (ACS) in Las Vegas last month, researchers described recent work on polymers and polymer composites designed for dental use. Although these materials have cosmetic advantages over metal, those currently available are not strong enough for use in all teeth.

Other chemists are focusing on ceramics and new metal amalgams. Ceramics are more brittle than dentists would like, and traditional amalgams have been accused of leaking mercury into the body.

Improving the material properties of fillings, inlays, and crowns can reduce the number of trips a patient has to make to the dentist. On average, most traditional dental materials last about 10 years, says Stephen Bayne of the University of North Carolina at Chapel Hill School of Dentistry "We'd love for that to be 20 to 25 years."

Teeth are made mostly of a natural ceramic and a natural composite: enamel and dentin, respectively. The thin surface layer of enamel consists mainly of a calcium-based mineral called hydroxyapatite. Underneath, the bulk of the tooth is made of dentin--a mix of hydroxyapatite, collagen, water, and salts. A third type of tissue, cementum, lines the dentin under the gum line.

The mouth is a harsh environment. Acids from foods and plaque-forming bacteria erode not only teeth but dental materials too (SN: 10/8/88, p. 238). The act of chewing hits a restoration or filling with what amounts to a series of hammer blows. To choose appropriate materials, dentists have to assess what mechanical stresses the material will encounter.

For example, a molar in the back of the mouth, which does a lot of chewing, experiences greater stresses than a tooth in the front. Also, the center of a tooth surface flexes more than the edges, so a restoration located there runs the risk of popping out if not properly bonded, Bayne says. Artificial dental materials are some of the most durable substances used in the body. "People say that if it works there, it'll work anywhere else," Bayne remarks.

Gold is the ultimate filling material, at least for durability, but it is prohibitively expensive, says Jack Ferracane, president of the Academy of Dental Materials and a biomaterials researcher at the Oregon Health Sciences University in Portland. On the other hand, dental amalgam--an alloy of silver, tin, copper, zinc, and mercury--is reasonably priced, strong, and dependable, which has made it the workhorse material for the past century.

Recent concerns that mercury may leach out and cause adverse health effects have fueled some of the research into alternative filling materials (SN: 4/10/93, p. 230). Scientists at the National Institute of Standards and Technology (NIST) and the American Dental Association Health Foundation are developing metal alloys, based on silver and tin, that do not contain mercury. The challenge is to get the new mixtures to solidify properly. In the old mix, mercury, a liquid at room temperature, coats the other metals and helps them all to solidify.

Many scientists say that the risk of mercury seeping out of the solid filling is low. "Once it's fully set, the mercury is bound up and won't come out," says Bayne. In the past, the greatest danger was to the dentists who prepared the amalgams. Now, all the materials are combined in a sealed capsule to minimize exposure, says research chemist Joseph M. Antonucci of NIST.

Safety concerns aside, newer materials have been gradually replacing traditional amalgams because of their cosmetic advantages. Composites, generally made of silica glass particles bound with a polymer resin, look more natural than metal because the color can be matched to the patient's teeth. The dentist mixes the silica and polymer, applies an adhesive to the tooth, fills the cavity, and shines a high-intensity light on the material to set it. The light causes the polymer molecules to react with one another, linking them to form a solid resin.

During the setting process, however, the polymers tend to shrink, thus weakening the bond to the tooth. In addition, tiny gaps created between the filling and the tooth can harbor cavity-causing bacteria, says Ferracane.

The trick, then, is to produce a polymer that doesn't shrink as it sets. Several research teams are investigating one approach, which is to add ring-shaped monomers such as spiral orthocarbonates to the resins. As these molecules react with each other, they open up, expanding the material's volume slightly.