Dental researchers improving methods of treating cavities

By Jennifer Lorenzo

Current resin-based filling materials shrink, leak, and wear away. This process allows decay to develop, and eventually the pulp may become infected.

The new nanofiller and liquid-crystal filling material reduce leakage by preventing shrinkage and resisting wear. Before the cavity is filled with this resin, a new treatment is applied to the pulp chamber to encourage dentin growth and protect the pulp, if leakage does occur
Very few American adults have never had a cavity.

According to estimates from the National Institute of Dental and Craniofacial Research (NIDCR), 94 percent of Americans over age 18 have had either fillings or untreated dental decay. Around 100 million of those fillings are the traditional silver amalgam type.

Amalgam fillings have the advantage of being inexpensive, durable and strong, but most people consider them unsightly. Some also consider them an environmental health risk because of the mercury they contain. Although the fillings are safe as long as they remain in the teeth, they could pose an environmental hazard once they are removed.

Traditional silver fillings have largely given way to plastic-based composite resins that match the color of teeth. These resins, however, tend to shrink, which leads to tiny gaps between the tooth and the filling, allowing bacteria to creep back in to the tooth and cause more decay. They also are weaker than amalgam fillings and tend to wear away and discolor after several years, so they need to be replaced.

Enter new shrinkage-resistant materials, currently being tested and developed by an interdisciplinary group of scientists from the Health Science Center and Southwest Research Institute. The long-term study, funded by the NIDCR, will lead to filling materials that are far superior to anything now being used and significant improvements in the way cavities are treated.

The Advanced Dental Restorative Systems (ADRS) project has three components in various stages of development, said project director H. Ralph Rawls, Ph.D., professor and head of the Division of Biomaterials in the Department of Restorative Dentistry. "Each is important by itself," said Dr. Rawls, "but all three combined add up to more than the sum of the parts." 

Less (shrinkage) is more

Two of the research projects are devoted to improving the two components of the composite resins themselves - the plastic matrix that binds them all together and the inorganic filler that provides strength and aesthetics.

The component closest to being realized is based on liquid crystal monomers, and is being developed by a team headed by Barry K. Norling, Ph.D., associate professor of restorative dentistry. Monomers are molecules that can be chemically linked together to form a polymer. The resulting material is malleable enough to pack into a cavity, where it will harden when exposed to a "curing" lamp in the dentist's office. But the hardening is accompanied by shrinkage.

The new resins reduce shrinkage from the current 8 percent to 2 percent. Dr. Rawls explains, "Imagine a bowl of spaghetti. The cooked spaghetti takes up more volume than uncooked spaghetti; the strands are all twisted and curled around each other, with gaps between them. Dry spaghetti, on the other hand, is aligned and packs together tightly. What we're doing with these materials is making them more like dry spaghetti."

Dr. Norling explained that the aligned molecules become unaligned during curing to a solid polymer, losing the initial tight packing and yielding a kind of virtual expansion that compensates for much of the shrinkage that inevitably occurs when monomers are converted to polymers.

The smaller the better

A second team, headed by Dr. Rawls, is developing a reinforcing filler, or nanofiller, which will strengthen the polymer and make it even more shrinkproof. "When we add filler, we're starting where the new polymer leaves off, at about 2 percent. With the right filler we can get shrinkage down below 1 percent," he said.

Dr. Rawls likens nanofillers to the sand and gravel added to cement to make a stronger concrete. Unlike gravel, though, these nanofillers are almost unimaginably small - less than 1/100,000th the diameter of a human hair. The smaller the particles, the stronger, smoother and more translucent the filler. Yet the polymer composite can hold just so many nanoparticles before it becomes too viscous to handle. 

In addition to being manageable, the new filler must be radiopaque, that is, it must be able to absorb enough radiation to provide good x-ray contrast. Too much radiopacity will mask what's behind the filling, while too little will not provide enough contrast to make decay visible.

The new filler is based on zirconium oxide rather than the heavy-metal-containing glass particles that are currently added to the plastic-based resins for radiopacity. Current fillers provide sufficient strength, malleability and radiopacity but are too soluble, degrading the polymer material over time.

The challenge facing the scientists is to create radiopaque filler with all three attributes: the finest particles, most malleable texture and least leachability possible. "Our main thrust now is developing a process for making the materials that would make it attractive commercially," he said.

Helping teeth

The ultimate restoration - a brand-new tooth - is still a long way off, but Mary MacDougall, Ph.D., associate dean for research in the Dental School and professor in the Department of Pediatric Dentistry, is developing a cavity treatment that will stimulate the growth of dentin, the hard tissue under the enamel that surrounds and protects the pulp of the tooth. The final component of the study, the new treatment eventually will be applied to the cavity before it is filled, thus helping the tooth to heal itself.

New dentin grows in the pulp chamber naturally in response to trauma, such as cavity formation or drilling, but the process can be very slow. A growth factor known as OP1 (osteogenic protein 1), which stimulates new bone growth, also has been shown to stimulate the growth of this reparative, or tertiary, dentin.

Dr. MacDougall and her colleagues are working on stabilizing this factor and delivering it to the pulp chamber. "We want to be able to deliver it so it will diffuse down to the pulp in sufficient concentration," said Dr. Rawls, "and stimulate the tissue to replace itself."

However, other work being conducted at the Health Science Center may one day make this research on treating cavities obsolete. "With the explosion in current genetic knowledge, we may see the day when dentures, crowns and fillings are no longer needed," Dr. MacDougall said.

The ability to regenerate teeth may still be years from being realized, but Dr. MacDougall has just received NIDCR funding for a five-year study on tooth development in mice.

The study, "Gene Expression and Regulation during Odontogenesis," will focus on the regulation and function of matrix proteins in tooth formation. It is a large collaborative research effort between investigators in the Department of Pediatric Dentistry and the University of Missouri at Kansas City.

Some day people may be able to regenerate teeth. Until then however, everyone will be able to benefit from the work being conducted today by Health Science Center researchers.

Barry K. Norling, Ph.D. (from left), associate professor of restorative dentistry; H. Ralph Rawls, Ph.D., professor and head of the Division of Biomaterials in the Department of Restorative Dentistry; and Mary MacDougall, Ph.D., associate dean for research and professor of pediatric dentistry, are working with an interdisciplinary group of scientists with the goal of creating new filling materials and improving treatments for cavities.

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