Local scientists win $3.5 million dental grant 7/16/97
Materials currently used in dental fillings have many shortcomings. Silver fillings contain mercury which, while safe, isn't environmentally compatible and the silver color is no longer acceptable. Plastic-based composite resin materials tend to shrink after placement, cause stains, discoloration and eventually, more tooth decay. These materials also weaken more readily than silver fillings when used on the chewing surfaces of the teeth.
To help address these problems, the National Institute of Dental Research (NIH/NIDR) has awarded a $3.4 million grant to a group of scientists from The University of Texas Health Science Center at San Antonio (UTHSCSA) and Southwest Research Institute (SwRI). Using a combination of disciplines, these five investigators are blending their expertise and knowledge to develop a material for dental fillings far superior to anything which currently exists.
=46rom the Health Science Center, H. Ralph Rawls, PhD, professor and head of the division of biomaterials in the department of restorative dentistry, is director of the research project. Dr. Rawls, who supervises the entire program as well as nanofiller (extremely small, such as one billionth of a meter) particle development, is collaborating with several scientists. Barry K. Norling, PhD, and Joo L. Ong, PhD, are also from the department of restorative dentistry. Dr. Norling is responsible for the new liquid crystal monomer development. A monomer is a molecule that can be chemically bound as a unit of a polymer. From pediatric dentistry, Mary MacDougall, PhD, oversees the dentin stimulating liner project and Don Ranly, DDS, PhD, is co-investigator on the tertiary dentin stimulating project and co-director and supervisor of research and development. David Carnes, PhD, from the department of endodontics, will assay biocompatibility of new materials to ensure safety. From SwRI, Steven Wellinghoff, PhD, is synthesizing both the new resin monomers and the nanofiller particles while James Lankford, PhD, conducts extensive physical and mechanical property characterizations to ensure better performance.
"Neither silver fillings nor composite resin materials are optimally X- ray opaque," Dr. Rawls said, "making it difficult for dentists to see cavities that develop adjacent to fillings. The ideal material would have the aesthetics of the current composite resins and the functional lifetime of silver fillings and it would be opaque, easy to place in the cavity and stimulate the tooth to repair itself.
The scientists are developing a new composite resin, a new polymer resin and a new filler particle. A new cavity liner which actually stimulates the tooth to create new dentin and helps the tooth to heal itself is also being developed.
"The polymer will be based on liquid crystal monomers," Dr. Rawls explained. "When carefully selected, these monomers can actually expand on polymerization, offsetting the natural shrinkage that occurs when liquid monomers are converted to solid polymers. Liquid crystal monomers also impart a desirable rheology, or flowability, to material used for fillings. This material is almost rigid when put into the tooth but malleable enough for the dentist to work with, and it hardens when exposed to bright blue light."
The scientist explained that the rheology of the material is like a good latex ceiling paint; it flows freely but once applied, it doesn't drip.
Current composite resin filling materials use glass particles as the filler, to improve strength and provide translucency for aesthetics and radiopacity. The new filler will be based on tantalum oxide nanoparticles because this material is sufficiently radiopaque (is impervious to radiation) for good X-ray contrast, but is not radiopaque enough to mask what's behind it. Nanoparticles, less than 1/100,000th the diameter of a human hair, improve strength, wear-resistance and aesthetics.
Dr. Wellinghoff originally developed tantalum oxide nanofillers while conducting research for NASA. Early astronauts noted strange flashes of light, caused by energetic particles passing through the space suit face shield, then through the optic nerve. To block these particles, NASA developed optically transparent face shields loaded with heavy metal. Dr. Wellinghoff's innovative solution was to load a standard phenoxy resin with large amounts of tantalum oxide nanoparticles.
While modern composite resins are usually bonded to the underlying tooth structure, the bonding is tenuous and doesn't last in large fillings, according to Dr. Rawls. Modifications made by these scientists will make a bonded, stress-bearing filling possible for the first time, resulting in a much longer lifetime without the problems of amalgam.
"Another factor which is designed to increase the life of the filling is a dentin-stimulating liner" Dr. Rawls said. "Any tooth will naturally respond to trauma, such as cavity formation or the preparation done by the dentist when he repairs it, by growing more dentin in the pulp cavity, the internal space occupied by the nerve.
"This process can be very slow," he continued. "Scientists have recently discovered that the same protein growth factors that stimulate new bone growth also stimulate growth of tertiary dentin. The liner we develop will stabilize these growth factors and permit them to be delivered to the pulp after they are placed in the prepared cavity, prior to the tooth being filled.''
Over the course of the five-year program several matrix resin monomers and nanofillers will be synthesized and evaluated. In addition, these materials will be combined to produce a spectrum of experimental composite resin fillings which will be tested to ensure safety and effectiveness.
"The best combination would provide a durable, functional restoration with enhanced longevity in a strengthened, fracture-resistant tooth," Dr. Rawls concluded. "It is just such a restorative system that this research program is designed to develop."
Dr. Rawls received his PhD in physical chemistry from the Institute of Molecular Biophysics at Florida State University and a Research Career Development Award for Caries and Restorative Materials Research from the National Institute of Dental Research. He developed a diagnostic aid for incipient caries, fluoride-ion releasing dental resins, X-ray opaque biomedical plastics, and improvements in toothbrushes and other dental hygiene products.
Contact: Jan Elkins (210) 567-2570