Scientists report in this month's issue of the journal Dental Materials that two synthetic molecules designed in their laboratory to improve the durability of composite fillings had acceptable strength and good biocompatibity during initial tests.
According to the scientists, these results suggest the structure of these so-called oxirane, or epoxy, molecules can be further refined in the laboratory to produce a safe, non-shrinking resin matrix, the chemical backbone of a composite filling. "There has been a need in restorative dentistry for a safe, non-shrinking composite matrix," said Dr. David Eick, a scientist at the University of Missouri at Kansas City and lead author on the study. "These results mark a small, but important, research step toward meeting this need."
Dental composites are the white, resin-based fillings that have become a mainstay of restorative dentistry. In 1999 alone, the American Dental Association estimated that over 85 million composite fillings were placed in the United States, ranking them as the number one dental restorative in the country.
Despite the aesthetic advantages of tooth-colored composites over amalgam, many dentists say they remain hesitant to place these fillings in decayed back teeth, where the mechanical stresses of chewing are greatest.
They say the problem is composite fillings, which are typically inserted into cavities as a viscous-plastic gel, shrink by about three percent as they polymerize and harden in the tooth. Though three percent might seem trivial, composites are sealed tightly on all four sides of the tooth's inner surface. As the composite shrinks, it can pull away from the seal, creating an constant stress that may eventually crack the tooth or open a seam for bacteria to cause secondary caries.
To eliminate the problem, scientists say they need to improve the chemistry of the composite's matrix backbone, the main source of the shrinkage. One hope is to develop a suitable synthetic molecule, or monomer, that polymerizes without losing volume as it forms chemical bonds with other monomers.
According to Eick, a strong candidate is oxirane, which polymerizes cationically. That is, its monomers open their aromatic rings and expand to form chemical bonds. Current composites that are based on free-radical chemistry, in which their rings contract during bond formation, resulting in a slight loss of volume.
"The typical polymerization stress for most current composites is in the neighborhood of 20 megapascals," said Eick, referring to a standard unit measure of stress. "The oxiranes by themselves are maybe 8 megapascals and, when other expanding monomers are added to the oxirane, the measure is down to less than one. So, we are getting very close to having a zero stress polymer.
To reach this point, however, has been no stroll through Kansas City's Country Club Plaza. Given the many issues that arise in designing synthetic materials, Eick and colleagues have assembled an interdisciplinary research team, which, according to Eick, "brings a synergistic scientific focus" to the project. Included on the research team are material scientists, computer scientists, toxicologists, synthesis chemists, and important industry collaborators.
"The team starts out by modeling a variety of molecules on the computer that we think might expand and which appear to have a biocompatible structure," said Eick. "Biocompatibility is a huge issue - as it is in the development of all dental materials - because residual monomers could leach from the composite into the body. For this reason, the toxicologists run these molecules through a range of initial biocompatiblily tests that ultimately leave us with only the most viable molecules.
"These molecules are then given to the synthesis chemists," continued Eick. "Because the molecules are extremely large, it can really be a horrendous synthesis job to produce some of these materials. For the easiest molecules, it can take three to six months just to synthesize them."
Following this strategy, Eick and colleagues first reported in 1999 that oxirane monomers, blended with polyol, had superior biocompatibility profiles compared to oxirane alone and two other oxirane mixtures. Polyol, a generic term for polymers with a large number of hydroxyl groups, serves as a so-called "carrier" resin, helping to cross link the matrix network and modify its chemical properties.
The following year, the group published its prototype of a light-initiated, oxirane-polyol composite. It consisted of an oxirane (UVR-6105); a polyol (pTHF-250); about 75 percent quartz/fumed silica filler, the inorganic bulk material that adds strength to the matrix; and a cationic photo-initiator and sensitizer system, which, when exposed to UV light, generates an acid that catalyzes the polymerization process.
Now, in this month's issue of Dental Materials, Eick et al. report on the initial biocompatibilty and physical strength of three experimental oxirane-polyol composites. Each composite contained the same chemical components as the previously reported prototype, with one exception. The third composite had the low-viscosity acrylate, Ebecryl 1830, added to increase the degree of polymerization.
After a battery of laboratory tests, the scientists found that the first two composites - 4016E and 4016G - had acceptable compressive strengths, a gauge of their potential durability in the mouth, that were comparable to their earlier oxirane/polyol prototype. These composites also were shown to be non-cytotoxic in standard cell culture assays.
However, the third composite, known as 4016GB, which included Ebecryl 1830, did not fare as well. It scored poorly in compressive strength, indicating it likely lacks the necessary structural durability for long-term placement in the mouth. It also was found to be mildly cytotoxic in the cell assays.
Based on their positive results with 4016E and 4016G, the scientists concluded that "suitable oxirane/polyol formulations can be designed and optimized to serve as matrix resins . . . with acceptable mechanical properties and good biocompatibility profiles." Studies are ongoing to further refine these and other oxirane/polyol composites for eventual clinical evaluation.
Published in the July 2002 issue of Dental Materials, the paper is titled, "In vitro biocompatibility of oxirane/polyol dental composites with promising physical properties." The authors are: J.D. Eick, E.L. Kostoryz, S.M. Rozzi, D.W. Jacobs, JD Oxman, C.C. Chappelow, A.G. Glaros, and D.M. Yourtee. The research was funded by the National Institute of Dental and Craniofacial Research and 3M Company.