Novel resins could reduce shrinkage, toughen composites

2009 04 01 10 04 53 41 Teeth 70

Esstech, a biomedical materials company, has found evidence that its novel line of Exothane elastomers fared better than traditional polymeric matrix resins used in dental composites.

Esstech's results suggest that these materials could significantly improve dental composites, according to Jim Duff, a research chemist at Esstech. Duff presented his research, conducted with assistance from the University of Colorado, during a poster session at the recent American Academy for Dental Research (AADR) annual meeting in Tampa, FL.

"It'll solve a couple of different problems," Duff told "The first and foremost is the issue of shrinkage, one of the biggest issues in restoratives. I think these materials will help out in the industry by having the low volumetric shrinkage and the high conversion. Additionally, they will help toughen up the composite so that they tend to flex rather than break or crack."

Leakage of uncured or residual monomer, a potential outcome of low conversion, can be associated with allergic reactions and sensitization, he and his colleagues noted.

“It'll solve a couple of different problems.”
— Jim Duff, Esstech

Their research found that Exothanes could enhance the strength and limit shrinkage in traditional dental restoratives utilizing methacrylate chemistry, which the industry is already familiar with. The material outperformed the controls displaying higher conversion, lower volumetric shrinkage, and 80% lower shrinkage stress in addition to "superior results in toughness and percent elongation," they reported.

Duff and his team suggest two means of harnessing the potential of the resins: They could be used neat, as a liner material, where their high elongation and toughness will help prevent cracking and caries formation; and they could be used as functional additives to improve the physical properties of existing formulations.

'Interesting molecules'

The company's foray into this aspect of restoratives was a result of Duff's enthusiasm for chemistry.

"The project came out of an unrelated side project I was working on with some interesting molecules that I thought I could make," Duff explained. "So I made them and did some testing on their mechanical properties. At that point, I got my boss' attention, and we sent them out for shrinkage and conversion testing -- and that's when everybody got really interested in them because they came back with such good numbers."

Esstech combined Duff's efforts with previous work done on Exothane 10, the company's first attempt at making a low-shrinkage, high-conversion material.

"We had that one on the shelf in research and development for four years, but we didn't really know what to do with it, to be honest" Duff said. "The low-refractive index was a challenge for us, because dental materials companies want something that has a higher refractive index for the surface layer of the composite."

The results from testing efforts changed their minds, and the company created other Exothanes, numbered 8, 9, 26, and 32. One other material, Exothane 24, performs differently due to its "higher functionality," according to Duff.

Side-by-side comparison

For the AADR study, researchers compared the six Exothane elastomers to a BisGMA:TEGDMA blend (70:30) resin and a urethane dimethacrylate (UDMA) resin. Conversion and reaction kinetics were monitored with the near-infrared reflectance spectrum using Fourier transform infrared spectroscopy. They found that, overall, the Exothanes displayed considerably higher final conversion levels ranging from 94% to 98%.

The researchers also used a linometer to test volumetric shrinkage and found that the BisGMA:TEGDMA blend, UDMA, and Exothane 24 resulted in the greatest volumetric shrinkage and the lowest conversion values. The Exothane 24 possesses higher volumetric shrinkage and lower conversion due to its increased methacrylate functionality, they noted. In addition, "the low conversion does not necessarily indicate poor reactivity," they wrote.

Finally, the researchers used a tensometer to determine polymerization shrinkage stress, noting that "the highly functional Exothane 24 had the highest shrinkage stress" followed by the two control materials. During tensile testing, in which data were obtained by curing the Exothanes and the traditional polymeric matrix resins under a 600-W UVA light, all Exothanes proved to be significantly tougher than the controls, they noted.

"All of these attributes suggest that Exothanes would increase the durability of dental composites," the researchers concluded.

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