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EPR detects release of hydroxyl free radicals from dental resin

“The results of this paper present sufficient evidence to affirm that dental resins release hydroxyl radicals”

Dental resins are widely used for restorative dental work. They have several properties that make them desirable for filling small cavities, forming the bases of dental bridges and crowns and cosmetic reshaping of teeth. Dental resins have a good tooth-like appearance, are readily pliable, bond to the repaired tooth, and are reasonably inexpensive.

They are formed by photopolymerization of dimethacrylate monomers to create an organic matrix, which is filled with an inorganic compound such as silica. Free radicals produced during the polymerization reaction become trapped within the substance of the resin. The high degree of entanglement of the cross-linked matrix prevents these free radicals from leaving the resin, but their concentration has been shown to reduce overtime until none is detectable.

This disappearance of the free radicals is thought to occur through further reactions with the methacrylate polymer. Such reactions are likely to produce more free radicals, indeed, there is evidence supporting the presence of peroxyl free radicals. This supports oxidation as the key pathway for the post-polymerization decrease in free radical concentration. However, although the role of oxidation in the reduction of trapped radicals has gained widespread acceptance, the exact mechanism involved has not yet been described.

Spin-trapping and electron paramagnetic resonance (EPR) spectroscopy have been used to assess the potential release of free radicals from dental resin. Samples of recently formed dental resin were immersed in ethanol containing the spin-traps EMPO and DEPMPO. EPR assessments were made at room temperature for up to 2 hours after photopolymerization using a bench-top EPR spectrometer.

The release of hydroxyl radicals from the methacrylated resin was detected. These subsequently reacted with the surrounding ethanol molecules to produce secondary ethoxy free radicals. The amount of hydroxyl free radicals produced was not quantified, but there is the potential for these highly reactive hydroxyl radicals to damage surrounding tissues on their released from the dental resin.

It is already known that unconverted monomers remaining after incomplete photopolymerization are cytotoxic and can have harmful effects in vivo. The release of free radicals within the mouth could pose an additional safety concern for the use of dental resins. It is thus important that further research assesses whether the free radicals from dental resins are released in quantities likely to affect the biocompatibility.

This detection of free radical release from dental resin provides important new information for understanding the aging process of such resins and highlights a new potential source of concern for the safety of dental resins that warrants further investigation.

Lamblin G, et al. Acta Biomaterialia 2010;6:3193–3198