Impact of Dissolved Helium on Polymerisation Shrinkage of Dimethacrylate Resin-blends

Objectives: To study the impact of the gaseous environment for storage and mixing of pre-cure high and low viscosity dimethacrylate resin-blends on post-cure polymerization shrinkage.
Methods: Experimental dimethacrylate resins, BisGMA and TEGDMA in weight percentage ratios of 60:40 and 40:60, respectively and 0.2% camphorquinone and 0.8% dimethylaminoethyl methacrylate (DMAEMA) were mixed in a light-proof ambient environment. The mixed resin-blends were divided into two groups and stored in either a normoxic atmosphere (G1) or in a helium atmosphere with <1.5% O₂ for 24h. A third group was separately manufactured by mixing in the helium-rich atmosphere (G3). The polymerization shrinkage percentage (PS) was calculated using an image measurement device (ACUVOL - Bisco Dental). Baseline volume measurements were obtained for 8±1 μL of each resin blend and subsequently cured using an irradiance of 1500mW/cm² at ~470nm for 20 s. Volume measurements were repeated after 5 min and PS values calculated. The PS instrumentation was calibrated by performing identical measurements on two commercial resin-based composites with low and high viscosities, as it is expected that the lower viscosity presents the highest shrinkage.
Results: Significantly increased PS was observed for the lower viscosity resin-blend (40:60 BisGMA: TEGDMA) for groups G1 (mixed and stored in normoxic conditions) and G3 (mixed in a helium-rich atmosphere, p<0.05). For G2, the helium-rich atmosphere reduced PS for the 40:60 resin-blend when compared with all other mixing/storage conditions. For the high viscosity resin-blend, it seems that the inert atmosphere had no influence on the PS, considering 24h storage or the mixing into helium. PS measurements for the commercial resin-composites used as instrument controls were within expected values.
Conclusions: The incorporation of dissolved gases in dental resin-based materials is inevitable during manufacturing and storage, but infrequently considered. Exposure of a low viscosity dimethacrylate resin-blend to a helium-rich atmosphere for 24 h resulted in a significant modification in PS demonstrating a direct effect on the reaction chemistry. The findings imply that manufacturing and storage conditions can be manipulated to improve the properties of polymerized dental resin composites.