Nanotechnology influence on polymerization shrinkage of composites

Objective: The aim os this study was to evaluate the volumetric and linear polymerization shrinkage and contraction stress of four nanohibrid composite resins. Materials and methods: Four nanohybrid resins and one microhybrid were selected. To measure free linear contraction, five specimens of each resin were made on a teflon mold, 6mm length, 3mm width and 2mm depth. The specimens were filled to achieve a smooth surface. The mold-specimen set was observed under microscope and the gaps between the mold and the resin were measured in micrometers. To measure volumetric contraction, five resin discs of each resin were prepared (1mm thick, 1g weight). The measurements were performed with the analytic scale, using the Archimedes method. Contraction stress set-up was performed bonding composites between stainless steel plates (C Factor 1,5) with one of the plates connected to a load cell of 50N, attached to a universal testing machine EMIC DL500. Contraction stress analisis was iniciated the moment the light activation was performed (40 seconds) and carried on until 120 seconds. Results: One way ANOVA and Tukey's test showed that all nanohybrid resins presented lower values of linear contraction (11.2, 15.6, 23.2, 24.2µm) compared to the microhybrid resin (33.4µm), with no significant difference between TPH3 and Z350. The values of volumetric contraction were lower with nanohybrid resins (-1.61±0.04, -1.63±0.03, -1.68±0.05, -1.71±0.03) compared to the microhybrid resin (-1.99±0.11). Contraction stress analisis showed no statystical difference between groups. Conclusion: Nanohybrid resins showed a smaller degree of contraction on both linear and volumetric shrinkage tests applied, compared to a microhybrid resin, but contractions stress of polymerization was similar to all composites. Grandio and Tetric Evoceram obtained the lowest values of volumetric contraction. The size and amount of particles seem to have influenced the polymerization contraction.