Highly translucent dental resin composites through refractive index adaption using zirconium dioxide nanoparticles and organic functionalization

Objectives: For dental resin composites, high translucency is important. Therefore, the aim of the study was to create a biocompatible and highly translucent resin-based composite, and to investigate the effect of material thickness on translucency. Methods: A biocompatible ORMOCER® resin matrix was reinforced with dental glass powder as fillers. To reach a high translucency, refractive index matching of the matrix and fillers was done in the two ways: (1) Highly refractive ZrO2 nanoparticles were incorporated into the resin. (2) The resin was modified via addition of 4-Methylthiophenol. The corresponding refractive indices were acquired on an Abbe refractometer (n = 5). In both cases, the dental glass powder was added and translucency of the resulting minifilled and nanohybrid composites were measured using spectral photometry (n = 5). Additionally, the translucency of the experimental composites was determined as a function of specimen thickness in the range 10 mm-2 mm (n = 5). One-way ANOVA was performed to determine the significant differences in various optical parameters among different amounts of modifications and thicknesses at a = 0.05. Furthermore, cytotoxicity tests (extract and direct contact tests) were conducted according to ISO 10993 to classify the biocompatibility of the composites (n = 6). Results: The translucency values of the composites with 47 wt.-% dental glass powder and a specimen thickness of 2 mm, could be increased from 26% up to 71% by increasing the refractive index of the matrix through incorporating ZrO2 nanoparticles. Moreover, it can also be increased to 67% via addition of 4-Methylthiophenol. Further results showed that the translucency significantly depended on the sample thickness following an exponential function. The effect of all tested parameters was significant among the materials (p < 0.001). The composites did not show any cytotoxic effect. Significance: Highly translucent and biocompatible resin composites were developed. They show attractive properties for the use as dental enamel material in direct and indirect restorations.