System compliance dictates the effect of composite filler content on polymerization shrinkage stress

Abstract

Objective The effect of filler content in dental restorative composites on the polymerization shrinkage stress (PS) is not straightforward and has caused much debate in the literature. Our objective in this study was to clarify the PS/filler content relationship based on analytical and experimental approaches, so that guidelines for materials comparison in terms of PS and clinical selection of dental composites with various filler content can be provided. Methods Analytically, a simplified model based on linear elasticity was utilized to predict PS as a function of filler content under various compliances of the testing system, a cantilever beam-based instrument used in this study. The predictions were validated by measuring PS of composites synthesized using 50/50 mixtures of two common dimethacrylate resins with a variety of filler contents. Results Both experiments and predictions indicated that the influence of filler content on the PS highly depended on the compliance of the testing system. Within the clinic-relevant range of compliances and for the specific sample configuration tested, the PS increased with increasing filler content at low compliance of instrument, while increasing the compliance caused the effect of filler content on the PS to gradually diminish. Eventually, at high compliance, the PS inverted and decreased with increasing filler content. Significance This compliance-dependent effect of filler content on PS suggests: (1) for materials comparison in terms of PS, the specific compliance at which the comparison being done should always be reported and (2) clinically, composites with relatively lower filler content could be selected for such cavities with relatively lower compliance (e.g. a Class-I cavity with thick tooth walls or the basal part in a cavity) and vice versa in order to reduce the final PS.

Polymerization shrinkage and shrinkage force kinetics of high- and low-viscosity dimethacrylate- and ormocer-based bulk-fill resin composites

“…While the elastic modulus has been revealed as the decisive factor affecting shrinkage stress formation in these highly rigid testing systems, shrinkage stress development depends more on polymerization shrinkage than on elastic modulus when more compliant (semi-rigid) measuring devices are used [48][49][50]. Consequently, conflicting results on the effect of inorganic filler content on shrinkage stress might be ascribed to differences in the compliance of measuring devices [51]. Since real teeth and their cavities are not completely rigid, but show elastic and visco-elastic characteristics [52], in the current experimental set-up, axial deformation of the specimens was only partially restricted, with the load cell being axially displaced by 0.4 µm per Newton force.…”

“…Since real teeth and their cavities are not completely rigid, but show elastic and visco-elastic characteristics [52], in the current experimental set-up, axial deformation of the specimens was only partially restricted, with the load cell being axially displaced by 0.4 µm per Newton force. According to Wang and Chiang [51], an instrument compliance of 0.4 µm/N falls in the category of low compliance, in which the stress should increase with the filler content.…”

Polymer-Based Direct Filling Materials

“…Especially for class I and class V restorations, the cavity configuration factor (C-factor, the ratio of surface area of bonded to non-bonded interfaces in a preparation) is high and this has been correlated, in general, with increase in stress [70]. This correlation is not without controversy, as it is often seen as an oversimplification of the subject, since it overlooks the volume of the restoration [70] and the condition of the remaining tooth structure [71–73]. However, at least for conventional composites, because of the C-factor vs. stress correlation, the use of incremental placement is still recommended to minimize the bonded surface on each increment, reducing the relative C-factor in each increment, and therefore, reducing overall stress [60].…”

Nanoindentation study on mechanical properties and curing depth of dental resin nanocomposites

“…Moreover, the reported degree of polymerization of the dental resin composites was in a range of 35–77%, and it was found to decreases with the depth . So we assume that, as the depth becomes deeper, the conversion of resin monomers reduces, and the reinforcement effect of the silica nanoparticles on the matrix would be enhanced due to an increasing ratio of the modulus of the particle to that of the matrix . In this way, the improved reinforcement effect with the depth could become more intensive as the particle content increases, thereby probably giving rise to an increasing depth of cure when subjected to a higher particle content in Fig.…”

“…The experimental organic matrix contained Bis‐GMA and TEGDMA monomers (both from BOC sciences Inc., USA). A visible light initiator system consisted of photoinitiator camphorquinone (CQ) and ethyl 4‐dimethylaminobenzoate (EDAB) (both from Sigma–Aldrich, Milwaukee, WI) . The silanized silica nanoparticles (Evonik Inc., Germany) with an average diameter of 20 nm and a density of 2.2 g/cm 3 were used as the inorganic reinforcement of the composites.…”