Filler characteristics of modern dental resin composites and their influence on physico-mechanical properties

The mechanical properties of dental resin-based composites (RBCs) are highly dependent on filler characteristics (size, content, geometry, composition). Most current commercial materials are marketed as "nanohybrids" (i.e. filler size <1μm). In the present study, filler characteristics of a selection of RBCs were described, aiming at identifying correlations with physico-mechanical properties and testing the relevance of the current classification.

Micron/sub-micron particles (> or <500nm) were isolated from 17 commercial RBCs and analyzed by laser diffractrometry and/or electron microscopy. Filler and silane content were evaluated by thermogravimetric analysis and a sedimentation technique. The flexural modulus (E_flex) and strength (σ_flex) and micro-hardness were determined by three-point bending or with a Vickers indenter, respectively. Sorption was also determined. All experiments were carried out after one week of incubation in water or 75/25 ethanol/water.

Average size for micron-sized fillers was almost always higher than 1μm. Ranges for mechanical properties were: 3.7flex^water<16.3GPa, 86<σ_flex^water<161MPa and 23.7water<108.3HV0.2/30. Values generally decreased after storage in ethanol/water (Δmax=86%). High inorganic filler contents (>75wt%) were associated with the highest mechanical properties (E_flex and σ_flex>12GPa and 130MPa, respectively) and lowest solvent sorption (∼0.3%).

Mechanical properties and filler characteristics significantly vary among modern RBCs and the current classification does not accurately illustrate either. Further, the chemical stability of RBCs differed, highlighting differences in resin and silane composition. Since E_flex and sorption were well correlated to the filler content, a simple and unambiguous classification based on such characteristic is suggested, with three levels (ultra-low fill, low-fill and compact resin composites).