Aim: Bulk-fill resin composites have a high depth of cure, and cavity with a depth even higher than 4 mm can be restored though a single step with the bulk-fill technique. A main concern of bulk-fill composites is the shrinkage stress developing at the composite-tooth walls interface, potentially higher than that developed through the incremental layering technique. The aim of this study is to investigate cusps deformation caused by material shrinkage during photopolymerization.
Materials and Methods: Two bulk-fill composite materials, with high-viscosity and low-viscosity and a different material composition, were characterised through linear shrinkage and compressive tests. Using both the bulk-fill and the incremental layering techniques for the restoration of mesio-occlusal-distal cavities of human premolars, cusps distance variation was evaluated.
Results: The low-viscosity composite presents a significantly higher shrinkage value (p<0.05) than the high-viscosity composite, while mechanical properties such as strength and the Young’s modulus of the high-viscosity composite are significantly higher (p<0.05). No significant difference has been observed for cusps deformation in both the composite materials according to mesio-occlusal-distal cavities restored through the bulk-fill or the incremental layering techniques.
Conclusions: Although for the high-viscosity composite, a lower cusps deformation would be expected, no significant difference has been observed between the composite materials. This result can be ascribed to the higher Young’s modulus value of the high-viscosity composite. This suggests that the high-viscosity composite is stiffer, while the low-viscosity composite is more compliant, thus balancing the cusps distance variation.