Sufficiency of curing in high-viscosity bulk-fill resin composites with enhanced opacity

The study aims analyzing if improved opacity in modern high-viscosity bulk-fill resin composites (BF-RBC) contradicts with the sufficiency of curing and to assess material's tolerance to less ideal curing conditions.

Simulated large cavities (10 × 6) mm were filled in one increment with three BF-RBCs (Filtek One, FO; Tetric Evo Ceram Bulk Fill, TEC-BF; SonicFill2, SF2). One central and two peripheral (4 mm apart from the center) micromechanical property line-profiles (HV, Vickers hardness; Y_HU, indentation modulus) were measured in 0.2-mm steps at 24 h post-polymerization (n = 6). Depth of cure (DOC) was calculated from the HV variation in depth. A scratch test (DOC_{scratch test}) estimated the tolerance in polymerization when simulating clinically relevant curing conditions (exposure distance up to 7 mm; centered and with a 3-mm offset placement of the LCU). Irradiance and spectral distribution of the used light curing unit (LCU) were assessed at various curing conditions.

DOC varied among 3.6 mm (SF2, peripheral) and 5.7 mm (FO, central). The BF-RBC influences DOC stronger (p < 0.001, ηP² = 0.616) than the width (p < 0.001, ηP² = 0.398). Significant lower DOC (t test) was measured peripheral compared to center in all materials. Y_HU was more sensitive to the varied parameters as HV. DOC_{scratch test} varied among 2.4 mm (SF2, 3-mm offset, exposure distance 7 mm) and 3.9 mm (FO, center, 0 mm).

Whether opacity competes with DOC is material dependent. BF-RBCs tolerate small variations in LCU's centricity better than variations in exposure distance.

The upper incremental thickness threshold of 4 or 5 mm was not reached in all BF-RBCs under simulated clinically relevant curing conditions.