In Vitro Evaluation of Marginal Adaptation in Large Direct Class II Restorations Using Single- and Multi-Increment Approaches with “Bulk-Fill” Composites Under High Cycling Load

Objective: Evaluating the in vitro marginal adaptation of conventional, flowable and restorative bulk-fill resin composites placed in large class II cavities with supra- and sub-gingival margins, using a bulk-fill or layering approach, before and after thermo-mechanical loading (TML) simulating parafunctional forces. A total of 40 prepared teeth were divided and assigned to each of the five experimental groups. In group 1, restorations were made of layered high-viscosity conventional composite (Tetric EvoCeram); in groups 2 and 3 restorations were made of a high-viscosity bulk-fill composite (Tetric Powerfill) applied in one (group 2) or three layers (group 3); in groups 4 and 5 restorations were made of a flowable bulk-fill composite (SDRflow) applied in one (group 4) or two layers (group 5), underneath a layer of high-viscosity composite (Ceram-X Spectra ST). The same adhesive (OptiBond FL) was used in all groups. All specimens were submitted to a TML comprising a loading phase of 250,000 cycles at 100 N combined to 1675 thermal cycles (5 to 55 °C). The proximal tooth-restoration interfaces were analyzed quantitatively by SEM, prior and after TML. Results: Repeated measures ANOVA followed by Fisher’s LSD (Least Significant Difference) post hoc tests served for comparing inter-group marginal adaptation percentages between the pre- (T0) and post- (T1) loading conditions and intra-group marginal adaptation percentages. The lowest pre-loading values were for the cervical dentin adaptation ranging from 94.79% (SDRflow layered) to 66.06% (Tetric Powerfill layered) while the post-loading values of continuous cervical dentin adaptation varied from 61.20% (SDRflow layered) to 33.36% (SDRflow Monolayer). TML with higher axial forces led to a marked reduction in continuous adaptation at enamel or dentin margins in all groups. Overall, the low-viscosity bulk-fill SDRflow layers showed the best behavior while other products showed varying levels of degradation. Conclusions–Clinical significance: Simulated bruxism loading conditions induced severe marginal adaptions of class II composite restorations, which could potentially impact their lifespan.