Linear Shrinkage Stress and 3D-Gap Evaluation in High C-Factor Cavities

Objectives: The aim was evaluating linear shrinkage stress of different bulk-fill composites and if the tridimensional interfacial gap in deep class I cavities is affected by material, curing techniques and layering strategies.
Methods: Linear shrinkage stress evaluation was performed with a universal testing machine, considering the contraction force generated by composite specimens placed between metal cylinders. Four materials were tested: SDR (SDR, G1), SonicFill2 (SF, G2), Admira Fusion X-Tra (AFXT, G3), Filtek Supreme XTE (FS, G4). For 3D gap analysis n=80 human molars were selected and prepared with standardized deep class I cavity and adhesive procedure. Four subgroups (n=5) were created according to layering and curing techniques: 2+2mm with soft start curing (SG1), 2+2mm with conventional curing (SG2), 4mm with soft start curing (SG3), 4mm with conventional curing (SG4). All samples underwent micro-CT scans and afterwards, voids surrounding restorations underwent automatically thresholding procedure (Mimics, Materialise) in order to analyze 3D interfacial gap. Statistics were performed using one-way ANOVA with post-hoc Bonferroni correction and three-way ANOVA with Tukey test.
Results: Concerning linear shrinkage, SDR presented significantly lower stress during irradiation, while FS showed a significantly higher stress. Concerning interfacial gap, significant differences were reported between materials, with all bulk groups performing better than FS. Curing technique did not influence significantly interfacial gap in any group. Layering technique influenced interfacial gap only in FS group, with SG1 and SG2 performing better. Regarding gap localization, bottom of the cavity resulted to be the critical area in all samples.
Conclusions: Low-viscosity bulk-fill composites seem to possess the best performances for deep class-I cavities.