Micro-computed tomography evaluation of polymerization shrinkage of bulk fill composites cured by different light curing modes

Objectives: This study evaluated the volumetric polymerization shrinkage (VPS) of two bulk-fill resin composites, one packable and one flowable, at two different locations, light-cured by two different modes, using micro-computed tomography (uCT), a non-destructive analysis.
Methods: Standardized class-I preparations (4mm depth x 4mm length x 4mm wide) were performed in 24 human molars and randomly divided in 4 groups (n=6/group), according to the type of composite and light-curing mode used: Tetric EvoCeram Bulkfill + High mode (10sec - 1,200 mW/cm²); Tetric EvoFlow Bulkfill + High; Tetric EvoCeram Bulkfill + Turbo mode (5sec - 2,000 mW/cm²); Tetric EvoFlow Bulkfill + Turbo. All groups were light-cured by the same light-curing unit (Bluephase Style 20i). Each tooth was scanned 3 times by uCT: 1- after the cavity preparation, 2- after cavity filling (uncured), and 3- after composite curing. The uCT images were imported into a 3D rendering software; VPS percentages and their location (regarding top and bottom/sides of the restoration) were calculated for each sample. Data were submitted to one-way ANOVA and post-hoc comparisons.
Results: When High curing mode was evaluated, the packable bulk-fill material presented lower polymerization shrinkage than the flowable one (p=.032); however, when Turbo mode was evaluated, both materials presented no significant differences (p=.557). For both composites, High and Turbo modes did not present statistical significant differences (p>.05). For all materials and curing modes, higher VPS percentages were observed on the occlusal (88.81-98.74%) than in the pulpal surface of the cavities (p<.05).
Conclusions: Different curing modes did not influence VPS of class-I bulk-fill composite resin restorations, either conventional or flowable. The conventional bulk-fill composite presented lower VPS than its flowable counterpart when High mode was used. For both types of light-curing modes and composites, volumetric shrinkage was predominantly located on the occlusal surface of the restoration, while less shrinkage occurs at the pulpal surface.