Influence of Bulk-fill Restoration on Polymerization Shrinkage Stress and Marginal Gap Formation in Class V Restorations

Abstract

Purpose: This study evaluated the influence of Class V cavity extension and restorative material on the marginal gap formation, before and after aging, and the theoretical polymerization shrinkage stress distribution in a tooth restoration. Methods and Materials: Class V cavities with the depth of 2 mm, cervical/incisal distance of 4 mm, and margins located in the enamel 1 mm above the cementoenamel junction were prepared in 60 bovine incisors in two mesiodistal dimensions (n=30): 2.9-mm large extension cavities (LE) or 1.4-mm small extension cavities (SE). The cavities' depths were validated using a periodontal probe, while the mesiodistal and cervical/incisal distances were measured using a stereomicroscope. After adhesive application (Clearfil SE Bond), each group was randomly divided into two groups (n=15) according to the restorative material: Filtek Z350 XT (N) or Filtek Bulk Fill Posterior (BF). The marginal gap formation between the tooth structure and the restorative material was evaluated using a stereomicroscope before and after thermocycling for 15,000 cycles (5 degrees C and 55 degrees C). Data were analyzed using repeated-measures analysis of variance (ANOVA) and Tukey test for multiple comparisons (alpha=0.05). A three-dimensional geometric model with the same dimensions as the experimental test was created for each cavity, and the restorations were modeled for each restorative material. In the analysis software, the finite element mesh was created with tetrahedral quadratic elements, and the polymerization shrinkage was simulated by thermal analogy. The maximum principal stress was used to express the tensile stress in the adhesive interface through colori-metric graphs. Results: For the marginal gap, the repeated-measures ANOVA revealed a significant effect only for the factors composite resin (df=1, F=4.09, p =0.04) and thermal aging (df=1, F=44.35, p<0.001). For all numerical simulations, higher stress concentration occurred at the enamel margin, and the stress peak decreased in the following sequence: LE-N (17.0 MPa) > SE-N (15.0 MPa) > LE-BF (9.1 MPa) > SE-BF (8.2 MPa). Conclusion: Marginal gaps in the specimens fell between approximately 12 and 17 mu m; however, the regular bulk-fill composite showed less gap formation and better stress distribution around the cavity margin than the regular nano-filled composite, regardless of the cavity extension.