The ability of highly-filled flowable composites in preventing marginal gap in class V restorations: an optical coherence tomography study

Background: Despite improvements in adhesive systems and restorative materials, interfacial gap opening and subsequent microleakage are major factors in the failure of resin-based composite (RBC) restorations. This study evaluated the marginal gap in class V restorations using two highly-filled flowable RBC with varying viscosities, both before and after aging process. Methods: Standardized class V cavities were created on the buccal side of intact single-root human upper premolars (n = 48). Specimens were randomly divided into three groups (n = 16) considering the selected restorative materials: a nanohybrid packable RBC serving as a control group (G1, Clearfil Majesty ES2, Kuraray Noritake) and two highly-filled RBCs with different viscosities (G2, Majesty ES Low Flow, Kuraray Noritake; G3, Majesty ES Super Low Flow, Kuraray Noritake). The initial marginal adaptation, both at the enamel and dentin substrate, was evaluated using optical coherence tomography (OCT). Specimens were then divided into two subgroups (n = 8) according to the aging process performed: thermocycling (TC) and thermomechanical cycling (TMC). After TC and TMC specimens were scanned again with OCT to evaluate margin degradation. Using a dedicated program, 2D cross-sectional images were obtained and the images were processed and quantitatively analyzed using Image J software. The interfacial gap between tooth and composite was linearly measured at baseline and after aging. A three-way analysis of variance (ANOVA) and a post-hoc Tuckey test were used to statistically analyze the data (Stata 17.0 software package). Results: ANOVA statistics regarding baseline and post aging results indicated that the employed material and the substrate were influent on interfacial gap (p ≤ 0.05). The ANOVA test also showed that TMC induce significantly higher gap opening than the TC. G2 (Majesty ES Low Flow) performed significantly better than G1 (Clearfil Majesty ES2) and G3 (Majesty ES Super Low Flow) at baseline and then G1(Clearfil Majesty ES2) after the aging process in terms of interfacial adaptation. Dentin showed significantly lower adaptation at both the baseline and post aging process. Moreover, the thermomechanical cycling induced a significantly higher gap opening than the thermocycling alone. Conclusion: Highly-filled flowable RBC showed promising results in terms of interfacial gap adaptation both at the baseline and after the aging process. The presence of a cervical substrate and the mechanical aging worsen the marginal adaptation.