Cuspal Deflection and Marginal Adaptation in Premolars Restored with Low-Shrinkage Resin Composites

Objectives The aim of this in vitro study was to evaluate cuspal deflection and marginal adaptation in premolars restored with conventional and low-shrinkage resin composites (RCs). Materials and Methods Fifty-six maxillary first premolars with standardized MOD cavities were randomly assigned to four groups (n ¼ 14): Clearfil Majesty Posterior (CMP), Charisma Diamond (CHD), Beautifil II LS (BL), and Filtek Z550 (FZ). Cuspal deflection was measured using a digital micrometer as the linear distance between two reference points at baseline, 5 minutes, and 24 hours after restoration. Deflection values were calculated relative to the baseline. Three samples per group were evaluated using scanning electron microscopy to assess marginal adaptation. Statistical Analysis Data were analyzed using IBM SPSS Statistics V23.0. Data distribution was evaluated using the Shapiro-Wilk test. Kruskal-Wallis and one-way ANOVA tests were used according to data distribution, followed by appropriate post hoc tests. The significance level was set at p < 0.05. Results The amount of cuspal deflection at 5 minutes was greater than that observed at 24 hours in all groups. CMP, CHD, and BL showed lower cuspal deflection compared with FZ at all time intervals (p < 0.001). CMP exhibited the lowest cuspal deflection values, although no significant differences were found among the low-shrinkage RCs. Conclusion Modifications in the organic matrix or material formulation of the RCs can contribute positively to the control of cuspal deflection and support improved marginal adaptation. The reduced cuspal movement produced by low-shrinkage RCs may increase the likelihood of clinical success by helping preserve restoration integrity. © 2026. The Author(s).