Effect of Beveling Large Class II Cavities on the Enamel Marginal Quality of Direct Resin-Based Restorations

Background/Objectives: It is unclear whether enamel margins should be beveled in direct resin-based restorations. This study evaluated the influence of enamel beveling on the marginal quality of mesio-occluso-distal (mod) cavity boxes. Methods: Seventy-five caries-free human molars were divided into three groups. Mod-cavities with the entire margin in the enamel were prepared ± proximal bevel (n = 25). Twenty-five beveled mod-cavities served as control. Each group was restored with five material combinations: micro hybrid composite with etch-and-rinse (ER) or self-etch (SE) adhesive, compomer with ER or SE, and low-shrinkage composite with ER. A complex filling technique was used in the control. After artificial aging (1000 thermal cycles, 5/55 °C), the percentage of continuous margins (PCM) of the proximal boxes was analyzed by scanning electron microscopy using epoxy replicas (×300), and the marginal seal was assessed by light microscopy after dye penetration (×64). Statistical analysis was performed using Kruskal–Wallis and Mann–Whitney U tests (p < 0.05). Results: No significant differences in PCM were found between the restorative systems (p = 0.075). The composite with low shrinkage showed the highest mean PCM of all groups (78.1%). Significant differences in marginal seal were observed between the restoratives after bevel preparation (p < 0.05). Beveling significantly improved the PCM only for the hybrid composite (p < 0.05), whereby the effect on marginal seal was less pronounced. Conclusions: Clinicians should be aware that beveling mod-cavities does not necessarily improve the marginal quality of direct resin-based restorations bonded with well-established adhesives and may be more beneficial for traditional hybrid composites.