Influence of Cusp Coverage Design and Hybrid Resin–Ceramic Materials on the Biomechanical Performance of Partial Coverage Restorations

Restoration of structurally compromised teeth often requires cusp coverage, yet the influence of preparation design and material type on performance remains unclear. This study evaluated the effect of cusp coverage design and hybrid resin–ceramic material on the marginal adaptation and fracture resistance of partial coverage restorations in mandibular molars. Eighty extracted teeth were prepared for indirect restorations and allocated to four groups (<i>n</i> = 20) according to design, either functional cusp coverage (FC) or complete cusp coverage (CC) and material, either GC Cerasmart (CS) or VITA Enamic (EN). Restorations were bonded with dual-cure resin cement, thermocycled, and subjected to cyclic loading. Fracture load, marginal adaptation, and failure mode were evaluated (α = 0.05). CC-CS and CC-EN exhibited significantly higher fracture loads than FC-CS and FC-EN (<i>p</i> < 0.001), while no difference was found between materials within each design. For marginal adaptation, CS showed significantly greater marginal gaps than EN in both designs (<i>p</i> < 0.001). CC designs demonstrated a higher proportion of repairable failures (Type I and II), whereas EN showed more catastrophic fractures. Within the limitations of this in vitro study, cusp coverage design significantly affected fracture resistance, while material type primarily influenced marginal adaptation. Both hybrid resin–ceramics provided acceptable mechanical performance for partial coverage restorations.