Optimal cuspal coverage of ceramic restorations using CAD/CAM: Biomechanical characteristic analysis by 3D finite element analysis and in vitro investigation

Aim: The present in vitro study aimed to evaluate the stress distribution patterns, resistance to fracture, and failure modes of endodontically treated molars restored with different cuspal coverage options. Materials and methods: Three-dimensional models of mandibular first molars with six kinds of typical cuspal coverage were generated: T1: mesiobuccal cuspal coverage; T2: coverage of all buccal cusps; T3: mesiolingual cuspal coverage; T4: coverage of all lingual cusps; T5: mesiobuccal and mesiolingual cuspal coverage; T6: coverage of all cusps. All restorations were fabricated with zirconia-reinforced lithium silicate ceramic. The stress and its distributions under axial and oblique loading were analyzed by finite element analysis (FEA). Sixty human mandibular molar samples were randomly allocated into six groups (n = 10) to simulate the application of six types of restorations with different cuspal coverage, as in the FEA analysis, and were then subjected to a compressive test. All fractured specimens were subjected to fractography. Data were analyzed by one-way analysis of variance (ANOVA), the Tukey post hoc test, and the Fisher exact test (alpha = 0.05). Results: The T2 and T6 groups presented superior stress distribution patterns under both axial and oblique loading compared with the other models. The fracture loads in the T2 (1627 +/- 358 N) and T6 (1639 +/- 355 N) groups were significantly higher than those in the other groups (P < 0.05). The T2 and T6 groups exhibited more restorable failure modes. Fractography showed more cracks below the cementoenamel junction in the T3, T4, and T5 groups. Conclusions: Onlay restorations with whole functional cuspal coverage provided comparable effects to coverage of all cusps in endodontically treated molars, and both methods exhibited a more even stress distribution and fracture resistance and better mechanical performance in high occlusal areas than other types of cuspal coverage.