Fracture resistance of mandibular molar endocrown with different preparation designs: an in vitro study

Background Endocrowns have gained popularity as a conservative option for restoring endodontically treated molars because their success rates are comparable to those of conventional crowns. However, there is limited evidence on the effectiveness of different endocrown marginal designs, particularly concerning their fracture resistance and failure modes under axial and oblique loading. This in vitro study aimed to evaluate the fracture resistance and failure modes of three different endocrown marginal designs on mandibular molars and compare them with endocrowns under axial and oblique loading. Methods Sixty-four extracted mandibular molars were root-treated and lined with flowable composite (1.5 mm thickness) before being embedded in epoxy resin blocks. The samples were divided into four groups ( n = 16): three endocrown groups with different preparation designs (butt-joint, 0.8 mm chamfer, and 1.0 mm shoulder) and one group with conventional crowns. Crown preparations were performed with a high-speed handpiece mounted on a modified milling device, targeting a 2.0 mm anatomical occlusal reduction for conventional crowns and a 4.0 mm central chamber depth with a 2.0 mm axial wall height (ferrule) for endocrowns (except the butt-joint design, which had no ferrule preparation). Restorations were fabricated using the Chairside Economical Restoration of Esthetic Ceramics (CEREC) Computer-Aided Design and Computer-Aided Manufacturing (CAD/CAM) system with lithium disilicate (IPS e.max CAD; minimum occlusal thickness: 1.5 mm) as the restorative material and cemented with the RelyX Ultimate dual-cure resin adhesive. After 1,500 thermocycling cycles, the samples were divided into two subgroups ( n = 8) for axial and oblique loading tests. The fracture resistance was tested using a universal testing machine at a crosshead speed of 0.5 mm/min, applying a load of up to 5,000 N until fracture. Failure modes were examined using a stereomicroscope at 5x magnification and categorized as catastrophic or non-catastrophic. The mean fracture resistance was compared using one-way Analysis of Variance (ANOVA) with Bonferroni post-hoc tests, and Fisher’s exact test was used to assess the association between preparation designs and failure modes ( p < 0.05). Results Endocrowns with shoulder margins showed significantly higher fracture resistance (3,042.83 ± 850.76 N) than conventional crowns (1,908.28 ± 635.82 N) under axial loading ( p = 0.02). No significant differences were found under oblique loading between the preparation designs. The preparation design significantly influenced the failure mode under axial loading ( p = 0.049). Endocrowns exhibited a higher rate of catastrophic fractures than conventional crowns, regardless of loading direction. Conclusions Endocrowns provide a conservative alternative to conventional crowns, offering a higher fracture resistance under axial loading. Further research is needed to evaluate the endocrown performance under oblique loading.