METHODS: Three hundred samples were prepared (6 × 2 mm disc shape) and divided into five groups of denture polymers (n = 60) and further subjected into five treatment groups (Polident®, Steradent, distilled water, eugenol 5-minutes, and eugenol 10-min). Three samples were extracted from each treatment group for baseline data (n = 12). Baseline data were used to calculate the initial number of C. albicans adherence. A 0.5 ml immersion solution from each specimen was cultured on YPD agar and incubated for 48 h at 37 °C. Visible colonies were counted using a colony counter machine (ROCKER Galaxy 230).

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.