Fracture Resistance and Stress Distribution of Weakened Bovine Roots Restored with Different Glass Fiber Post Systems

Background/Objectives: The restoration of structurally weakened roots remains a clinical challenge. This in vitro study evaluated the fracture resistance, failure pattern, and stress distribution of weakened bovine roots restored with two glass fiber post systems under different remaining root dentin thicknesses. Methods: Sixty bovine incisor roots were divided into six groups (n = 10): G1, 2 mm of remaining root dentin without a post; G2, 3 mm without a post; G3, 2 mm with an Exacto post; G4, 3 mm with an Exacto post; G5, 2 mm with a Splendor-SAP post; and G6, 3 mm with a Splendor-SAP post. Specimens underwent root canal treatment and standardized restoration, followed by 1,200,000 mechanical cycles at 88 N and 3.8 Hz and fracture testing at 45°. Finite element models were subjected to an 88 N oblique load. Fracture resistance was analyzed using Welch’s one-way ANOVA followed by Games–Howell post hoc comparisons, whereas failure patterns were analyzed using the Fisher–Freeman–Halton exact test. Results: Fracture resistance differed significantly among groups (p < 0.001). G1 (88.7 N) and G2 (95.6 N) showed the lowest and statistically similar values; G3 (670.9 N) and G5 (691.9 N) showed intermediate and statistically similar values; and G4 (786.2 N) and G6 (784.8 N) showed the highest and statistically similar values. Failure-pattern distribution was not significantly associated with the experimental groups (p = 0.582). Finite element analysis showed stress concentration predominantly in the cervical region of the root, with G5 showing the highest maximum principal stress in root dentin. Conclusions: Glass fiber posts substantially increased the fracture resistance of weakened roots. Among post-restored specimens, roots with 3 mm of remaining dentin showed higher fracture resistance than those with 2 mm, whereas no significant difference was observed between the Exacto and Splendor-SAP systems under the same remaining-dentin condition.