Evaluation of the Fracture Resistance of Endocrown Restorations Prepared at Different Heights Above the Enamel-Cement Junction

Objective: This study evaluated the fracture resistance of endocrown restorations prepared at different heights above the enamel-cement junction (ECJ). It sought to elucidate the effects of restoration margins on biomechanical behavior and the success of restorations. Materials and Methods: Eighty extracted mandibular molars were divided into four groups. In Group I, restorations were prepared at the ECJ level; in Group II, 1 mm above; in Group III, 2 mm above and Group IV, 4 mm above the ECJ. Restorations were fabricated from lithium disilicate ceramics using CAD/CAM technology and cemented with appropriate adhesive protocols. The fracture resistance of the restorations was tested using a universal testing machine. Data were assessed for non-normal distribution using the Shapiro-Wilk test, and intergroup comparisons were conducted using the Kruskal-Wallis H test, followed by Dunn’s test. Results: The fracture resistance of Group II (1 mm) and Group III (2 mm) was significantly higher compared to the other groups, with mean values of 1423 ± 75 N and 1389 ± 68 N, respectively (p < 0.05). Group I (at the ECJ level) exhibited the lowest fracture resistance, with a mean of 1023 ± 95 N. Group IV (4 mm) demonstrated a mean fracture resistance of 1225 ± 81 N, which was significantly lower than Groups II and III (p < 0.05). These findings indicate that positioning the restoration margins closer to the enamel-dentin transition enhances biomechanical stability. Conclusion: Endocrown restorations prepared 1–2 mm above the ECJ demonstrated superior fracture resistance within the scope of this study. The findings emphasize the significance of preparation height in achieving optimal biomechanical performance. While lithium disilicate ceramics were utilized in this research, the study did not compare different material types. Therefore, further investigations are necessary to evaluate the influence of alternative restorative materials. Additionally, long-term clinical studies are required to validate these findings under intraoral conditions.