Influence of Cement Thickness, Dentine Thickness, and Intracoronal Depth on the Fracture Resistance of 3D-Printed Endocrowns: A Pilot In Vitro Study.

Abstract

Background/Objectives: Endodontically treated molars are structurally weakened due to internal tissue loss, increasing their risk of fracture. Endocrowns, developed as a conservative alternative to post-core systems, have gained popularity with the rise of digital dentistry, CAD/CAM workflows, and 3D-printed restorations. In this context, the aim of the present pilot study was to investigate the influence of cement layer thickness, intracoronal depth, and dentine wall thickness on the fracture resistance of molars restored with 3D-printed endocrowns. Methods: Twelve extracted human molars were endodontically treated and restored with endocrowns fabricated from a 3D-printed resin material, SprintRay Crown^TM (SprintRay Inc. Los Angeles, CA, USA), via masked stereolithography (MSLA) on a Prusa SL1 printer. Cementation was performed using RelyX Universal Resin Cement (3M, Maplewood, MN USA). Cone beam computed tomography (CBCT) was used to measure the dentine thickness and intracoronal depth before cementation and cement thickness after cementation. The fracture resistance was evaluated using a universal testing machine. For each variable (Td, Dp, Tc), the 12 specimens were divided into two groups (n = 6). Statistical analysis included Pearson correlation, a one-way ANOVA, and the non-parametric Mann-Whitney U test. Results: Within the limitations of this pilot in vitro study, cement thickness demonstrated a strong positive correlation with fracture resistance (r = 0.577) and was the only variable showing statistical significance in the ANOVA (F = 7.847, p = 0.019). In contrast, intracoronal depth and dentine wall thickness exhibited weaker and nonsignificant correlations. No significant mechanical advantage was observed from increasing the pulp chamber depth or peripheral dentine wall thickness. This result was further supported by nonparametric Mann-Whitney U testing (p = 0.015). Conclusions: Cement layer thickness is a key biomechanical factor influencing the fracture resistance of endocrown restorations. Preparation depth and dentine wall geometry appear to have a less direct impact.