Influence of Endodontic Access and Thermocycling on the Fracture Resistance of CAD/CAM Lithium Disilicate Anterior Crowns.

Objectives: Catastrophic failures in lithium disilicate single-unit prostheses in the mandibular anterior region, particularly when subjected to endodontic access for root canal treatment, represent a significant clinical concern. This in vitro study evaluated the fracture resistance of CAD/CAM-fabricated lithium disilicate intact and endodontically accessed crowns, with and without aging through thermocycling. Methods and Materials: A mandibular right central incisor typodont was prepared and scanned for CAD/CAM fabrication of a lithium disilicate crown. Eighty crowns were milled, treated, etched, primed, and cemented to printed resin dies. The crowns were divided into eight groups based on endodontic access location (facial, incisal, or lingual) and thermocycling condition (presence or absence). All groups underwent vertical loading until fracture, and fracture resistance was measured. Fractured crowns were analyzed for crack patterns and failure modes using scanning electron microscopy. Two-way ANOVA assessed the effects of access type on fracture resistance, while one-way ANOVAs with Tukey's HSD post hoc tests analyzed fracture resistance across aging conditions. Results: Fracture resistance was significantly affected by endodontic access type and thermocycling (p < 0.001). Non-thermocycled, intact crowns had the highest fracture resistance (1218.0 ± 174.8 N), while thermocycled crowns with incisal access demonstrated the lowest resistance (720.9 ± 98.3 N). Lingual access crowns displayed higher fracture resistance than facial and incisal access crowns, although resistance significantly decreased with thermocycling (p = 0.008). SEM analysis revealed that lingual access crowns visually appeared to exhibit less irregular crack patterns compared to facial and incisal access crowns, which displayed more numerous and irregular crack lines. Conclusions: Endodontic access reduced fracture resistance; however, thermocycling had a statistically significant effect only in the lingual access group. A conservative lingual access preserved integrity similarly to intact crowns, making it preferable for mandibular restorations. All crowns met minimum clinical fracture resistance standards, suggesting that crown replacement may not be necessary with conservative access designs. Variations between aged and non-aged specimens should be considered when interpreting in vitro data.