Tehran University of Medical Sciences Journal of Craniomaxillofacial Research 2345-5489 13 3 2026 10 21 338 344 Hamed Kazemi Student Research Committee, Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. Amir Reza Khazaei Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. Amir Hossein Khazaei Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. Fatemeh Rashki Ghaleh No Department of Endodontics, Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. 2026 02 16 2026 08 30 Introduction: This study aimed to evaluate the influence of different access cavity designs (conservative versus traditional) and the impact of thermocycling and load cycling on the fracture resistance of endodontically treated molars. Materials and Methods: In this in vitro investigation, sixty extracted human molars were selected and allocated into six groups based on access cavity design and subsequent thermocycling (1000 cycles between 5 °C and 55 °C) and load cycling (200,000 axial cycles at 80 N) protocols. Root canal treatment was performed on all specimens using standardized endodontic procedures. Fracture resistance was subsequently assessed using a universal testing machine. Results: The mean fracture resistance of the teeth with conservative access cavity preparation was significantly higher than that of traditional preparation (P = 0.044). Furthermore, the application of thermocycling and load cycling significantly reduced fracture resistance across all tested groups (P < 0.05). Conclusion: These findings underscore the critical roles of access cavity design and simulated functional aging conditions in determining the long-term structural durability of endodontically treated teeth.  Keywords: Root canal therapy; Fracture resistance; Load cycling; Thermocycling; Access cavity Preparation.  https://jcr.tums.ac.ir/index.php/jcr/article/view/629 https://jcr.tums.ac.ir/index.php/jcr/article/download/629/513