Influence of Cavity Designs on Fracture Behavior of a Mandibular First Premolar with a Severely Curved h-shaped Canal

Teeth are more susceptible to fracture after root canal therapy1, resulting in failure of the treatment and even extraction. Loss of dental hard tissue is 1 main cause contributing to the decreased fracture resistance2. To minimize the removal of dental hard tissue in an access cavity opening, Clark and Khademi3 proposed the conservative endodontic cavity (CEC). CEC preparation, without the completely unroofed and straight-line access needed for the traditional endodontic cavity (TEC)4, only removes the tissue needed for instrumentation entering the canal orifices. Although the CEC offers the benefit of maintaining hard tissue, its capability of preserving the fracture strength of mandibular premolars is Introduction: This study aimed to investigate the fracture behavior of a mandibular first premolar with a severely curved h-shaped canal using the extended finite element method. Methods: Following the micro-computed tomographic data, models of the intact tooth, the conservative endodontic cavity (CEC), the modified conservative endodontic cavity (MCEC), and the traditional endodontic cavity (TEC) were created. All models were subjected with a total load of 600 N perpendicularly to the contact areas. The distributions of maximum prin-cipal stress were recorded. The evolution of cracks in the enamel and dentin was simulated with the extended finite element method. Results: The intact tooth showed the highest crack initiation load and the smallest stress concentration area. The CEC and MCEC showed higher crack initiation loads and smaller stress concentration areas compared with the TEC. On the occlusal surface, tensile stress was centralized around the distal fossa and the distal margins of cavities. In the root, the stress was concentrated at the mesiolingual side for all models and at the internal surface on the bifurcation section for the CEC. Cracks originated at buccal side of the distal fossa and microcracks were formed confined to the enamel, and then cracks occurred in the dentin below the bone level. Conclusions: For the mandibular first premolar with a severely curved h-shaped canal, the MCEC preserved the fracture resistance equally as well as the CEC and reduced the stress concentration on the bifurcation section. The fracture initiated in the enamel, forming microcracks on the buccal side of the distal fossa and then occurred as an irreparable fracture in the dentin. (J Endod 2021;47:1000-1006.)