Effect of Apical Limit Settings on the Occurrence of Apical Dentinal Microcracks in Procedures up to Root-end Filling

抄録

 Purpose: In this study, teeth in which root canal shaping was performed with different apical limit settings were used to clarify the timing of development of apical dentinal microcracks (microcracks). The microcracks occurring during root canal shaping were observed using micro-computed tomography (micro-CT). Root canal obturation of the experimental teeth was followed by root-end resection and cavity preparation for the same teeth, and the occurrence of microcracks on the resected root surfaces was evaluated.

 Methods: Nineteen human-extracted mandibular canines and premolars with similar root morphology were used. Root planting models with exposed apex (test piece) were prepared for the teeth. Once a tooth was removed, micro-CT scanning was performed before the procedure. The apical limits were set in three groups (n＝6): at the anatomical apical foramen (Af), at 1 mm inner Af (Af−1), and at 1 mm outer Af (Af＋1). After root canal shaping with nickel-titanium rotary files under wetting, micro-CT scans were performed, and the presence of a microcrack was evaluated in each image. After root canal obturation, the apex was resected perpendicularly to the tooth axis using carbide burs at 3 mm from the apex. The resected root surfaces were stained with methylene blue solution, and stereomicroscopic observation was performed under light-emitting diode (LED) transillumination to record the occurrence and direction of microcracks. The length of microcracks from the root canal wall was also measured. Following these observations, 3 mm depth root-end cavities were prepared along the root canal axis using an ultrasonic tip. Subsequently, staining and transillumination observations were performed using the preview method, and the occurrence and direction of microcracks on the resected root surface were recorded. The length of microcracks from the wall of the root-end cavity was also measured.

 Results: Micro-CT images demonstrated no new microcracks during root canal shaping. After root-end resection, the number and length of microcracks increased in the order of the Af−1, Af and Af＋1 groups. After root-end cavity preparation, the detected incomplete cracks ran in the labiolingual direction and the complete crack ran in the mesiodistal direction. Furthermore, the length of microcracks after root-end cavity preparation was significantly greater in the Af＋1 group than in the Af−1 group (p＜0.05).

 Conclusion: The results suggest that root canal shaping with apical limit settings outside the root causes thinning of the dentin and increases the probability of microcracks during ultrasonic root-end cavity preparation.