Introduction The purpose of this study was to investigate the effects of an isthmus on the success rate of surgically treated molars. Methods The study included 106 maxillary and mandibular first molars with endodontic lesions limited to the periapical area. Endodontic microsurgical procedures were performed according to the Yonsei protocol reported in a previous study. When an isthmus was observed after a high-magnification inspection, it was included in the retrograde preparation design. When an isthmus was not observed, only the main root canal space was prepared. The patients were followed up periodically every year after treatment. Success was defined as the absence of clinical signs and symptoms and radiographic evidence of complete or incomplete healing. Results Of the 106 teeth included in the study, 72 teeth had an isthmus, and 34 did not. Kaplan-Meier analysis revealed that the cumulative survival rate after surgery was 61.5% and 87.4% for 4 years when an isthmus was present and absent, respectively. A multivariate Cox proportional hazards regression analysis showed that the adjusted hazard ratio for failure was 6.01 times higher for the isthmus-present teeth than for the isthmus-absent teeth (P <.05). Conclusions In this study, the success rate for endodontic microsurgery on isthmus-absent teeth was higher than that for isthmus-present teeth. Considering the success rate and potential risk of weakening the root after preparation, the techniques of isthmus preparation need to be improved.
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