Abstract: Objective: To observe the treatment outcome of crown lengthening on mandibular second molars, determine the distribution of risk factors, and explore the influence of the number of risk factors on the effect of crown lengthening. Methods: A total of 314 mandibular second molars were included from the patients visited our hospital from 2019 to 2021. At 6-week follow-up, the patients were divided into successful group and failed group. The risk factors that affected the curative effect of two groups were screened, and the correlation between the number of them and the curative effect was analyzed. Results: Of the 314 teeth, 88.54% had good results and 11.46% had poor results. The distance between gingival margin and the broken end (P<0.01), the distance between gingival margin and crest (P=0.032), and the shape of root (P=0.006) were the risk factors influencing the postoperative outcome. As the number of risk factors increased, the rate of surgical failure increased linearly, and when the number of risk factors reached 3, the risk of failure increased significantly (P<0.01). There was a positive correlation between them (P<0.01). Conclusion: The distance between gingival margin and broken ends, the distance between preoperative broken ends and crest, and the shape of root are the risk factors that influence the effect of crown lengthening. There was a positive correlation between the number of risk factors and the rate of surgical failure (P<0.01). When the number of risk factors reached 3, the risk of surgical failure increased significantly, and the indication should be chosen carefully.

Key words: mandibular second molars, crown lengthening surgery, treatment outcome, risk factors