·From September 2018 to April 2020, 21 cases of M3M germs with impaction tendency were included in the experimental group at the Department of Oral Surgery, Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine. At the same time, 21 adult M3Ms which were completely impacted were included as the control group. M3Ms were extracted by ultrasonic osteotome and turbine drill. The patients were followed up 1 d, 3 d, 7 d, 3 months and 6 months after surgery. The surgery time, bone defect depth (BDD), probing depth (PD), plaque index (PLI), gingival index (GI), pain degree (visual analog score, VAS), degree of swelling, mouth opening restriction degree, and adverse reactions were compared between the two groups. IBM SPSS statistics 25 software was used for statistical analysis, and the quantitative data were expressed in x±s. The independent sample t-test or Mann-Whitney U test was used for the comparison between the two groups in primary and secondary outcomes. Kruskal-Wallis H test was used to analyze the intragroup changes of the primary outcomes, and Mann-Whitney U test was used for further pairwise comparison within the group. Statistical significance was accepted at a value of P<0.05.

·The surgery time of the experimental group was (7.62±1.50) min and that of the control group was (10.76±2.21) min. There was a significant difference in the surgery time between two groups (P=0.000). The BDD of the experimental group was (0.65±0.33) mm and that of the control group was (2.49±1.37) mm at 6 months after surgery. There were significant differences in BDD, PD, PLI and GI between the two groups at 6 months after surgery (P=0.000, P=0.012, P=0.011, P=0.042). The BDD of the two groups at 6 months after surgery was significantly higher than that at baseline, and the difference was statistically significant (P=0.009, P=0.000). There was a significant difference in VAS between the two groups at 3 d after surgery (P=0.044). At each follow-up period after surgery, there was a significant difference in the degree of swelling between the two groups (P=0.005, P=0.031, P=0.019). In the control group, 1 case of inferior alveolar nerve injury occurred, and the other patients did not have adverse reactions such as bleeding, nerve injury, dry socket disease, etc.

·Preventive extraction of M3M germ with impaction tendency can shorten the surgery time, and reduce postoperative reaction, which can obtain better long-term clinical effect compared with adult impacted M3M extraction.