·To evaluate the efficacy of the preservation of alveolar crest in situ for regeneration of periodontal osseous defect distal to the mandibular second molar (M2) after mandibular third molar extractions.

·After screening by evaluation, a total of 19 patients were enrolled and accepted the mandibular third molar extractions with modified ridge preservation at Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine from August 2020 to August 2021. Meanwhile, 8 cases of traditional tooth extraction were included in the control group. All the patients were followed up at 1 week, 3 months and 6 months after operation. Postoperative symptoms including pain, swelling and nerve injury were also recorded in detail. Intraoral clinical examination was performed on M2, including cold and heat stimulation, tooth percussion results and looseness. Osseous defect depth (ODD) on the disto-buccal aspect of M2 was taken as the primary outcome variable. Probing depth (PD) and clinical attachment level (CAL) were taken as the secondary outcome variables during the follow-up period. In the meantime, all the patients received cone beam computed tomography (CBCT), and the data were analyzed and reconstructed for the measurement of ODD through CS3DImaging software.

·No obvious postoperative complications such as infection or never injury were found in any patient. The PD, CAL and ODD on the disto-buccal aspect of M2 at 6 months after operation in the two groups were both improved compared with those at 3 months after operation (P<0.05 for all). Furthermore, all the data in the experimental group were lower than those of the control group at any time of the follow-up (P<0.05 for all).

·Modified ridge preservation has a notable improvement in regeneration of periodontal osseous defect distal to the M2 after mandibular third molar extractions.