Abstract: Objective This study aimed to investigate the clinical advantages and disadvantages of two flap advancement techniques, trapezoidal full-thickness flap (TFF) and combination flap (CF), in guided bone regeneration (GBR) in the posterior mandible, and to compare the differences in postoperative pain and swelling levels between the two techniques. Methods A retrospective analysis was conducted on 89 patients who underwent single posterior mandibular implant placement with simultaneously guided bone regeneration in our hospital from September 2023 to May 2024. The patients were divided into two groups according to the flap advancement technique: the TFF group (47 patients) and the CF group (42 patients). The two groups were compared in terms of the need for additional vertical incisions, the use of additional fixation devices such as membrane fixation screws, and the incidence of accidental flap perforation. Postoperative pain and swelling were recorded for one week following surgery in both groups. Results During surgery, all cases in the TFF group required vertical incisions extending beyond the mucogingival junction. In the CF group, 13 cases (31.0%) did not require additional vertical incisions, and 29 cases（69.0%）required small vertical incisions terminating at the mucogingival junction. No flap perforations occurred in the TFF group, while in the CF group, there were 2 cases (4.8%) of accidental flap perforation during periosteal incision and flap release. For collagen membrane fixation, in the TFF group, 22 cases (46.8%) used membrane fixation screws at the crest, and 25 cases (53.2%) used screws at the apex; in the CF group, 18 cases (42.9%) used membrane fixation screws at the crest, but no screws were needed at the apex. No postoperative infections or bone graft material exposure were observed in either group. Although pain levels were lower in the TFF group compared to those in the CF group on postoperative days 1, 3, 5, and 7, no statistically significant difference was observed between the two groups (P>0.05). Similarly, while swelling levels were lower in the TFF group than in the CF group on postoperative days 1, 3, and 5, no statistically significant difference was found between the two groups (P>0.05). However, on postoperative day 7, swelling was significantly lower in the TFF group than in the CF group (P<0.05). Conclusion Both techniques could achieve effective flap advancement and primary wound closure following GBR procedures. There was no significant difference in postoperative pain between the two groups during the entire observation period, and no significant difference in swelling on postoperative days 1, 3, and 5, except that swelling in the CF group was slightly more severe than in the TFF group on postoperative day 7. As a modified flap advancement technique, CF is more technically sensitive than the classic TFF. However, when CF is applied in the posterior mandible, it can reduce the use of additional vertical incisions, which benefits blood supply and reduces surgical invasiveness. Additionally, it can retain the collagen membrane by using the apical periosteal "pocket", thus eliminating the use of membrane fixation screws at the apex of the surgical area and decreasing the risk of damaging adjacent anatomical structures. Therefore, the combination flap technique has good clinical applicability and potential for widespread adoption.

Key words: Combination flap, Trapezoidal full-thickness flap, Guided bone regeneration, Flap advancement, Postoperative swelling, Postoperative pain