Objective The purpose of this study was to objectively evaluate the relationship between recovery of hypoesthesia of the lower lip after sagittal split ramus osteotomy (SSRO) with trigeminal somatosensory evoked potential (TSEP) and sagittal split area or plate screw position, using postoperative computerized tomography (CT). Study design The subjects consisted of 58 patients (116 sides) with mandibular prognathism who underwent SSRO. The patients were divided into 2 groups. The Ob group, consisting of 62 sides, underwent the Obwegeser method. The OD group, consisting of 54 sides, underwent the Obwegeser–Dal Pont method. Trigeminal nerve hypoesthesia at the region of the lower lip was assessed bilaterally by the TSEP method. An electroencephalograph recording system (Neuropack Sigma; Nihon Koden, Tokyo, Japan) was used to analyze the potentials. Each patient was evaluated preoperatively and then postoperatively at 1 week, 2 weeks, 1 month, 3 months, 6 months and 1 year. Postoperative CT was performed in all cases to measure the sagittal split area (SSA) as well as the distance between the plate (the most medial point of screw) and the mental foramen (PM). Then comparisons between the 2 groups in recovery period of the lower lip, SSA, and PM were performed. Furthermore, the relationships between recovery period of hypoesthesia of the lower lip and SSA or PM were evaluated statistically. Results The average measurable period and standard deviation of TSEP of the lower lip was 3.4 ± 5.5 weeks in the Ob group and 11.3 ± 13.7 weeks in the OD group. There were significant differences between the Ob group and the OD group ( P < .0001). The OD group showed significantly greater SSA and PM than the Ob group ( P < 0.0001). Conclusion This study suggested that recovery period of hypoesthesia of the lower lip after SSRO was strongly associated with SSA and PM.