Patients and surgical treatments
A total of 27 Japanese patients (13 men and 14 women) who were treated by SSRO at Tokyo Medical University Hospital between January and December 2020 were analyzed. Patients who underwent extraction of the lower third molars or advanced movement of the proximal segment during SSRO were excluded. Features of the jaw deformities were mandibular prognathism with/without maxillary retrusion and/or facial asymmetry. The age of the patients at the time of orthognathic surgery ranged from 17 to 51 years (mean: 25.5 years; median: 24.0 years).
SSRO procedures were performed basically according to the Hunsuck-Epker modification, which is known as short lingual osteotomy (SLO) [8, 9]. A modified version of the SLO was performed using an ultrasonic bonecutting device to determine the posterior osteotomy boundary . Briefly, vertical osteotomy of about 10 mm was performed from the posterior edge of the horizontal osteotomy, in close proximity to the mandibular foramen, toward the inferior border, using an ultrasonic bone-cutting device to split the sagittal surface of the ramus. Nine surgeons who were trained in oral surgery for 9 or more years performed all operations.
CT analysis of rami anatomy and buccal fracture of the rami
CT analyses were performed about 1 month before the surgery, and 4 or 5 days after the surgery, using a Revolution CT device (GE Healthcare) at Tokyo Medical University Hospital (tube voltage: 120 kV; tube current: auto mA; rotation time: 0.5 s/rotation; slice width: 2.5 mm; slice interval: 0.625 mm; field of view: 23 cm; reconstruction kernel: bone; scan pitch: 0.561).
Horizontal images were obtained at the height of the lingula of the mandible (Image A, Fig. 1A) and the mandibular foramen (Image B, Fig. 1B), and the landmarks for measurement were manually identified (Fig. 1C, D); in Image A, the forward point (A), the backward point (B), the intersection point of the perpendicular bisector of line AB and the lateral surface of the buccal cortical bone (C), in Image B, the medial point (D), the point on the medial surface 5 mm behind point D (E), the point of tangency of the mandibular foramen and point D (F), the center of the mandibular foramen (G), the point of tangency of the lateral surface of the lingual cortical bone and point F (H), the backward point (I), the point on the medial surface 5 mm in front of point I (J), the point on the lateral surface 5 mm in front of point I (K), the forward point (L), the point on the lateral surface 5 mm behind point L (M), and the intersection point of the perpendicular bisector of line IL and the lateral surface of the buccal cortical bone (N). Angle ACB as the curve of the medial region of the cortical bone and the thickness of the distal region of the cortical bone were measured in Image A. Angle DFH as the curve of the medial region of the cortical bone, angle LNI as the curve of the lateral region of the cortical bone, the distance of EM as the forward thickness of the ramus, the distance of JK as the backward thickness of the ramus, the distance of GI, and the thickness of the distal region of the cortical bone were measured in Image B. Whether or not the mandibular foramen was in direct contact with the buccal cortical bone was also evaluated in Image B. Bad splits in the buccal plate of the ramus were evaluated in Image A or B after surgery, when a split line appeared on the buccal cortical bone 3 mm or more lateral from point I (Fig. 1E). These measurements were conducted 3 times by 2 experienced oral surgeons, and the mean values were analyzed.
Statistical analysis was performed by the Student t-test or the Fisher exact test using Prism 8 software (GraphPad Software). A p-value of less than 0.05 was considered to indicate a statistically significant difference between 2 groups.