The purpose of this study was to compare computed tomography (CT) Hounsfield unit values of bone fragment gaps after sagittal split ramus osteotomy (SSRO) in patients with and without asymmetry, and to evaluate differences between the deviated and non-deviated sides in asymmetric patients. Thirty-two patients who underwent a bilateral SSRO were included in this retrospective study. Patients were divided into symmetric and asymmetric groups by cephalometric analysis. CT values of the bone fragment gap were measured at 1 week and at 1 year after surgery. There were significant differences between CT values obtained at 1 week and at 1 year after surgery for all measurement points. However, there were no significant differences in CT values between symmetric and asymmetric patients at either 1 week or 1 year after surgery. Among asymmetric patients, there were no significant differences between the deviated and non-deviated sides at 1 week or 1 year after surgery. This study showed ossification of the bone fragments and adaptation to change the mandible form in patients with and without asymmetry following SSRO.
The sagittal split ramus osteotomy (SSRO) is one of the preferred surgical procedures for the correction of various forms of mandibular prognathism. The advantage of SSRO is that it provides a large contact area of bone segments to advance or set back the jaw, including in cases with asymmetry.
With the conventional SSRO technique for asymmetric cases, the distal segment moves and rotates to the left or right side. The bony interference between the proximal and distal segments of the mandible causes a gap. This interference is prone to relapse after surgery, condylar disc displacement, or condylar resorption. Various methods have been proposed to solve this problem, including grinding the bony interference between the proximal and distal segments, bending the distal segment posterior to the last molar, and performing a bone graft in the area of the segment gap. Conventional SSRO with bent plate fixation is a method in which a gap between the proximal and distal segments is created with a bent plate, preventing internal rotation of the condylar head ( Fig. 1 ). In setback surgery, especially in cases with asymmetry, fixation with a bent plate between the segments can be performed without a bony contact to prevent large changes in the condylar position and angle. Furthermore, bone volume and facial contour can be adjusted without bone grafts, preventing postoperative temporomandibular disorders. However, the treatment of asymmetric cases with a variety of modified SSRO procedures can result in small or large gaps between the bone fragments ( Fig. 2 ).
The measurement of computed tomography Hounsfield unit values (CT values) and CT evaluation of the formation of bone and soft tissue has recently arisen as a useful way of evaluating bone healing. This method has been used to demonstrate that the gap between the proximal and distal segments can fill with new bone after SSRO. SSRO without rigid fixation of the segments has been shown to exhibit poor bone healing. Additionally, the types of fixation materials used in SSRO influence the healing of bone segments after the procedure.
This study aimed to use CT values (Hounsfield units, HU) to evaluate the healing of bone segments after SSRO in patients with and without asymmetry.
Materials and methods
This retrospective study included 32 patients (six men and 26 women; mean age 31.63 years, range 17–58 years) who were diagnosed with mandibular prognathism and underwent SSRO during the period November 2006 to August 2013. Inclusion criteria were: (1) no deformation of the midface (patients who did not undergo Le Fort I osteotomy); (2) no history of surgery involving the middle or lower face; (3) bent plate fixation was used on both sides; and (4) no bad fractures during surgery. Informed consent was obtained from all patients in accordance with the Declaration of Helsinki, and the study was approved by the necessary ethics committee. Cephalograms were obtained preoperatively, at 1 week postoperative, and at 1 year after surgery. CT scans were obtained at 1 week postoperative and at 1 year after surgery.
For all patients, the SSRO was performed by the same surgeon and assistants using the conventional Obwegeser method, with a 1.6-mm round bur used on the anterior side of the ramus, a Lindemann bur above the lingula of the mandible, and a reciprocating saw on the lateral cortex. The mandibular crack was split using an osteotome and a bone spreader. After the split, the proximal segments were tested for adequate mobility. At the time of fixation, the dental arch was secured to the maxillary arch with an interpositional splint and wire. An osseous step was formed at the site of fixation; this was dependent on the amount of setback required. The overlapping area consisted of the anterior part of the proximal segment and the distal segment, which was fixed without removing the overlap of the lateral cortex. Plates were bent to fit the step at the overlap and to maintain the condyle in its original position ( Fig. 1 ). A miniplate and four screws were placed monocortically in the mandibular angle region on each side. Twenty-one patients received an unsintered hydroxyapatite/poly- l -lactide (uHA/PLLA) miniplate (28 mm × 4.5 mm × 1.5 mm) and four uHA/PLLA screws (2 mm × 6 mm or 8 mm) (Super Fixsorb-MX; Takiron, Osaka, Japan) and the remaining 11 patients received one long titanium miniplate (4 holes/bur, 8 mm deep, 1 mm thick) and four titanium screws (2 mm × 7 mm) (Würzburg titanium miniplate system; Stryker Leibinger, Freiburg, Germany) via the conventional technique.
Lateral and frontal cephalograms were obtained for all patients preoperatively, at 1 week postoperatively, and at 1 year after surgery, for skeletal analysis ( Figs 3 and 4 ).
Cephalograms were obtained using a standardized cephalometric technique and were analyzed using CephaloMetrics A to Z software (Yasunaga Computer Systems Inc., Fukui, Japan) on a Microsoft computer. The skeletal occlusion in all patients was classified as class III on the basis of the lateral cephalometric analysis, with asymmetry taken into account for accurate frontal or axial cephalometric analysis. On the frontal cephalogram, the angle between the anterior nasal spine–menton line and the line perpendicular to the bilateral zygomatic frontal suture line was defined as the maxillomandibular (Mx–Md) midline angle. A positive Mx–Md midline angle represents mandibular deviation to the left, and a negative value represents mandibular deviation to the right ( Fig. 4 ). The Mx–Md midline angles were given positive values in all patients so that all consecutive measurements could be attributed to either the deviated or the non-deviated side. Patients were divided into two groups based on the Mx–Md midline angles: the asymmetry group included patients in whom the Mx–Md midline angle was greater than 2.5° (deviated side, n = 16; non-deviated side, n = 16), while the symmetry group included patients in whom the angle was less than 2.5° (both the right and left sides, n = 32).
One skilled observer performed all digitizations so that errors in the cephalometric analyses were small and acceptable for the purposes of this study. Error analysis by digitization and re-measurement of all cases generated an average error of less than 0.4 mm for linear measurements and 0.5° for angular measurements.
The patients were placed in the gantry with the tragal–canthal line perpendicular to the floor for CT scanning. They were instructed to breathe normally and to avoid swallowing during the scanning process. CT scans were obtained in the radiology department by skilled radiology technicians using a high-speed, advantage-type CT generator (LightSpeed Plus; GE Healthcare, Milwaukee, WI, USA) with each sequence taken 1.25 mm apart (120 kV, average 150 mA, 0.7 s/rotation, pitch factor 0.75). The CT value (HU) was measured at five points in the bone fragment gap using computer software (SimPlant 2011; Materialize Dental, Leuven, Belgium). The measurement points were defined as follows: (1) distal point: located in the bone fragment gap, at a minimum distance from the distal screw head in the proximal segment ( Fig. 5 ). (2) Proximal point: located in the bone fragment gap, at a minimum distance from the mandibular foramen ( Fig. 6 ). (3) Upper middle point: the midpoint of the proximal point and the distal point in the sagittal direction, and the upper end of the bone fragment gap in the axial direction. (4) Lower middle point: the midpoint of the proximal point and the distal point in the sagittal direction, and the lower end of the bone fragment gap in the axial direction. (5) Middle point: the midpoint of the upper middle point and the lower middle point in the axial and coronal direction ( Fig. 7 ).