This study aimed to evaluate the postoperative changes in masticatory function in patients with jaw deformities with or without asymmetry treated by orthognathic surgery. Thirty female patients who underwent a Le Fort I osteotomy with sagittal split ramus osteotomy (SSRO) were enrolled. The patients were divided into symmetry and asymmetry groups. The bite force, occlusal contact area, and bite force balance were measured before and at 1, 3, and 6 months and 1 year after surgery; these measurements were compared statistically within and between the two groups. In the symmetry group, there was a significant difference in the preoperative bite force and the 1 month postoperative bite force ( P = 0.0033). In the asymmetry group, the bite force before surgery was significantly different from that at 1 month ( P = 0.0375) and at 1 year ( P = 0.0353) after surgery. Significant differences in the bite force were also observed between the following time points: 1 month and 1 year ( P = 0.0003), 3 months and 1 year ( P = 0.0034), and 1 month and 6 months ( P = 0.0486). The occlusal contact area, bite force, and occlusal balance tended to change after Le Fort I osteotomy with SSRO, with a significantly improved bite force in patients with asymmetry before surgery.
The important goals of surgery for jaw deformity are improved masticatory function and facial aesthetics. The sagittal split ramus osteotomy (SSRO) is one of the preferred surgical procedures for correcting various forms of mandibular prognathism. The Le Fort I osteotomy is also used very frequently with SSRO (double-jaw surgery) in orthognathic surgery.
Numerous studies have investigated masticatory function before and after orthognathic surgery. One study reported a temporary decrease in the bite force and occlusal contact area immediately after surgery, with an improvement in both parameters over time. Another reported no differences in either parameter after SSRO or intraoral vertical ramus osteotomy (IVRO) with or without Le Fort I osteotomy. However, no study has reported the changes in masticatory function after surgery in patients with various types of jaw deformity.
Many studies have investigated occlusal problems. Occlusal instability, midline discrepancy, right–left differences in molar relationship, and inclination of the frontal occlusal plane have been considered important occlusal characteristics in patients. Differences in the heights of the right and left rami have also been suggested as important skeletal problems associated with temporomandibular joint (TMJ) pathologies. A similar tendency has been identified in patients with mandibular prognathism accompanied by asymmetry. A study reported that the occlusal contact area and occlusal force were lower in patients with asymmetry than in individuals with a normal skull shape, with a shift in the occlusal balance to the deviated side in the former group.
This study aimed to evaluate the postoperative changes in masticatory function in patients with jaw deformities with or without asymmetry treated by Le Fort I osteotomy with SSRO.
Materials and methods
This retrospective study included 30 female patients (mean age 27.33 years, range 15–49 years). Only female patients were included in this study as there are marked differences in bite force between males and females. The study patients were diagnosed with mandibular prognathism and underwent Le Fort I osteotomy with SSRO between November 2007 and August 2013. As there were clear differences between patients with mandibular prognathism and patients with an open bite in the bite force and bite force balance, the patients with an open bite were excluded from this study. Informed consent was obtained from all patients in accordance with the Declaration of Helsinki, and the study was approved by the ethics committee of Yamanashi University Hospital.
The patients underwent a standard Le Fort I osteotomy, with a periodontal incision in the anterior region and a vestibular incision in the posterior region to prevent postoperative scar formation in the labial gingival tissue. The maxillary segment was then repositioned with an intermediate occlusal split and fixed with miniplates, 16 monocortical screws, and four L-type plates (four-hole, unsintered hydroxyapatite/poly- l -lactic acid; Super FIXSORB MX, Takiron Co. Ltd., Osaka, Japan). The SSRO was performed according to the method described by Trauner and Obwegeser. The mandibular segment was then repositioned with an occlusal split and fixed with miniplates, eight monocortical screws, and two straight plates (four-hole). All patients underwent pre- and postoperative orthodontic treatment, with the latter beginning at 1 month after surgery and completed within 1 year.
Immediately after surgery, intermaxillary fixation (IMF) with elastic bands was performed to maintain an ideal occlusion in the same manner in all patients. There was no difference in the duration of IMF between the patients with and without asymmetry.
Lateral and frontal cephalograms were taken for all patients before and at 1 year after surgery for the skeletal analysis. Cephalograms were obtained using a standardized cephalometric technique and were analyzed using CephaloMetrics AtoZ 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. 1 ).
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 undeviated side. The 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 3.5° ( n = 15), while the symmetry group included patients in whom the angle was less than 3.5° ( n = 15).
One skilled observer performed all digitization 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.
A pressure-sensitive system to measure the occlusal bite force and occlusal contact area was used in this study. The pressure-sensitive system comprised a pressure-sensitive sheet (Dental Prescale; Fuji Photo Film Co., Tokyo, Japan) and an analysis apparatus (Dental Occlusion Pressuregraph FDP-705; Fuji Photo Film Co.) connected to a personal computer (LaVieC, LC50H/3, NEC, Tokyo, Japan) ( Fig. 2 A–C) . The reproducibility of these devices has been determined in previous studies.
Each patient was seated with her head in an unsupported natural position and a forward gaze. The Frankfort horizontal plane was maintained approximately parallel to the floor. The pressure-sensitive sheet was placed between the maxillary and mandibular arches, with the centre of the sheet between the central incisors. The patient was instructed to bite as forcefully as possible for approximately 3 s. The test was performed three times per subject, and the data were recorded as the mean of the three values. The sheet was read and analyzed by the Dental Occlusion Pressuregraph and the results were entered into the computer and visualized on the display screen. They represented the bite force, occlusal contact area, and bite force balance. The bite force balance was defined as the distance from the line passing through the centre of the left and right areas on the sensitive sheet to the centre of gravity of the bite force ( Fig. 3 ). The centre of gravity was calculated as follows:
r c = ∑ i m i r i M