The aim of this study was to conduct a prospective clinical trial comparing the neurosensory function of the inferior alveolar nerve (IAN) after mandibular advancement surgery with either bilateral sagittal split osteotomies (BSSO) or mandibular distraction ostoegenesis (MDO). 23 Class II mandibular hypoplasia patients requiring mandibular advancement were randomized into two groups for either BSSO or MDO. Subjective and objective neurosensory evaluations were performed preoperatively and at the following postoperative times: 2 weeks (TBD1), 6 weeks (TBD2), 12 weeks (TBD3), 6 months (TBD4) and 12 months (TBD5). Subjective evaluation included the use of a visual analogue scale (VAS). Objective evaluation included the use of light touch (LT), two-point discrimination (2PD) and pain detection threshold (PD) tests. Intra-operative or postoperative complications were recorded. Using a mixed model, no significant differences were reported in subjective VAS scores and objective LT, 2PD and PD scores between the BSSO and MDO groups over 12 months ( p > 0.05). Common postoperative complications included localized wound infection (BSSO = 2, MDO = 6) and condylar resorption (BSSO = 1, MDO = 1).
Persistent neurosensory disturbance of the inferior alveolar nerve (IAN) is a well-known complication of bilateral sagittal split osteotomies (BSSO). Since the inception of this technique, there have been numerous investigations into the risk factors and recovery patterns associated with this postoperative complication. The patient’s age, particularly for those over 40 years of age, has been reported in several studies to have a significant influence on persistent neurosensory deficits. Intra-operative risk factors such as excessive nerve manipulation, nerve laceration, dissection trauma to the medial soft tissue of the ramus, splitting technique, degree of mandibular advancement and unfavourable fractures have also been implicated . Anatomical variations of the mandible such as the corpus height and location of the IAN canal near the inferior border of the mandible have also been reported to be risks for nerve injury . Most risk factors can be minimized during surgery by experienced surgeons, but postoperative neurosensory disturbance remains a common postoperative morbidity.
In recent years, mandibular distraction osteogenesis (MDO) has been applied to treat patients with Class II mandibular hypoplasia. This technique has been reported to have a lower incidence of postoperative neurosensory disturbance of the IAN , but it has been associated with other minor complications. There has been no comparison of such morbidities between these two surgical techniques. This clinical trial aims to compare the neurosensory function of the IAN and the complications after mandibular advancement surgery with either BSSO or MDO in the treatment of patients with Class II mandibular hypoplasia.
Materials and methods
This was a prospective clinical trial and ethical approval was obtained from the Ethics Committee, Faculty of Dentistry, The University of Hong Kong. The inclusion criteria comprised patients with a skeletal diagnosis of Class II mandibular hypoplasia, aged 16 years or older with radiographic evidence of completion of skeletal growth using a hand-wrist radiograph and requiring surgical mandibular advancement. Patients with systemic diseases, facial asymmetry and craniofacial syndromes were excluded from the study. Patients fulfilling the inclusion criteria were randomly assigned to a BSSO or a MDO group based on a computer generated randomization table. Informed consent was obtained from each patient before any treatment was carried out.
23 subjects with a 6–12-month follow-up were presented, with 12 subjects (3 males, 9 females) receiving BSSO and 11 subjects (2 male, 9 females) receiving MDO. Additional maxillary surgery or lower anterior subapical osteotomies (Hofer) were carried out if necessary. Great care was taken during the Hofer osteotomy by keeping the osteotomy cuts 5 mm anterior to the mental foramen to avoid the anterior loop of the mental branch of the IAN and to protect and avoid excessive stretching of this branch. 9 subjects in the BSSO group and 7 in the MDO group had received a Hofer osteotomy. In the BSSO group, 3 subjects had undergone single jaw surgery while the remaining 11 subjects had undergone double jaw surgery. In the MDO group, 2 subjects underwent single jaw surgery and 9 subjects received double jaw surgery.
Standardized BSSO were performed in the BSSO group, with fixation achieved using titanium mini-plates. For the MDO group, bilateral vertical osteotomies were performed distal to the lower last molar. The distal and proximal segments were then mobilized to a limited extent to ensure that there were no bony hindrances and to check the integrity of the IAN. Unidirectional intra-oral distractors were placed with a distraction vector parallel to the upper occlusal plane. Fixation of the distractors was achieved with titanium mini-screws.
A 5–7-day latency period was observed postoperatively after which distraction was commenced two to four times a day at a rate of 1 mm/day. The distraction process was continued until a Class I occlusion was achieved. The distractors were kept in place for a 3-month consolidation period. Following radiographic evidence of bony healing, a second operation was performed to remove the distractors.
Evaluation of IAN function was performed with subjective and objective neurosensory tests. These tests were performed 1 week before surgery (T0) and at the following time periods after surgery for the BSSO group or after distraction for the MDO group: 2 weeks (TBD1), 6 weeks (TBD2), 3 months (TBD3), 6 months (TBD4) and 12 months (TBD5).
Subjective evaluation was performed using a patient questionnaire that consisted of a visual analogue scale (VAS), where a score of 0 was ‘normal sensation’ and 10 was ‘abnormal sensation in its greatest severity’.
Objective evaluation involved testing of the IAN function in its area of distribution on the mental region on both sides. Three sensory modalities were evaluated. First, light touch threshold (LT), which was evaluated using Semmes-Weinstein pressure aesthesiometer monofilaments (Stoelting Co., IL, USA). These monofilaments were used in increasing thickness and applied to the mental region with just enough pressure to bend the hair. The patient was asked if he or she was able to feel the monofilament as it was applied and the force ( g ) corresponding to that monofilament was recorded. Second, two-point discrimination (2PD) was evaluated using a pair of parallel pins mounted around a plastic disc. These pins were separated at increasing distances ranging from 2 to 20 mm. They were applied to the mental region in a horizontal direction and the patient was asked if he or she was able to discriminate whether the pin application felt like one or two points. The distance setting started from the maximum distance and decreased until the patient could no longer discriminate between the two points correctly. The distance between the pins at which the pins were felt as one point was recorded. Third, pain detection threshold (PD) was evaluated using a stress and tension gauge (Dentaurum, Ispringen, Germany). This gauge consisted of a pin calibrated to a gauge meter (10–100 g) that measured the amount of force applied by the pin to the test area. The pin was applied to the mental region with just enough pressure for the patient to feel sharp pain. The force indicated on the gauge was recorded.
All testing procedures were performed with the patient’s eyes closed. The tests were applied to one point in the centre of the mental region on both sides and were repeated three times, following which the readings were averaged.
Any intra-operative complications associated with the BSSO or MDO surgical techniques, such as IAN transaction (partial or total), hemorrhage and unfavourable fractures, were recorded. Postoperative complications were recorded for both groups at follow-up. For the MDO group, complications occurring during the distraction process were also recorded.
Data were analyzed with statistical analysis computer software (SPSS 17.0 for Windows © , Chicago, IL). The normal distribution of the quantitative variable values was confirmed using the Kolmogorov–Smirnov test. Paired t -tests were used to determine whether there were any differences between left and right sides at different time points in both groups. Mixed model was utilized to compare the postoperative subjective VAS and objective (LT, 2PD and PD) scores between the BSSO and MDO groups. Variance and covariance matrices were chosen based on the Bayesian Information Criteria (BIC). The presence or absence of a Hofer osteotomy was adjusted for in the model. Interaction was not evaluated due to the small sample size. A p -value of less than 0.05 was considered statistically significant.
The mean age in both groups was similar (26.5 years in the BSSO group and 25.3 years in the MDO group). Mean mandibular advancement (mean ± SD, mm) was comparable for both groups, 7.71 ± 2.19 mm in the BSSO group and 8.00 ± 2.47 mm in the MDO group ( p > 0.05).
No intra-operative complications were reported for either group. The IAN was exposed in 8 patients (2 unilateral and 6 bilateral) in the BSSO group and all patients in the MDO group.
VAS scores were elevated in the early postoperative/distraction period (TBD1) for both groups ( Fig. 1 ). These scores gradually reduced over time from TBD2 to TBD5 but did not revert back to normal preoperative values. From the mixed model, there were no significant differences in VAS scores between the BSSO and MDO groups over the 12-month postoperative period ( p = 0.69).
No significant differences were found between the left and right mental regions for all tests in either group ( p > 0.05). As such, data for the left and right mental regions were averaged and the mean used for data analyses. Both groups reported increased LT, 2PD and PD scores in the early postoperative/distraction period ( Figs. 2–4 ). These scores gradually returned to near normal values from TBD3 to TBD5 in both groups. The result from mixed model showed that the main effect of the Hofer osteotomy on LT, 2PD and PD score was not significant ( p = 0.69, p = 0.11, and p = 0.77, respectively). When comparing the BSSO and MDO groups, no significant differences were detected for LT, 2PD and PD scores throughout the 12-month postoperative period ( p = 0.55, p = 0.39, and p = 0.096, respectively). The mean differences for LT, 2PD and PD scores between these two groups were 0.02 ± 0.04 g, 1.00 ± 1.15 mm, and 3.78 ± 2.16 g, respectively.
In the BSSO group, localized wound infections were reported in 2 patients. These were treated with antibiotics and in one case, incision and drainage under local anaesthesia. 1 patient developed an increased overjet 1 year postoperatively (TBD5), which was confirmed by radiographic examination to be due to condylar resorption. In the MDO group, 1 patient required replacement of the distractors during the distraction period due to insufficient distractor length. This was attributed to over-activation of the distractor during the initial placement. During the consolidation period (TBD1–TBD3), 1 patient required early removal of the distractors and the use of heavy elastics to correct a bilateral posterior open bite that had developed during distraction. Persistent wound infection around the distractor rods was a common problem during the consolidation period ( n = 6). During the post-distractor removal period (TBD3–TBD5), 1 patient developed a progressive open bite and asymmetry, which was confirmed by radiographic examination and cephalometric analysis to be due to condylar resorption. Other complications are listed for both groups in Tables 1 and 2 .