This study aimed to investigate the incidence of neurosensory disturbance (NSD) after bilateral sagittal split osteotomy (BSSO) in different age groups and to assess the probability of sensory recovery in patients aged <19 years, 19–30 years, and >30 years. Hypoaesthesia of the lower lip was assessed subjectively and objectively immediately after BSSO and at 1 week and 1, 6, and 12 months after BSSO. Hypoaesthesia was considered permanent if it was present 1 year after BSSO. The frequency of NSD immediately after surgery was significantly higher in older patients. The cumulative incidence of recovery at 1 year was lower and the mean time to recovery was longer in the older patients, although these differences were not statistically significant. Older age was a significant risk factor for permanent hypoaesthesia, with an incidence of 4.8% per patient aged <19 years, 7.9% per patient aged 19–30 years, and 15.2% per patient aged >30 years. These findings show that the risk of NSD after BSSO is significantly higher in older patients. These results may aid surgeons in preoperative patient counselling and in deciding the optimal age at which to perform BSSO.
Neurosensory disturbance (NSD) of the inferior alveolar nerve (IAN) is one of the most frequently occurring complications of bilateral sagittal split osteotomy (BSSO). The most common manifestation is numbness of the lower lip (hypoaesthesia). In many patients, NSD resolves within several months after surgery. However, if it is still present a year after surgery, it is considered permanent. This permanent hypoaesthesia leads to significant morbidity and is therefore an important complication of the elective BSSO procedure that should be explained to the patient before obtaining informed consent.
During mandibular surgery, iatrogenic nerve damage and subsequent NSD can occur because of several factors. For example, IAN bruising can be caused by nerve compression during soft tissue dissection near the mandibular foramen, excessive nerve manipulation during splitting, the use of sharp instruments (chisels) during BSSO, or the incorrect placement of screws. Large mandibular advancements and increasing age have also been described as risk factors for NSD.
At the authors’ clinic, BSSO is performed using a sagittal splitter and separator (without the use of chisels) in an attempt to minimize the risk of hypoaesthesia. This retrospective study was performed to analyze the incidence of hypoaesthesia and time to recovery from hypoaesthesia after the use of this BSSO technique in different age groups. The aim of this study was to report the incidence of NSD of the IAN after BSSO in different age groups (<19 years, 19–30 years and >30 years) and further to investigate NSD in these different age groups in order to provide information that will aid surgeons in explaining age-specific risks to patients and deciding the optimal age at which to perform BSSO.
Materials and methods
A retrospective cohort study was conducted of patients who had undergone BSSO alone or bimaxillary procedures at the study university medical centre in the Netherlands. The patients’ clinical records were screened for details of sex, age at surgery, preoperative diagnosis, and concomitant procedures. The surgical reports were reviewed to assess the intraoperative status of the IAN, which was classified as follows: not visible in the distal segment, less than half visible in the distal segment, more than half visible in the distal segment, prepared from the proximal segment either by blunt dissection or with a bur, and visibly damaged. The patients were divided into three groups on the basis of age: group A <19 years, group B 19–30 years, and group C >30 years.
The prospective collection of data on NSD after the BSSO procedure was started at the study centre in 2004, thus all consecutive patients undergoing BSSO between January 2004 and January 2014 were included. Exclusion criteria were concomitant genioplasty, previous mandibular surgery, and pre-existing hypoaesthesia. A minimum follow-up of 6 months was necessary for inclusion in this study.
The medical files of 320 patients were reviewed retrospectively. From this series, a total of 57 patients were excluded: 37 who required a concomitant genioplasty, one with a previous history of orthognathic surgery, one with pre-existing hypoaesthesia, and 18 who were followed up for less than 6 months. All patients were treated according to the same procedures and received the same clinical care.
The BSSO was performed using a sagittal splitter and separator following the standard protocol at the centre. Fixation of the mandibular segments was performed with three bicortical position screws through the upper border of the buccal cortex into the lingual cortex (superior to the mandibular canal and nerve). The BSSO was performed by one of seven maxillofacial surgeons on one side and by a resident under close supervision of the surgeon on the other side. All patients were discharged within a week after surgery and were scheduled for clinical and radiographic evaluations at 1, 2, and 3 weeks and 1, 6, and 12 months after surgery. Although the last evaluation was scheduled to be performed 1 year after BSSO, not all patients wished to return after the 6-month evaluation appointment. However, if a patient experienced any NSD at 6 months postoperatively, they were always evaluated at 1 year after surgery.
Neurosensory function was tested preoperatively, immediately after surgery, and at clinical evaluation at 1 week and 1, 6 and 12 months after BSSO. Hypoaesthesia was considered permanent if it was present 1 year after surgery. Neurosensory testing was performed in a standardized manner, using sensory testing methods that are used widely in osteotomy studies. Sensory function was assessed subjectively by questioning the patient about altered sensation in the lower lip and by comparing contralateral sides. Light-touch detection was performed by the maxillofacial surgeon. This involved the surgeon softly touching the lower lip with cotton swabs and evaluating if the patient experienced reduced or altered sensation in the lower lip area. NSD was interpreted as a binary outcome measure: absent or present. If any disturbance or altered sensation was noted, hypoaesthesia was recorded as present.
This study was performed in accordance with the guidelines of the study institution and followed the Declaration of Helsinki on medical protocol and ethics. Because of the retrospective nature of this study, it was granted a written exemption by the institutional review board of the university medical centre.
Statistical analyses were performed using IBM SPSS Statistics for Windows version 20.0 (IBM Corp., Armonk, NY, USA) and R version 18 (The R Foundation, Vienna, Austria). Descriptive statistics were performed. χ 2 tests and Student t -tests were used where appropriate. Generalized linear mixed models (GLMM) were used to study the effect of age group on the status of the nerve and the status of the third molar. The same model has been applied to investigate the effect of third molar status and nerve status on permanent hypoaesthesia. These models were required since the status of the nerve and the status of the third molar were assessed per side and mixed models are necessary to account for the correlated nature of the left and right side measurement within each patient.
In this study, the three age groups were compared retrospectively; there was no control group. NSD was assessed at the patient level and therefore a univariate logistic regression model was employed to assess the effect of the three age groups on NSD after BSSO.
To study the effects of sex, type of malocclusion, and concomitant Le Fort I osteotomy on the occurrence of NSD at the patient level, univariate logistic regression models were estimated.
To investigate the effects of age (age groups) on the time to recovery of nerve function, a Cox regression proportional hazards model was used. Recovery was defined as the absence of any sensory dysfunction. Therefore, the outcome was analyzed at the patient level (NSD per patient). There was thus no correlated nature of the data (left and right side within one patient) in this analysis.
The study group comprised 263 patients (104 men and 159 women) who underwent 526 sagittal split osteotomies (SSOs/sites). Orthognathic surgery was performed to correct class II malocclusion in 226 patients (85.9%) and class III malocclusion in 37 patients (14.1%). The BSSO was combined with Le Fort I osteotomy (bimaxillary procedure) in 86 patients (32.7%). Mandibular third molars were present at 196 sites (37.3%). The mean duration of follow-up was 427.9 days (standard deviation (SD) 159.4 days, range 188–1465 days).
The characteristics of the patients in groups A, B, and C are presented in Table 1 . The status of the nerve during BSSO in each group is presented in Table 2 . There was a significant statistical association between the three age groups with regard to the presence of mandibular third molars ( P < 0.01) and status of the nerve ( P = 0.035). There was no significant effect of third molar status ( P = 0.433) on hypoaesthesia. If the nerve was separated from the proximal segment (either blunt or with a bur) or was visibly damaged during surgery, the risk of permanent NSD was significantly higher ( P = 0.01).
|Total number of patients (%)||63 (24.0)||101 (38.4)||99 (37.6)|
|Mean (SD)||17.1 (1.3)||22.7 (3.1)||40.6 (6.5)|
|Male||19 (30.2)||50 (49.5)||35 (35.4)|
|Female||44 (69.8)||51 (50.5)||64 (64.6)|
|Malocclusion class (%)|
|II||54 (85.7)||78 (77.2)||94 (94.9)|
|III||9 (14.3)||23 (22.8)||5 (5.1)|
|Third molars (% per site)|
|Present||94 (74.6)||69 (34.2)||33 (16.7)|
|Absent||32 (25.4)||133 (65.8)||165 (83.3)|
|Bimaxillary procedure (%)||19 (30.2)||40 (39.6)||27 (27.3)|
|Mean (SD)||418.5 (136.2)||434.1 (194.9)||427.6 (131.5)|
|Total number of sites||126||202||198|
|IAN not visible in the distal segment||21 (16.7)||36 (17.8)||24 (12.1)|
|Less than half of the IAN visible in the distal segment||30 (23.8)||35 (17.3)||27 (13.6)|
|More than half of the IAN visible in the distal segment||52 (41.3)||98 (48.5)||93 (47.0)|
|IAN separated by blunt dissection from the proximal segment||11 (8.7)||15 (7.4)||31 (15.7)|
|IAN from the proximal segment using a bur||10 (7.9)||18 (8.9)||20 (10.1)|
|IAN visibly damaged||2 (1.6)||0 (0.0)||3 (1.5)|
Univariate logistic regression models for NSD at the patient level were estimated. Risk factors used were sex (odds ratio (OR) 1.06, 95% confidence interval (CI) 0.58–1.89; P = 0.87), type of malocclusion (OR 1.12, 95% CI 0.51–2.49; P = 0.77), and concomitant Le Fort I osteotomy (OR 0.91, 95% CI 0.49–1.68; P = 0.76). The risk factors showed no significant association with permanent NSD.
The incidence of NSD in the three age groups during clinical follow-up is shown in Fig. 1 . Immediately after BSSO, NSD was present in 132 patients (50.2%): 26 in group A (13 left side, eight right side, five bilateral), 43 in group B (12 left side, 15 right side, 16 bilateral), and 63 in group C (19 left side, 24 right side, 20 bilateral). Accordingly, the incidence of immediate postoperative NSD was 41.3% per patient in group A, 42.6% per patient in group B, and 63.6% per patient in group C.
Logistic regression analysis was used to investigate the effects of age group on the occurrence of NSD immediately after BSSO. A significant association between age and NSD was found ( P < 0.01). With group A as the reference group, the ORs for groups B and C were 1.06 (95% CI 0.56–2.00) and 2.49 (95% CI 1.30–4.76), respectively.
One year after BSSO, hypoaesthesia was observed in 26 patients (9.9%): three in group A (two left side, one right side), eight in group B (three left side, four right side, one bilateral), and 15 in group C (six left side, nine right side). Accordingly, the incidence of permanent hypoaesthesia was 4.8% per patient in group A, 7.9% per patient in group B, and 15.2% per patient in group C.
Logistic regression analysis was employed to study the association between age group and permanent hypoaesthesia. A significant association between age and permanent NSD was found ( P = 0.05). With group A as the reference group, the ORs for groups B and C were 1.72 (95% CI 0.44–6.74) and 3.57 (95% CI 1.00–12.89), respectively.
The cumulative incidence of recovery at 1 year after BSSO was 0.833% in group A, 0.702% in group B, and 0.593% in group C. The cumulative incidence of postoperative sensory recovery in each age group is shown in Fig. 2 . The hazard ratio decreased with increasing age, implying that the older group experienced recovery at a later stage compared with the younger groups. However, the difference among groups was not statistically significant ( P = 0.33). The hazard ratios and mean time to recovery in each age group are shown in Table 3 .