Abstract
Postoperative inferior alveolar nerve (IAN) neurosensory impairment was prospectively evaluated in 20 consecutive patients with mandibular prognathism who underwent bilateral sagittal split osteotomy. Routine presurgical imaging was obtained for all patients in study and control groups (10 patients each). Cone beam CT of the mandibular ramus and body was performed in 10 randomly selected patients (study group) and the precise location of the IAN was determined preoperatively and intraoperatively. Nerve sensation was evaluated by subjectively monitoring the physical feeling of the lower lip and the chin skin preoperatively and at different times postoperatively. Exact nerve location was successfully determined in all 10 cases in the study group. There were almost no significant differences between patients’ sensation scores at the chin skin and lip sites. No significant differences were found between the two sides of the 20 patients. A significant increase in the score trend along the timeframes, in both groups, could be clearly seen together with a statistically significant difference ( P ≤ 0.004) between the study and the control groups. In conclusion, precisely locating the IAN using CT is a significant means for efficiently minimizing nerve damage during sagittal split osteotomy.
Bilateral sagittal split osteotomy (BSSO) of the mandible was first introduced by S chuchardt in 1942, but many surgeons have modified the procedure . The concepts and advantages of the procedure remain the same, including great flexibility in repositioning the distal segment of the mandible, broad bony overlap of the segments after repositioning the jaw and minimal alterations in the position of the muscles and temporomandibular joint .
The main postoperative side effect of the procedure is neurosensory impairment due to pressure or damage to the inferior alveolar nerve (IAN) . Recently, surgeons have focused on improving the technique to protect the neurovascular bundle and to minimize damage to it . The appearance of neurosensory disturbance following BSSO varies. The incidence of IAN sensory deficits ranges from 13% to 100% immediately after surgery and from 0% to 85% 1–2 years post-surgery . The cause of these variations may be the differences in surgical techniques used in various studies . Another reason for the variability may be the differences in neural sensation assessment methods used in the studies .
The lack of standardized research methods for IAN impairment prevents a valid comparison between the different studies evaluating neurosensory disturbance following BSSO . The aim of this study is to propose a standardized protocol that will accurately locate the IAN pathway in the mandibular body and ramus preoperatively using CT scans, to determine the BSSO location and to evaluate the postoperative effect of this procedure on IAN neurosensory impairment.
Material and methods
The authors conducted a prospective cohort study which included 20 patients, 12 females and 8 males (40 mandibular sides), aged 16–21 years, with mandibular prognathism and negative or zero overjet. The patients underwent a BSSO procedure for mandibular setback (mean 7.2 mm, range 6–8 mm) between May 2004 and February 2007. All the patients were examined and treated according to a specific ortho-surgical protocol by the same orthodontist and surgeon at the Department of Oral and Maxillofacial Surgery and at the Orthodontic and Craniofacial Department of the Rambam Health Care Campus, Haifa, Israel. The patients were diagnosed with skeletal Class III malocclusion with mandibular prognathism by clinical examination and complete orthodontic records. A presurgical orthodontic stage was performed 4–12 months prior to the surgical date using a 0.022 × 0.028 in. pre-adjusted fixed orthodontic appliance (Omniarch ® GAC company). Ten patients (7 females and 3 males), serving as a study group, were randomly selected to undergo cone beam computerized tomography (CBCT) of the mandibular ramus and body in addition to routine presurgical imaging. The remaining patients (5 females and 5 males) served as a control group. The inclusion criteria were generally good health, normal jaw growth pattern and normal IAN functioning as determined by preoperative assessment of the lower lip and chin skin sensation. Evidence of a syndromic condition, congenital anomalies or traumatic injury to the jaw and face were exclusion criteria.
The patients mean age, ANB angle values, overjet measurement scores and duration of presurgical orthodontic preparation are presented in Table 1 .
| Gender | |||||||
|---|---|---|---|---|---|---|---|
| Group | Number of patients ( N ) | Female ( n ) | Male ( n ) | Mean age (years) | Mean ANB angle (degrees) | Mean overjet (mm) | Mean duration of presurgical orthodontic preparation (months) |
| Study group | 10 | 3 | 7 | 17.9 ± 1.45 | −2.50 ± 1.08 | −2.30 ± 1.88 | 8.00 ± 1.82 |
| Control group | 10 | 5 | 5 | 18.3 ± 1.25 | −3.40 ± 1.57 | −3.00 ± 2.45 | 8.10 ± 2.33 |
| Total | 20 | 8 | 12 | 18.1 ± 1.33 | −2.95 ± 1.39 | −2.65 ± 2.15 | 8.05 ± 2.04 |
The Human Subjects Review Board (IRB) of The Rambam Health Care Campus approved the study. Principles outlined in the Declaration of Helsinki were followed throughout this study.
Imaging
Standard imaging of the BSSO procedure, which included lateral cephalograms and orthopantograms, was performed for all the participants. According to the study’s protocol CBCT of the mandibular body and ramus was performed using a PLANMECA ProMax 3D CBVT ® machine for mandibular canal (IAN pathway) imaging on ten randomized participants (study group) in parallel to the Frankfort horizontal plane at 2 mm intervals (by the same technician). Two examiners, who work as senior oral and maxillofacial surgeons, separately carefully examined the CBCT of the patients in the study group and measured the width of the marrow space between the mandibular canal and the internal and external cortical bone for each of the CT slices. A bone reconstruction mode was used by a laser imager to predict the exact IAN course for each patient, allowing precise prediction of the mandibular bone osteotomies according to the anticipated course of the IAN. Each of the osteotomy sites that seemed to endanger the IAN was identified and clearly marked on the CT to draw the surgeon’s attention to these sites whilst performing the procedure. The level of reliability between the two examiners was tested and was found to be 96%.
Surgical technique
The operation was performed using the Obwegeser–Dal Pont technique with the modification described by H unsuck . Precise exploration of the mandibular ramus, body and the mandibular angle was performed after their exposure for comparison with the CT and to identify and mark the suspected sites of IAN damage during the BSSO. The IAN was exposed from the lingual aspect at the mandibular foramen. Using a long Lindemann bur, osteotomy of the lingual cortical bone was performed halfway between the sigmoid notch and the mandibular foramen. After releasing the masseter muscle sling, osteotomy of the buccal cortical bone was performed downward on the lateral crest of the alveolus, along the oblique ridge to the vestibular area of the second molar. The periosteum was reflected laterally to expose the body of the mandible down to the inferior border. The medial surface was also exposed with a retractor, and the medial cut was made through the medial cortex into the medullary bone from just above the lingula. The lingual and buccal osteotomy lines were then connected by a crestal osteotomy of the cortical bone. The vertical cut was made with a bur through the cortex on the buccal surface of the mandible between the first and the second molar. The two cuts were joined by a bur to cut the cortical bone in a sagittal direction. BSSO was accomplished with Lexar chisels (Martin). The fine, flexible chisel was driven directly into the inner aspect of the buccal cortex and slid down it closely following the buccal cortical wall so that it remained lateral to the neurovascular bundle. Care was taken to ensure that the chisels were in constant contact with the inner side of the buccal cortical bone. It has been shown that tracking the cortical plate in the manner reported results in a reduction of injuries to the IAN . The cortices were then separated, in search of the neurovascular bundle. Once located, the osteotome was placed beyond the nerve and the sagittal split was completed by prying apart the cut cortical plate using moderate force. The gap between the proximal and the distal segment was inspected and the neurovascular bundle was identified. The nerve course was observed in its intraosseous bed as preoperatively assessed by the CT imaging of the study group. Nerve continuity was preserved in all 40 sagittal splits (20 patients). Bony irregularities of the interior surface of the two contacting segments were carefully removed to prevent torquing of the segments and compression of the neurovascular bundle. After repositioning the mandible by means of a surgical splint and manual positioning of the proximal segment in the correct position, fixation was carried out in all cases using 4-hole upper border miniplates and 6 mm monocortical screws to eliminate the need for intermaxillary fixation following surgery.
Neurosensory assessment
A self-assessment questionnaire was used to evaluate the IAN neurosensory disturbance after the BSSO procedure for the study and control groups; it included a 5-point scale according to W estermark et al. . The IAN dermatome area was divided into two areas (lip and chin skin) ( Fig. 1 ), totalling 4 sites per patient: site 1, right chin skin; site 2 left chin skin; site 3, right lower lip; site 4, left lower lip. The participants were asked to choose a score for each one of the tested sites from a scale of 1 to 5 where: 1, completely numb; 2, almost no sensation; 3, reduced sensitivity; 4, almost normal sensitivity; 5, completely normal sensitivity.
The subjective evaluation of the neurosensory status was done preoperatively and postoperatively at 1 week, 1 month, 3 months, 6 months, and 1 year (24 scores were recorded for each patient). Before surgery, no patient had impaired sensation in the lower lip and chin (score 5). Follow-up was continued for 1 year postoperatively. The final orthodontic treatment stage was initiated 3 months postoperatively and was completed 6–18 months postoperatively.
Statistical and data analysis
The data were evaluated using SPSS software, version 17 (SPSS Inc. Chicago, IL, USA).
The differences between the sensation level scores in the chin skin and lower lip on each side (innervated by the same IAN) after BSSO at the stated times (1 week, 1 month, 3 months, 6 months, and 1 year following surgery), were assessed for each group by means of the non parametric paired marginal homogeneity test ( P < 0.05).
The self-assessment five point scale scoring system was unified into two categories: abnormal and normal sensitivity according to W estermark et al. . Completely numb, almost no sensation and reduced sensitivity (scores 1, 2, and 3) were considered abnormal, whilst the almost normal sensitivity and completely normal sensitivity categories (scores 4 and 5) were considered normal. Cohen’s kappa was assessed to measure the agreement between the sensation scores of the chin skin and the lip areas on each side.
The final differences in the sensation level scores of the groups, for each time point were evaluated by means of the Mann–Whitney U -test and the relative risk of sensation impairment in the absence of a CT scan was calculated.
Results
There were almost no significant differences between the patients’ sensation score assessments for the chin skin and lip sites on each side at the designated times in either group ( Table 2 ).
| Study group | Control group | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Mean right side (SD) | Mean left side (SD) | Mean right side (SD) | Mean left side (SD) | |||||||
| Chin | Lip | Chin | Lip | Significance (right, left) | Chin | Lip | Chin | Lip | Significance (right, left) | |
| Pre surgery | 5 ± 0 | 5 ± 0 | 5 ± 0 | 5 ± 0 | NS, NS | 5 ± 0 | 5 ± 0 | 5 ± 0 | 5 ± 0 | NS, NS |
| After 1 week | 2.70 ± 1.16 | 2.80 ± 1.03 | 2.60 ± 1.51 | 2.60 ± 1.35 | NS, NS | 1.20 ± 0.42 | 1.50 ± 0.52 | 1.40 ± 0.51 | 1.30 ± 0.48 | NS, NS |
| After 1 month | 4.10 ± 0.74 | 3.80 ± 1.13 | 3.20 ± 1.69 | 2.90 ± 1.59 | NS, NS | 2.00 ± 0.47 | 2.00 ± 0.67 | 2.20 ± 0.63 | 1.90 ± 0.73 | NS, NS |
| After 3 months | 4.30 ± 0.48 | 4.20 ± 0.42 | 4.50 ± 0.85 | 4.10 ± 0.87 | NS, NS | 2.70 ± 0.48 | 2.70 ± 0.82 | 2.50 ± 0.70 | 2.30 ± 0.67 | NS, NS |
| After 6 months | 4.70 ± 0.48 | 4.50 ± 0.52 | 4.60 ± 0.52 | 4.20 ± 0.63 | NS, 0.046 (*) | 2.80 ± 0.63 | 3.00 ± 0.67 | 2.90 ± 0.56 | 2.70 ± 0.67 | NS, NS |
| After 1 year | 4.80 ± 0.42 | 4.50 ± 0.53 | 4.80 ± 0.42 | 4.50 ± 0.53 | NS, NS | 3.30 ± 0.82 | 3.40 ± 0.84 | 3.30 ± 0.82 | 3.20 ± 0.92 | NS, NS |
6 months after surgery there were significant differences ( P = 0.046) on the left side between the chin and lip scores only in the study group. This weak significance can be interpreted by the small sample size (10 patients). There is a resemblance between the averages and their standard deviation (4.20 ± 0.63 vs. 4.60 ± 0.52). Unification of the five point sensation assessment scale into two categories: abnormal sensitivity (scores 1–3) and normal sensitivity (scores 4–5) in this group demonstrated Kappa’s values on the left side according to the tested times of 100% pre-surgery, 74% 1 week post-surgery, 80% 1 month post-surgery, 74% 3 months post-surgery, 100% 6 months post-surgery and 100% 1 year post-surgery; Kappa’s values for the right side were 100%, 52%, 38%, 100%, 100%, 100%, respectively.
As depicted in Table 2 and in the strong Kappa values, the authors conclude that there were no significant differences between the chin skin and lip sensation scores in the study or control groups. Thus, unification of the chin and lip score values into one averaged value for each side in both groups could be done. In this way the patient’s sensation could be assessed by only one unified average score per side in comparison to the two original scores per side ( Table 3 and Fig. 2 ).
| Study group | Control group | |||
|---|---|---|---|---|
| Mean right side (SD) | Mean left side (SD) | Mean right side (SD) | Mean left side (SD) | |
| After 1 week | 2.75 ± 0.95 | 2.60 ± 1.37 | 1.35 ± 0.33 | 1.35 ± 0.33 |
| After 1 month | 3.95 ± 0.79 | 3.05 ± 1.55 | 2.00 ± 0.52 | 2.05 ± 0.64 |
| After 3 months | 4.25 ± 0.43 | 4.30 ± 0.79 | 2.70 ± 0.63 | 2.40 ± 0.51 |
| After 6 months | 4.60 ± 0.46 | 4.40 ± 0.52 | 2.90 ± 0.61 | 2.80 ± 0.58 |
| After 1 year | 4.65 ± 0.34 | 4.65 ± 0.33 | 3.30 ± 0.82 | 3.25 ± 0.85 |
There was a significant increase in sensation during the whole process on both sides in the two groups. A significant increase (by a marginal homogeneity test) was recorded during the period between 1 week and 1 month ( P = 0.012) on the right side and between 1 month and 3 months ( P = 0.019) on the left side of the study group. A comparison between all timeframes on both sides of both groups revealed a significant difference between the groups (0.004 ≤ P ≤ 0.046). No significant differences were found with time when comparing the increasing score trend of the sensation assessment between the right and left sides in both groups (including the difference of 3.95 vs. 3.05 1 month post-surgery in the study group). This non-significant difference allowed unification of the two sides whereby each patient had one sensation score for each timeframe tested ( Table 4 ).
| Study group | Control group | ||
|---|---|---|---|
| Mean (SD) | Mean (SD) | Statistical significance | |
| Pre | 5 ± 0 | 5 ± 0 | p = 1.00 |
| After 1 week | 2.67 ± 1.10 | 1.35 ± 0.21 | p = 0.004 |
| After 1 month | 3.50 ± 0.89 | 2.02 ± 0.52 | p = 0.001 |
| After 3 months | 4.27 ± 0.47 | 2.55 ± 0.47 | p < 0.001 |
| After 6 months | 4.50 ± 0.39 | 2.85 ± 0.55 | p < 0.001 |
| After 1 year | 4.65 ± 0.29 | 3.27 ± 0.83 | p = 0.002 |
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