Prediction of neurosensory deficit in the lower lip and chin after sagittal split ramus osteotomy (SSRO) is challenging. This study aimed to elucidate factors related to the development and improvement of neurosensory disturbance (NSD) after SSRO with respect to surgical procedure and the anatomical and structural characteristics of the craniomaxillofacial skeleton. Subjects comprised 50 patients treated by a single experienced surgeon. Anatomical data and landmarks were obtained by computed tomography (CT) imaging. There was a significant difference between patients with or without NSD for the surgical space on the medial side of mandibular ramus 1 week after SSRO ( P = 0.006). Less than 15.0 mm between the lingula and mandibular notch (relative risk, 6.7; 95% CI, 1.7–33.8) and 195.0 mm 2 or more space on the medial side of the mandibular ramus (relative risk, 17.2; 95% CI, 3.9–100.4) indicated a significant risk of NSD development at 6 months postoperatively. These results suggested that the development of NSD is related to the surgical space on the medial side of the mandibular ramus and subsequent manipulation of the inferior alveolar nerve (IAN) in that region. Limited periosteal degloving prevents excessive stretching of the IAN during SSRO, thus lowering NSD incidence.
Sagittal split ramus osteotomy (SSRO) has gained widespread popularity in the field of orthognathic surgery, but previous studies have reported complications associated with this procedure. SSRO may damage the inferior alveolar nerve (IAN) and cause neurosensory disturbance (NSD) in the lower lip, which is one of the most common and unpleasant postoperative complications. The NSD caused by damage to the IAN is reportedly 9–84.6%. Even with careful surgery, injury to the IAN appears unpredictable. Multiple factors are considered responsible for the development of NSD after SSRO, including fixation methods, patient age, surgical procedures, improper splinting, magnitude of mandibular movement, experience of surgeons, and timing of the postoperative neurosensory evaluation. Although many investigators have reported NSD after SSRO, the precise factors remain to be elucidated. A few studies have examined the position and course of the mandibular canal in SSRO patients to elucidate any association with direct or indirect intraoperative damage to the IAN. Yamamoto et al. described the interface between the mandibular canal and the buccal cortical bone using computed tomography (CT) images, and the distance between the mandibular canal and the split surface was evaluated as a surgical risk factor for NSD after SSRO. The lower distance between mandibular canal and buccal cortical bone are also described as risk factors for NSD in other studies.
In addition to the above factors related to mandibular splitting, other factors known to cause postoperative NSD include the procedure of medial periosteal dissection and compression of the nerve on the medial side of the mandibular ramus by the protecting retractors. The space located in the medial aspect of the mandibular ramus is important for working space in SSRO; it is prepared for subperiosteal tunneling in order to insert a channel retractor just superior to the lingual to protect IAN and cut the mandible at the horizontal line. In this procedure the IAN is likely to be stretch excessively by the channel retractor to allow better visualization in this narrow surgical field. The relationship between the surgical space located medial to the mandibular ramus and the development of NSD remains to be evaluated. The present study aims to investigate the factors associated with the development and subsequent improvement of NSD on the basis of mandibular morphology on CT images and the surgical space located medial to the mandibular ramus.
Materials and methods
The study group comprised 50 patients (32 women and 18 men; aged 17–44 years) who underwent consecutive series SSRO (100 SSROs in total) for the correction of mandibular prognathism ( n = 48) and retrognathism ( n = 2) between November 2006 and January 2010. All patients were candidates for mandibular osteotomy alone, and cases requiring surgery in both jaws were excluded from the study. All impacted third molars were surgically extracted at least 1 year before surgery. Demographic data, such as age, sex, height, weight, and body mass index (BMI) were collected for all patients.
The standardized surgical procedures of the Obwegeser method, wherein a separator is used for splitting the ramus, were followed. All patients were operated on by a single experienced surgeon. Efforts were made to protect the IAN from damage in every phase. The medial aspect of the mandibular ramus was prepared for subperiosteal tunneling, followed by insertion of a channel retractor just superior to the lingula. The mandible was cut at the horizontal and vertical osteotomy line and at the anterior border of the ascending ramus using a long Lindemann bur and a no 702 tapered fissure bur, respectively. A thin sharp osteotome was malleted to section the body of the mandible at the junction of the lateral cortical plate and the intramedullary bone in the vertical and horizontal bone cut. No IAN injuries, such as lacerations or cuts, occurred during the actual split. Fixation of the lateral cortical bone was performed using positioning screws ( n = 48) and plates ( n = 52) on both sides.
In order to elucidate the surgical difficulty, overall surgical duration including the time required for medial ramus ablation and mandibular cuttings as well as surgical blood loss were measured. Nurses in the operating room timed each procedure in seconds using a stopwatch. The time required for medial ramus ablation and bone cutting was measured from the start of detachment of the muscle and periosteum to placement of the cut on the medial aspect of the mandibular ramus. Total blood loss was calculated by measuring the suctioned blood and weighing the gauze after adjusting for the volume of normal saline solution used for irrigation during surgery.
Maxillofacial anatomy related to the SSRO procedure was evaluated using CT images obtained using a helical CT scanner (HiSpeed NX/I Pro, GE Healthcare, USA). Each sequence was taken 1.25 mm apart for three-dimensional (3D) reconstruction (120 kV; average, 100 mA; helical pitch, 1.0 mm). All scans were performed in a standard fashion with patients in the supine position. Their heads were held in place with a special head frame. Table 1 shows the reference points, and the measurement regions analyzed were mainly based on the SSRO osteotomy sites. A horizontal image consistent with the horizontal osteotomy line and parallel to the Frankfurt plane, one slice above the level of the lingula of the mandibular ramus as observed on CT images, was selected for measurement ( Fig. 1 ). The resultant slice image data were converted to 3D CT images in the digital imaging and communications in medicine format and reconstructed by Osirix (version 3.6.1) ( Figs 2A, C and 3 ). The deepest position at the level of the mandibular notch as observed on coronal images was selected to measure the distance between the mandibular notch and the lingula ( Fig. 2 B). Another horizontal image parallel to the Frankfurt plane and just below the level of the lingula was selected for measurement ( Fig. 2 D). Among the measurement regions, (5)–(7) were referred from previous studies and (1)–(4) and (8) was newly specified for this study. In this study, the space enclosed by the mandible, maxilla and zygomatic bone was considered as the surgical space medial to the mandibular ramus (8) ( Fig. 3 ). In total, 100 surgical sites were evaluated. Two maxillofacial surgeons (T.K. and N.K.) measured all CT images in a blinded fashion. The mean value of each parameter was used for measuring numerical data.
|Setup of specified points|
|Point a||Anterior border of the ramus|
|Point b||Posterior border of the ramus|
|Point c||Edge of the medial oblique line|
|(1) Points a–b||The distance from points a–b – antero-posterior length of the lateral ramus|
|(2) Points a–c||The distance from points a–c – thickness of anterior ramus|
|(3) Points b–c||The distance from points b–c – antero-posterior length of the medial ramus|
|(4) Maxillary tuberosity to the mandible||The distance from the edge of the medial oblique surface to the maxillary tuberosity|
|(5) Lingula to the mandibular notch||The distance from the mandibular notch to the lingula|
|(6) Thickness of the mandible||Horizontal thickness of the mandible at the lowest point of the lingula|
|(7) Width of the bone marrow space at the buccal side||The bone marrow space between the outer mandibular canal and the inner surface of the buccal cortical bone|
|(8) Space on the medial side of the ramus||The area enclosed by the mandible, the maxilla and the zygomatic bone|
Clinical sensory testing was performed at 1 week, 6 months, and 1 year after surgery using Semmes–Weinstein pressure esthesiometer filaments (Stoelting Co., USA) widely used to assess the pressure/touch perceptions of the hand and face. This screening device consists of 20 nylon filaments of differing calibres. With the patient’s eyes closed, the monofilament was applied to the lower lip and chin area with just enough pressure to bend the filament slightly. The procedure was started with the smallest monofilament (1.65) and performed twice in each of nine areas in the mental nerve region, and the value obtained was recorded. A single examiner who was not involved in the surgeries performed these evaluations (T.K.). The highest threshold value on the lower lip and chin was recorded. Normal sensation values were considered to range from 1.65 to 2.83. If a value higher than 2.83 was obtained in at least one region, NSD was considered positive.
Correlations among parameters related to NSD development were analyzed by Student’s t -test and Fisher’s exact test. Applying the logistic regression model, the factors associated with NSD were used for multivariate analysis. Correlations among parameters related to NSD improvement (6 months later) were analyzed by Student’s t -test, and the factors associated with NSD improvement were used for multivariate analysis. All data were statistically analyzed by the JMP software program v8 (SAS Institute Inc., Cary, NC, USA). Differences were considered statistically significant at P < 0.05.
Hypoesthesia of the lower lip at 1 week and 6 months after SSRO was observed in 33.0% (33/100) and 11.0% (11/100) of cases, respectively ( Table 2 ). NSD in the lower lip at 1 year after SSRO was detected for 2 (2%) surgical sites. In the prognathism, hypoesthesia of the lower lip at 1 week and 6 months after SSRO was observed in 34.4% (33/96) and 11.5% (11/96) of cases. None of the NSD was seen in the surgery for retrognathism. There was no significant difference statistically in NSD between surgery for prognathism and retrognathism. As shown in Table 3 , the mean total surgical duration was significantly longer in patients who developed NSD in the lower lip at 1 week after SSRO than in those who did not (118.5 vs. 103.1 min, P = 0.019).
|Follow-up period||Appearance of sensory disturbance|
|n = 100 sides|
|Variables||No. cases ( n = 50) mean, SD||Sensory disturbance at PO1W||Sensory disturbance at PO6M|
|Appearance ( n = 26) mean, SD||No appearance ( n = 24) mean, SD||P -value||Appearance ( n = 8) mean, SD||No appearance ( n = 42) mean, SD||P -value|
|Age||28.6, 7.6||28.7, 7.9||28.5, 7.3||0.959||–||–||–|
|Height (cm)||165.3, 8.5||165.5, 7.6||165.0, 9.5||0.838||–||–||–|
|Weight (kg)||57.0, 9.1||56.7, 8.4||57.2, 10.1||0.851||–||–||–|
|BMI (%)||20.8, 2.4||20.7, 2.6||20.8, 2.2||0.764||–||–||–|
|Total surgical time (min)||111.8, 23.4||118.5, 26.4||103.1, 16.9||0.019||105.9, 10.3||112.1, 25.1||0.499|
|Blood loss (ml)||73.3, 61.4||93.8, 77.0||51.0, 24.3||0.012||77.5, 40.4||72.5, 65.0||0.835|
Similarly, patients who developed NSD at 1 week after SSRO experienced significantly greater blood loss compared with those who did not (93.8 ml vs. 51.0 ml, P = 0.012). There were no significant differences in fixation method, sex, height, weight, BMI, surgical site, time required for medial ablation and bone cutting, and magnitude of mandibular movement between patients with and without NSD ( Tables 3 and 4 ). There were significant differences between patients with NSD and without NSD at 1 week ( P = 0.006) and 6 months ( P = 0.001) after SSRO for the surgical space on the medial side of the mandibular ramus ( Table 4 ). Among other measurements, the distance between the maxillary tuberosity and the mandible, the distance between the lingula and the mandibular notch, mandibular thickness, width of the bone marrow space on the buccal side of the mandible were significant differences for NSD only at 6 months later as shown in Table 4 . In multivariate analysis, a distance of 11.0 mm or more between the maxillary tuberosity and the mandible (relative risk, 3.7; 95% CI, 1.1–15.5; P = 0.049), a distance of less than 15.0 mm between the lingula and the mandibular notch (relative risk, 6.7; 95% CI, 1.7–33.8; P = 0.005), a mandibular thickness of less than 8.7 mm (relative risk, 5.7; 95% CI, 1.4–24.3; P = 0.016), a distance of less than 1.5 mm between the buccal aspect of the mandibular canal and the buccal cortex (relative risk, 4.4; 95% CI, 1.2–18.4; P = 0.032), and a space of 195.0 mm 2 or more on the medial side of the mandibular ramus (relative risk, 17.2; 95% CI, 3.9–100.4; P < 0.001) indicated a statistically significant risk of NSD ( Table 5 ).
|Measurement||No. cases ( n = 100) mean, SD||Sensory disturbance at PO1W||Sensory disturbance at PO6M|
|Appearance ( n = 33) mean, SD||No appearance ( n = 67) mean, SD||P -value||Appearance ( n = 11) mean, SD||No appearance ( n = 89) mean, SD||P -value|
|Magnitude of mandibular movement (mm)||5.72, 3.7||6.6, 2.9||5.3, 4.0||0.101||5.7, 2.3||5.7, 3.8||0.936|
|Time required for ablation of the medial ramus and the cutting bone (s)||532.5, 213.2||540.9, 218.2||528.4, 212.3||0.783||474.5, 96.1||539.7, 222.8||0.343|
|Anatomical variables (mm)|
|(1) Points a–b||33.9, 3.0||33.3, 2.6||34.2, 3.2||0.203||32.3, 3.0||34.1, 3.0||0.074|
|(2) Points a–c||10.2, 2.2||10.0, 2.3||10.3, 2.1||0.409||10.1, 2.8||10.2, 2.1||0.871|
|(3) Points b–c||26.7, 3.6||26.4, 3.0||26.9, 3.8||0.521||25.4, 3.1||26.9, 3.6||0.203|
|(4) Maxillary tuberosity to the mandible||10.4, 2.4||10.8, 2.9||10.1, 2.2||0.157||11.8, 1.9||10.2, 2.4||0.031|
|(5) Lingula to the mandibular notch||16.1, 3.6||15.3, 3.4||16.5, 3.6||0.102||13.5, 2.8||16.4, 3.5||0.011|
|(6) Thickness of the mandible||9.5, 1.2||9.3, 1.2||9.6, 1.2||0.181||8.6, 1.4||9.6, 1.2||0.011|
|(7) Width of the bone marrow space at the buccal side||2.0, 1.1||1.8, 1.1||2.1, 1.1||0.170||1.3, 1.1||2.0, 1.0||0.042|
|(8) Space on the medial side of the ramus||159.2, 52.9||179.9, 56.8||149.0, 48.1||0.006||207.5, 43.6||153.2, 51.0||0.001|
|Variables||No. cases||Sensory disturbance||Univariate analysis||Multivariate analysis|
|( n = 100)||Appearance No. ( n = 11)||No appearance No. ( n = 89)||Relative risk||95% CI||P -value||Relative risk a||95% CI||P -value|
|(4) Maxillary tuberosity to the mandible (mm)||Low (<11.0)||65||4||61||1.0||1.0|
|(5) Lingula to the mandibular notch (mm)||High (15.0≦)||64||3||61||1.0||1.0|
|(6) Thickness of the mandible (mm)||High (8.7≦)||80||6||74||1.0||1.0|
|(7) Width of the bone marrow space at the buccal side (mm)||High (1.5≦)||67||4||63||1.0||1.0|
|(8) Space on the medial side of the ramus (mm 2 )||Low (<195.0)||76||3||73||1.0||1.0|