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Bilateral sagittal split osteotomies versus mandibular distraction osteogenesis: a prospective clinical trial comparing inferior alveolar nerve function and complications
Int. J. Oral Maxillofac. Surg. 2010; 39: 756–760 doi:10.1016/j.ijom.2010.04.001, available online at http://www.sciencedirect.com
Leading Clinical Paper Orthognathic Surgery
Bilateral sagittal split osteotomies versus mandibular distraction osteogenesis: a prospective clinical trial comparing inferior alveolar nerve function and complications
A. Ow, L. K. Cheung Discipline of Oral and Maxillofacial Surgery, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR
A. Ow, L. K. Cheung: Bilateral sagittal split osteotomies versus mandibular distraction osteogenesis: a prospective clinical trial comparing inferior alveolar nerve function and complications. Int. J. Oral Maxillofac. Surg. 2010; 39: 756–760. # 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. 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).
0901-5027/080756 + 05 $36.00/0
Keywords: bilateral sagittal split osteotomies; mandibular distraction osteogenesis; inferior alveolar nerve; complications. Accepted for publication 6 April 2010 Available online 7 May 2010
# 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Comparison of BSSO and MDO Persistent neurosensory disturbance of the inferior alveolar nerve (IAN) is a wellknown 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 studies1,12,15 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 implicated1,3,7,10–12,15. 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 injury11. 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 IAN13, 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 con-
sent was obtained from each patient before any treatment was carried out. 23 subjects with a 6–12-month followup 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
757
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
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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 pvalue of less than 0.05 was considered statistically significant. Results
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
Fig. 2. Light touch (LT) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05).
Fig. 3. Two-point discrimination (2PD) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05).
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.
Fig. 1. Visual analogue scale (VAS) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05).
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.
Comparison of BSSO and MDO
Fig. 4. Pain detection threshold (PD) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05). Table 1. Postoperative complications in the BSSO group. Postoperative complications
No. of BSSO patients
Postoperative 0–2 weeks (TBD1) Bilateral posterior open bite* Localized wound infectiony
1 2
Postoperative 6 months to 1 year (TBD4–TBD5) Condylar resorption Mild anterior open bite
1 1
* y
Intermaxillary fixation applied. 1 patient required incision and drainage under local anaesthesia.
Table 2. Postoperative complications in the MDO group. Postoperative complications
No. of MDO patients
During distraction Insufficient length of distractors*
1
Consolidation period (2 weeks to 3 months; TBD1–TBD3) Bilateral posterior open bitey Persistent wound infection around distractor rodsz
1 6
Post-distractor removal period (3 months to 1 year; TBD3–TBD5) Condylar resorption Mild anterior open bite
1 1
*
Re-operation with replacement of distractors. Re-operation with early removal of distractors and use of heavy elastics to correct open bite. z 2 patients required incision and drainage under local anaesthesia; 1 patient with orocutaneous fistula. y
Discussion
When compared with BSSO, the incidence of persistent IAN disturbance after MDO is expected to be lower. OW & CHEUNG9 reported a lower incidence of persistent IAN disturbance (3%) after MDO compared with BSSO, of which the incidence was 28%. Animal studies5,8 have shown that the IAN undergoes gradual stretching during the distraction process, which allows the nerve to adapt better and avoid any permanent damage when compared with the abrupt stretching of the IAN in BSSO advancement. This adaptation is influenced by the distraction rate, with a higher rate resulting in more nerve damage. Surgical technique is another contributing factor. With MDO, soft tissue dissection is
minimal and limited to the buccal mucoperiosteum on the lateral body and part of the lingual surface, with no dissection at the medial ramus. After the osteotomy is performed, gradual mobilization of the segments is carried out to ensure adequate separation. Excessive separation of the bone segments to check for the position of the IAN is not required. In contrast, the IAN is frequently trapped in the proximal segment with BSSO and surgical dissection is required to free the nerve. There is less traumatic manipulation of the IAN with MDO, but the number of clinical studies investigating persistent IAN disturbance after MDO is low. In a retrospective study, 13 VAN STRIJEN et al. reported persistent IAN disturbance in only 3 of 70 patients. In contrast, in a case series of 5 patients by
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WHITESIDES & MEYER14, minor parasthesia was reported in all patients after MDO using bilateral vertical body osteotomies. The reason for this difference could be attributed to the younger patient pool in the former study, of whom a better adaptive and regenerative capacity is expected. Similarly, in this study, patients reported mildly elevated subjective VAS scores even at postoperative/distraction 1 year, indicating that some degree of persistent neurosensory disturbance existed, even for MDO. The reason for this could be related to the adaptive and regenerative capacity of these patients, who were older compared with the sample as reported by VAN STRIJEN et al.13 and other studies9. The use of the Hofer osteotomy in this study could have had an effect on neurosensory recovery, although mixed model analysis had shown this effect to be insignificant. The number of patients in this study is small, so whether MDO results in a better recovery pattern than BSSO remains to be seen. Regarding other complications, in the MDO group, persistent wound infection around the distractor rods was a common finding despite meticulous oral hygiene. This was attributed to food trapping in and around the coils of the distractor rods, providing a portal of bacterial ingress into the wound. 2 patients required incision and drainage under local anaesthesia and 1 patient developed an oro-cutaneous fistula. Prompt resolution of the infections occurred on distractor removal. In later MDO patients, this coiled type of distractor was replaced with a smooth-surfaced type. There have not been any reports of severe infection around these smooth-surfaced distractor rods. Condylar resorption was seen in 1 BSSO and 1 MDO patient in the late postoperative period. OW & CHEUNG9 reported a 6% and 1% incidence of condylar resorption after BSSO and MDO, respectively. Risk factors for condylar resorption have been well-reported for BSSO and have included having a high mandibular plane angle, a posteriorly inclined condylar neck, large advancement, screw fixation and pre-existing temporomandibular joint (TMJ) internal derangement4,6. For MDO, risk factors for condylar resorption have not been elucidated. During the distraction process, MDO results in superior and posterior displacement of the condyle process. Similar to the situation in BSSO, this condylar displacement is thought to produce unfavourable loading of the condyle and disk, and may result in condylar resorption once the adaptive capacity of the condyle is exceeded. AZUMI et al.2 reported 4 patients with condylar resorption after MDO for lengthening.
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All these patients were female and had preoperative TMJ internal derangement. In these cases, control of the rate and rhythm of distraction may be important to allow physiologic adaptation of the condyle during the distraction process. The use of Class II elastics to pull the mandible forward during the activation period may also reduce the risk of condylar resorption by preventing the condyle from compressing against the fossa. Further long-term studies are required to address this area. In summary, this randomized clinical trial compared the incidence of persistent neurosensory disturbance between BSSO and MDO in the treatment of Class II mandibular hypoplasia. At present, some degree of postoperative neurosensory disturbance exists for patients in both groups 1 year postoperatively, although no significant differences between the two groups were found. Despite its low incidence, condylar resorption was reported in both groups and both techniques may share common risk factors for such a complication. A larger sample size is needed to compare the morbidities associated with these two techniques further. Competing interests
None declared. Funding
None. Ethical approval
Yes. Acknowledgements. The authors would like to thank Dr. Shen Liang and Mr. Shadow Yeung for their contributions to the manuscript.
References 1. August M, Marchena J, Donady J, Kaban L. Neurosensory deficit and functional impairment after sagittal ramus osteotomy: a long-term follow-up study. J Oral Maxillofac Surg 1998: 56: 1231– 1235. 2. Azumi Y, Sugawara J, Takahashi I, Mitani H, Nagasaka H, Kawamura H. Positional and morphologic changes of the mandibular condyle after mandibular distraction osteogenesis in skeletal class II patients. World J Orthodon 2004: 5: 32–39. 3. Brusati R, Fiamminghi L, Sesenna E, Gazzotti A. Functional disturbances of the inferior alveolar nerve after sagittal osteotomy of the mandibular ramus: operating technique for prevention. J Maxillofac Surg 1981: 9: 123–125. 4. Cutbirth M, Van Sickels JE, Thrash WJ. Condylar resorption after bicortical screw fixation of mandibular advancement. J Oral Maxillofac Surg 1998: 56: 178–182. 5. Hu J, Tang Z, Wang D, Buckley MJ. Changes in the inferior alveolar nerve after mandibular lengthening with different rates of distraction. J Oral Maxillofac Surg 2001: 9: 1041–1045. 6. Hwang SJ, Haers PE, Seifert B, Sailer HF. Non-surgical risk factors for condylar resorption after orthognathic surgery. J Craniomaxillofac Surg 2004: 32: 103–111. 7. MacIntosh RB. Experience with the sagittal osteotomy of the mandibular ramus: a 13-year review. J Maxillofac Surg 1981: 9: 151–165. 8. Makarov MR, Harper RP, Cope JB, Samchukov ML. Evaluation of inferior alveolar nerve function during distraction osteogenesis in the dog. J Oral Maxillofac Surg 1998: 56: 1417–1423. 9. Ow ATC, Cheung LK. Skeletal stability and complications of Bilateral Sagittal Split Osteotomies (BSSO) and Mandibular Distraction Osteogenesis (MDO): An evidence-based review. J
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Oral and Maxillofac Surg 2009: 67: 2344–2353. Panula K, Finne K, Oikarinen K. Neurosensory deficits after bilateral sagittal split ramus osteotomy of the mandible—influence of soft tissue handling medial to the ascending ramus. Int J Oral Maxillofac Surg 2004: 33: 543–548. Teerijoki-Oksa T, Jaaskelainen SK, Forssell K, Forssell H, Vahatalo K, Tammisalo T, Virtanen A. Risk factors of nerve injury during mandibular sagittal split osteotomy. Int J Oral Maxillofac Surg 2002: 31: 33–39. Van Sickels JE, Hatch JP, Dolce C, Bays RA, Rugh JD. Effects of age, amount of advancement, and genioplasty on neurosensory disturbance after a bilateral sagittal split osteotomy. J Oral Maxillofac Surg 2002: 60: 1012– 1017. van Strijen PJ, Breuning KH, Becking AG, Perdijk FB, Tuinzing DB. Complications in bilateral mandibular distraction osteogenesis using internal devices. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003: 96: 392–397. Whitesides LM, Meyer RA. Effect of distraction osteogenesis on the severely hypoplastic mandible and inferior alveolar nerve function. J Oral Maxillofac Surg 2004: 62: 292–297. Ylikontiola L, Kinnunen J, Oikarinen K. Factors affecting neurosensory disturbance after mandibular bilateral sagittal split osteotomy. J Oral Maxillofac Surg 2000: 58: 1234–1240.
Address: Lim Kwong Cheung 2/F Oral and Maxillofacial Surgery Faculty of Dentistry Prince Philip Dental Hospital 34 Hospital Road Hong Kong SAR Fax: +852 25599014 E-mail: [email protected]
Leading Clinical Paper Orthognathic Surgery
Bilateral sagittal split osteotomies versus mandibular distraction osteogenesis: a prospective clinical trial comparing inferior alveolar nerve function and complications
A. Ow, L. K. Cheung Discipline of Oral and Maxillofacial Surgery, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR
A. Ow, L. K. Cheung: Bilateral sagittal split osteotomies versus mandibular distraction osteogenesis: a prospective clinical trial comparing inferior alveolar nerve function and complications. Int. J. Oral Maxillofac. Surg. 2010; 39: 756–760. # 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. 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).
0901-5027/080756 + 05 $36.00/0
Keywords: bilateral sagittal split osteotomies; mandibular distraction osteogenesis; inferior alveolar nerve; complications. Accepted for publication 6 April 2010 Available online 7 May 2010
# 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Comparison of BSSO and MDO Persistent neurosensory disturbance of the inferior alveolar nerve (IAN) is a wellknown 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 studies1,12,15 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 implicated1,3,7,10–12,15. 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 injury11. 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 IAN13, 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 con-
sent was obtained from each patient before any treatment was carried out. 23 subjects with a 6–12-month followup 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
757
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
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Ow, and Cheung
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 pvalue of less than 0.05 was considered statistically significant. Results
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
Fig. 2. Light touch (LT) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05).
Fig. 3. Two-point discrimination (2PD) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05).
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.
Fig. 1. Visual analogue scale (VAS) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05).
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.
Comparison of BSSO and MDO
Fig. 4. Pain detection threshold (PD) scores (mean SE) from TBD1 to TBD5; BSSO group versus MDO group; No significant differences between groups over the 12-month postoperative period (p > 0.05). Table 1. Postoperative complications in the BSSO group. Postoperative complications
No. of BSSO patients
Postoperative 0–2 weeks (TBD1) Bilateral posterior open bite* Localized wound infectiony
1 2
Postoperative 6 months to 1 year (TBD4–TBD5) Condylar resorption Mild anterior open bite
1 1
* y
Intermaxillary fixation applied. 1 patient required incision and drainage under local anaesthesia.
Table 2. Postoperative complications in the MDO group. Postoperative complications
No. of MDO patients
During distraction Insufficient length of distractors*
1
Consolidation period (2 weeks to 3 months; TBD1–TBD3) Bilateral posterior open bitey Persistent wound infection around distractor rodsz
1 6
Post-distractor removal period (3 months to 1 year; TBD3–TBD5) Condylar resorption Mild anterior open bite
1 1
*
Re-operation with replacement of distractors. Re-operation with early removal of distractors and use of heavy elastics to correct open bite. z 2 patients required incision and drainage under local anaesthesia; 1 patient with orocutaneous fistula. y
Discussion
When compared with BSSO, the incidence of persistent IAN disturbance after MDO is expected to be lower. OW & CHEUNG9 reported a lower incidence of persistent IAN disturbance (3%) after MDO compared with BSSO, of which the incidence was 28%. Animal studies5,8 have shown that the IAN undergoes gradual stretching during the distraction process, which allows the nerve to adapt better and avoid any permanent damage when compared with the abrupt stretching of the IAN in BSSO advancement. This adaptation is influenced by the distraction rate, with a higher rate resulting in more nerve damage. Surgical technique is another contributing factor. With MDO, soft tissue dissection is
minimal and limited to the buccal mucoperiosteum on the lateral body and part of the lingual surface, with no dissection at the medial ramus. After the osteotomy is performed, gradual mobilization of the segments is carried out to ensure adequate separation. Excessive separation of the bone segments to check for the position of the IAN is not required. In contrast, the IAN is frequently trapped in the proximal segment with BSSO and surgical dissection is required to free the nerve. There is less traumatic manipulation of the IAN with MDO, but the number of clinical studies investigating persistent IAN disturbance after MDO is low. In a retrospective study, 13 VAN STRIJEN et al. reported persistent IAN disturbance in only 3 of 70 patients. In contrast, in a case series of 5 patients by
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WHITESIDES & MEYER14, minor parasthesia was reported in all patients after MDO using bilateral vertical body osteotomies. The reason for this difference could be attributed to the younger patient pool in the former study, of whom a better adaptive and regenerative capacity is expected. Similarly, in this study, patients reported mildly elevated subjective VAS scores even at postoperative/distraction 1 year, indicating that some degree of persistent neurosensory disturbance existed, even for MDO. The reason for this could be related to the adaptive and regenerative capacity of these patients, who were older compared with the sample as reported by VAN STRIJEN et al.13 and other studies9. The use of the Hofer osteotomy in this study could have had an effect on neurosensory recovery, although mixed model analysis had shown this effect to be insignificant. The number of patients in this study is small, so whether MDO results in a better recovery pattern than BSSO remains to be seen. Regarding other complications, in the MDO group, persistent wound infection around the distractor rods was a common finding despite meticulous oral hygiene. This was attributed to food trapping in and around the coils of the distractor rods, providing a portal of bacterial ingress into the wound. 2 patients required incision and drainage under local anaesthesia and 1 patient developed an oro-cutaneous fistula. Prompt resolution of the infections occurred on distractor removal. In later MDO patients, this coiled type of distractor was replaced with a smooth-surfaced type. There have not been any reports of severe infection around these smooth-surfaced distractor rods. Condylar resorption was seen in 1 BSSO and 1 MDO patient in the late postoperative period. OW & CHEUNG9 reported a 6% and 1% incidence of condylar resorption after BSSO and MDO, respectively. Risk factors for condylar resorption have been well-reported for BSSO and have included having a high mandibular plane angle, a posteriorly inclined condylar neck, large advancement, screw fixation and pre-existing temporomandibular joint (TMJ) internal derangement4,6. For MDO, risk factors for condylar resorption have not been elucidated. During the distraction process, MDO results in superior and posterior displacement of the condyle process. Similar to the situation in BSSO, this condylar displacement is thought to produce unfavourable loading of the condyle and disk, and may result in condylar resorption once the adaptive capacity of the condyle is exceeded. AZUMI et al.2 reported 4 patients with condylar resorption after MDO for lengthening.
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Ow, and Cheung
All these patients were female and had preoperative TMJ internal derangement. In these cases, control of the rate and rhythm of distraction may be important to allow physiologic adaptation of the condyle during the distraction process. The use of Class II elastics to pull the mandible forward during the activation period may also reduce the risk of condylar resorption by preventing the condyle from compressing against the fossa. Further long-term studies are required to address this area. In summary, this randomized clinical trial compared the incidence of persistent neurosensory disturbance between BSSO and MDO in the treatment of Class II mandibular hypoplasia. At present, some degree of postoperative neurosensory disturbance exists for patients in both groups 1 year postoperatively, although no significant differences between the two groups were found. Despite its low incidence, condylar resorption was reported in both groups and both techniques may share common risk factors for such a complication. A larger sample size is needed to compare the morbidities associated with these two techniques further. Competing interests
None declared. Funding
None. Ethical approval
Yes. Acknowledgements. The authors would like to thank Dr. Shen Liang and Mr. Shadow Yeung for their contributions to the manuscript.
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Address: Lim Kwong Cheung 2/F Oral and Maxillofacial Surgery Faculty of Dentistry Prince Philip Dental Hospital 34 Hospital Road Hong Kong SAR Fax: +852 25599014 E-mail: [email protected]