The bimaxillary distraction technique (BMD) was first described by Ortiz Monasterio et al. 1 for the correction of facial asymmetry in cases of hemifacial microsomia class IIa and b. This technique includes Lefort I osteotomy, oblique osteotomy along the angle of the mandible, followed by intermaxillary fixation to allow for the simultaneous distraction of both arches using a single distraction device fixed across the mandibular osteotomy. The major advantage of the technique was the ability to correct both facial asymmetry and occlusal canting without disrupting the existing stable occlusal contact in an adult population.
A few years later, the technique was used for the correction of other types of facial asymmetry including postankylotic asymmetry 2,3. In the last 10 years, BMD has gained wide-spread acceptance. However, there was no definitive guideline for the selection of the appropriate device. Early clinical trials advocated an oblique distraction vector with a unidirectional device, assuming simultaneous elongation of both the mandibular body and the ramus 3,4. A few years later, with the development of bidirectional devices, many clinical reports claimed achievement of superior esthetic results using more than one distraction arm with a single osteotomy line 5,6. To our knowledge, none of the present clinical reports have compared the outcome of unidirectional versus bidirectional devices or support osteotomy design over the other in postankylotic asymmetry.
The answer to these questions requires a critical analysis of the deformity in three dimensions (3D) including both the affected and the nonaffected sides. It is not a simple transverse discrepancy or asymmetry; it is a 3D deformity with the primary discrepancy on the affected side and secondary changes on the nonaffected side 7,8. This prospective clinical study was designed to evaluate the 3D correction of postankylotic deformity using the BMD technique with both unidirectional and bidirectional extraoral devices. The results of this study are expected to facilitate the selection of an appropriate device for each case and predict the outcome of the treatment.
Patients and methods
During the study period (2006–2008), six adult patients, five females and one male (mean age 23 years), were included. The patients involved in the study were presented to the Department of Oral and Maxillofacial Surgery, Ain Shams University with residual facial deformity after the successful release of unilateral TMJ bony ankylosis ( Table 1). The patients were randomly divided into two equal groups: group I was treated using the BMD technique using an extraoral unidirectional device, Molina adult size (KLS-Martin Company, Texas, USA), and group II was treated using the BMD technique using an extraoral bidirectional device (Stryker Leibinger Company, Tuttling, Germany). Patients were evaluated preoperatively, at the end of the consolidation period and followed up for 1 year (Table 2). Evaluation included clinical examinations, photographs, mounted study casts, direct soft tissue measurements (canthal commissure distance and incisal show), and radiographs (panorama, lateral, and posteroanterior cephalometry).
Under general anesthesia with nasotracheal intubation, an incision was made along the buccal sulcus; subperiosteal dissection was carried out to expose the maxilla. A standard Lefort I osteotomy with complete mobilization and down-fracturing of the upper jaw was carried out. Interosseous wires were used to suspend the maxilla on the nonaffected side to minimize rotation of the maxillary midline (Fig. 1a and b).
A standard submandibular incision was used to expose the mandibular angle ramus region. A corticotomy was conducted using a surgical bur extending from the retro-molar triangle to the mandibular angle (Fig. 2a and b). Intraosseous pins were inserted across the cheek on both sides of the corticotomy (Fig. 3a and b) (for the unidirectional device, two pins 3.2 mm, one on each side of the cortecotomy, and for the bidirectional device, four pins 2.0 mm, two on each side of the corticotomy, were used); a distractor device was mounted onto the intraosseous pins. The osteotomy was completed using bibeveled chisels. Intermaxillary fixation was completed using upper and lower arch bars. The wound was closed in the standard manner.
The distraction procedure was initiated on the fifth postoperative day at a rate of 1.5 mm/day. For the bidirectional device, the activation of linear distraction was started to achieve ramus lengthening and correction of occlusal canting (first 15 mm). Later transverse and angular vectors were activated by molding the gonial angle and medializing the chine point. A consolidation period of 10–12 weeks was allowed for both the groups. Intermaxillary fixation was maintained during active distraction and the consolidation period. At the end of consolidation, the distractor device and arch bars were removed under local anesthesia.
All patients tolerated the surgical as well as the distraction procedures. There was no evidence of permanent nerve injury (neither the inferior alveolar nerve nor the marginal mandibular branch of the facial nerve). The mean distraction distance was 23 mm for group I and 27 mm for group II. Skeletal changes were evaluated in three planes (Fig. 4); there was marked clinical improvement in the facial asymmetry in both the groups. Group I patients presented with higher correction of the ramus and facial heights (Fig. 5), whereas group II showed better correction of the mandibular midline and canthal commissure distances (Fig. 6). There were minimal changes in the dental occlusion including increased overjet, overbite crowding of lower anterior teeth and lingual tipping of the ipsilateral lower molars. However, none of the patients developed a posterior open bite. There was insignificant relapse in the 1-year follow-up record, except for patient no. 3 in group II, in whom partial relapse was detected.
An intraexaminer reliability study was carried out to determine the landmark identification error. Skeletal changes were evaluated in three planes. There was an insignificant difference among two different readings. The Mann–Whitney U-test was used to compare percentages of changes in the two groups.
Comparison between the two groups
In the vertical dimension, group II showed a higher percentage of change in both the maxillary height and the gonial height. However, these differences were not statistically significant.
Conversely, group I cases showed significantly greater elongation of the mandibular ramus (P=0.05) (Fig. 7); at the same time, they show higher elongation of the facial heights than group II (Fig. 8).
In the anteroposterior dimension, the patients in group I (Fig. 9) showed less correction of the SNB angle than the patients in group II (Fig. 10); however, the differences were not statistically significant.
In the medio-lateral dimension, patients in group II presented with superior skeletal correction of facial asymmetry; the percentage of change in shifting angle was 67.6±18.4. The percentage of change in group I was 32±4.8. Yet, there was no statistically significant difference between the two groups. Correction of occlusal plane canting was almost the same in the two groups.
Changes in the soft tissue measurements: As patients in group II presented superior correction of skeletal asymmetry, higher correction of the canthus commissure distances was expected. The percentage of change in the distraction side lateral canthus commissure was 9.6±11.7 for group II versus 8.6±6 for group I and medial canthal commissure was 9.3±5.4 for group II versus 3.4±3 for group I.
The use of a unidirectional versus a bidirectional device for BMD was investigated in this study. Molina et al. reported successful corrections of asymmetry with a unidirectional device 9–11. Despite these reports, there is disagreement about the use of a unidirectional distractor for 3D correction of a deformity. It was believed that a bidirectional distractor offers simultaneous elongation of both the mandibular body and the ramus 6,12.
A unidirectional device used with oblique angle osteotomy allowed the successful correction of vertical ramus deficiency, with a concomitant improvement of posterior facial height and occlusal canting. Yet, there is insignificant correction of mandibular retrognathia and mandibular midline shift 9. In contrast, the use of a bidirectional device with a single oblique osteotomy provides better means for advancement of the distal segment and centralization of the chin. However, it is less effective in elongation of the deficient ramus and correction of occlusal canting. Gradual distraction of proximal and distal segments across an angle with bending of the newly formed callus minimizes the linear elongation of both the mandibular body and the ramus. In our cases, we started distraction with an angle of 180° between two arms of the distractor (using it as a single unidirectional distractor) until correction of occlusal canting was achieved for about 10–12 mm. Further activation was carried out for advancement and medialization of the distal segment 13. From these findings, it was concluded that the bidirectional distractor is a better alternative for 3D correction of facial asymmetry secondary to unilateral TMJ ankylosis.
Another option for the treatment of this deformity is the use of a multifocal distraction device with two osteotomy lines. This technique has been shown to be sufficient for complete mandibular reshaping. However, simultaneous movements at several distraction sites result in interfering vector forces, making the adjustment of multifocal distraction device slightly difficult. Despite accurate planning three dimensional distraction may result in deviation from planned result. However, the use of callus remodeling during early consolidation phase facilitate correction of these inaccuracies 14.
The BMD technique offers complete correction of occlusal canting and maxillary deficiency as well as equalization of the mandibular ramus height, and yet, it offers limited correction of the mandibular midline and the soft tissue component of the deformity. Unfortunately, simultaneous distraction techniques do not correct the mandibular retrognathia associated with postankylotic deformity. Unilateral distraction with oblique angle osteotomy lengthens the vertical ramus with nonsignificant advancement of the mandible. If distraction osteogenesis is to be used for the correction of mandibular retrognathia, it should be carried out in a second stage using bilateral distraction with vertical body osteotomy 15,16,17. Moreover, the contra-lateral mandibular body needs further interventions such as fat augmentation or bone grafting to restore normal body convexity.
In this study, the correction of soft tissue asymmetry as measured by the canthal commissure distance was statistically insignificant. Nevertheless, there is considerable improvement in esthetics clinically, with the lateral canthus commissure showing better correction than the medial canthus. At the same time, group II patients presented with greater correction of the midline showed corresponding greater correction of canthal commissure distances.
Conflicts of interest
There are no conflicts of interest.
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