Three-dimensional changes of proximal segments in facial asymmetry patients after bilateral vertical ramus osteotomy

Abstract

The intraoral vertical ramus osteotomy (IVRO) is a useful technique for mandibular setback surgery. However, there is a tendency for lateral flaring of the proximal segments on the non-deviation side after the correction of mandibular asymmetry with this technique. The purpose of this retrospective study was to evaluate the positional changes of the proximal segments after IVRO setback in skeletal class III patients with asymmetry, using preoperative and postoperative computed tomography scan data, and to apply the results in clinical practice. A total of 28 skeletal class III patients with asymmetry who underwent bimaxillary orthognathic surgery were included. A three-dimensional cone beam computed tomography scan was obtained preoperative, at 1 month postoperative, and at 1 year postoperative. At 1 month after the surgery, the proximal segments showed an outward rotation, lateral flaring, and anterior rotation of the condylar head. All postsurgical directional changes had returned to the preoperative state at 1 year postoperative, and there was no statistically significant difference in postoperative angulation changes between the two sides. The results showed no statistical differences in the positional changes of the proximal segments between the deviation and non-deviation sides. This study reaffirms the benefits of the IVRO for a minimal bony interference between the proximal and distal segments in three dimensions, including mandibular asymmetry cases.

The intraoral vertical ramus osteotomy (IVRO) is a procedure used routinely for mandibular setback for the correction of mandibular prognathism. The IVRO tends to require a longer duration of intermaxillary fixation than the sagittal split ramus osteotomy (SSRO), which can be troublesome and inconvenient for the patient; however, it offers some advantages over the SSRO including being a technically easier procedure with a shorter operative time, the lower risk of inferior alveolar nerve injury, and the ability to reposition the condyle .

The IVRO is useful in patients with temporomandibular joint (TMJ) dysfunction, since the condylar repositioning away from the disc and posterior attachment that occurs postoperatively results in TMJ decompression . The TMJ decompression phenomenon has increased clinical interest in the postoperative changes occurring in the proximal segments. Additionally, an increase in intergonial width during IVRO mandibular setback has been reported, due to the proximal segments being positioned lateral to the distal segments .

There is a potential worsening of lateral flaring on the non-deviation side after the correction of asymmetry. The objective of mandibular setback surgery is not only to correct the skeletal malocclusion, but also to improve facial aesthetics; thus worsening asymmetry due to lateral flaring can be problematic . Only a few studies have addressed the postoperative changes in the gonial region after the correction of mandibular asymmetry. Furthermore, the changes in the proximal segments after IVRO have mostly been studied using two-dimensional (2D) cephalometric Images , and only a very few studies have utilized three-dimensional (3D) images from a computed tomography (CT) scan.

Cone beam computed tomography (CBCT) provides reliable data on the status of the joints as well as the postoperative bony remodelling changes, and CT-generated data are more reliable than 2D cephalometric data . The purpose of this study was to evaluate the changes in the proximal segments after IVRO setback in skeletal class III patients with asymmetry using preoperative and postoperative CT scan data retrospectively and to apply the results in clinical practice.

Materials and methods

This study followed the Declaration of Helsinki regarding medical protocol and ethics, and was approved by the regional Ethics Review Board of Yonsei Dental Hospital Institutional Review Board. Patients who were diagnosed with a skeletal class III dentofacial deformity at the study institution between 2013 and 2014 and who underwent bimaxillary orthognathic surgery performed by one surgeon were included. Patients with a history of prior orthognathic surgery and craniofacial anomalies such as cleft lip and palate were excluded.

All patients underwent presurgical orthodontic treatment until the desired post-surgical occlusion was achieved, following which virtual surgical planning was performed. Bimaxillary surgery including a Le Fort I osteotomy and conventional bilateral IVRO were performed on all patients by one surgeon (Y.S.J.). The Le Fort osteotomy was followed by rigid internal fixation. In accordance with the protocol, postoperative maxillomandibular fixation was maintained and released after 12 days, after which active physiotherapy was performed using class II elastics and the final wafer . Physiotherapy was continued until adequate mouth opening and a favourable occlusion was maintained. This was followed by postoperative orthodontic treatment. 3D CBCT scans (Alphard 3030; Asahi Roentgen Ind. Co., Ltd, Kyoto, Japan) were obtained for all patients at three time points: preoperative (T0), 1 month postoperative (T1), and 1 year postoperative (T2). 3D CBCT data were stored in DICOM files (digital imaging and communications in medicine), and the Simplant Pro 14.0 software program (Materialise Dental NV, Leuven, Belgium) was used to reconstruct 3D models. Reference points and planes were determined by one examiner using the CT sectional images and 3D reconstructed model.

Reference planes were set using the following landmarks: nasion (Na): centre point of the nasofrontal suture; nasal tip (NT): most prominent point of the nasal bone; frontozygomatic suture (FZS): junction point of the frontozygomatic suture and the orbital wall; optic canal (Oc): most inferior point of the optic canal; foramen magnum (FM): most postero-inferior point of foramen magnum; MP/LP: medial pole/lateral pole of the condyle; CP: most posterior point of the condyle; AP: most posterior point of the mandibular angle.

The reference planes are described in Fig. 1 . The horizontal plane (HOR) is the plane constructed by the three points of mid-optic canal and right and left frontozygomatic suture. The midsagittal plane (MSP) is the plane perpendicular to HOR and through nasion and foramen magnum. The coronal plane (COR) is the plane perpendicular to HOR and MSP, and through nasal tip. The mediolateral plane (MLP) is the plane constructed by the three points of medial pole of the condyle, lateral pole of the condyle, and most posterior point of the mandibular angle. The anteroposterior plane (APP) is the plane constructed by the three points of most posterior point of the condyle, most posterior point of the mandibular angle, and mid-condyle.

Fig. 1
Reference planes: (A) horizontal plane, midsagittal plane, and coronal plane; (B) mediolateral plane of the proximal segment; (C) anteroposterior plane of the proximal segment.

In order to monitor the 3D changes in the proximal segments, the angles between MSP and MLP/APP, as well as between HOR and MLP were measured at each time point. Patients with more than 3 mm of chin deviation from the midsagittal plane on reconstructed 3D CT images were assigned to the asymmetry group ( n = 28) and the ramus was categorized into a deviation side and a non-deviation side for comparison.

Mauchly’s sphericity test was used to validate a repeated measures analysis of variance (ANOVA), and the significance of progressive changes in the proximal segments was tested using repeated measures ANOVA. The Bonferroni method was used to counteract the problem of multiple comparisons. A P -value of <0.05 was considered statistically significant, and all statistical analyses were performed using SPSS version 18.0 software (SPSS Inc., Chicago, IL, USA).

Results

A total of 28 patients (11 male and 17 female) were included in this study. The average age was 21 years (range 17 to 34 years). There were no complications such as postoperative malocclusion or open bite. At 1 month after surgery, the proximal segments showed a condylar head with an outward rotation on horizontal view, lateral flaring on coronal view, and anterior rotation on sagittal view. All directional changes had returned to the preoperative state at 1 year postoperative.

The MSP to MLP angular measurement changes for the deviation side showed a 5.3° increase from T0 to T1 and then a 1° decrease from T1 to T2. On the non-deviation side, there was a 6.1° increase at T1 and then a decrease of 0.5° at T2. Neither the deviation nor the non-deviation side showed a statistically significant change in MSP to MLP angulation postoperative.

The MSP to APP angular measurement changes for the deviation side showed a 3.4° reduction (T0–T1), followed by 1° increase (T1–T2), and the non-deviation side showed a 5.3° reduction (T0–T1), followed by 0.4° increase (T1–T2). Neither the deviation nor the non-deviation side showed a statistically significant change in MSP to APP angulation postoperative.

The HOR to MLP angular measurement changes for the deviation side showed a 0.6° increase (T0–T1), followed by a 1.1° reduction (T1–T2), and the non-deviation side showed a 2.7° increase (T0–T1), followed by a 1.1° reduction (T1–T2). Again, there was no statistically significant change in HOR to MLP angulation postoperative on either the deviation side or the non-deviation side ( Tables 1 and 2 ).

Table 1
Angular measurements of the proximal segment in the asymmetry group; mean (standard deviation) values.
Time point a MSP to MLP MSP to APP HOR to MLP
Dev N-Dev P -value Dev N-Dev P -value Dev N-Dev P -value
T0 76.6 (7.0) 76.2 (5.8) 0.798 3.4 (5.7) 7.0 (4.2) 0.008 b 96.5 (6.3) 93.1 (6.3) 0.054
T1 81.9 (7.7) 82.3 (6.5) 0.827 0 (4.9) 1.7 (5.0) 0.195 97.1 (7.0) 95.8 (6.7) 0.485
T2 80.8 (8.1) 81.8 (6.2) 0.616 0.9 (5.0) 2.1 (4.3) 0.334 96.0 (5.9) 94.7 (5.6) 0.415
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Aug 10, 2020 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Three-dimensional changes of proximal segments in facial asymmetry patients after bilateral vertical ramus osteotomy
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