Patients
The protocol for this prospective study was approved by the hospital’s Institutional Ethics Committee. The inclusion criteria were as follows: (1) having an age of 18 years or older, (2) having skeletal class III deformity (i.e., A point–Nasion–B point angle ≤ 0°) and significant facial asymmetry (i.e., menton deviation ≥ 4 mm or lip cant ≥ 2 mm or significant contour asymmetry), (3) receiving Le Fort I osteotomy and BSSO setback through a surgery-first approach from the same team of surgeons, and (4) receiving postsurgical orthodontic treatment by one senior orthodontist. The study was conducted over a 3-year period. The attending surgeons were supervised by one senior surgeon who had more than 40 years of experience at the Chang Gung Craniofacial Center. The exclusion criteria were as follows: (1) having a history of craniofacial surgery, craniofacial anomaly, cleft, or genetic syndrome or (2) having a history of facial bone fracture.
Surgical Procedures
Surgery was performed under hypotensive general anesthesia in accordance with the maxilla-first sequence. The Le Fort I osteotomy was performed using a technique similar to that described by Bell.18 The technique for cutting and separating the bone was similar to an ordinary Le Fort I osteotomy except for the separation of the pterygomaxillary junction. An oscillating saw was used to make the initial cut, and the bony separation was then performed using a pterygomaxillary suture chisel.19 The maxillary segment and mandible were wired to the intermediate splint to form the maxillomandibular complex, which moved in accordance with the bilateral condyles and guided the position of the maxilla. Once the bony interferences had been removed and the maxilla properly positioned, the maxillary segments were fixed using four-hole L plates and four screws for each maxillary buttress. The jaws were then unwired to examine the occlusion before the intermediate splint was removed. The BSSO approach was modified from that of Hunsuck by extending the anterior cut of the osteotomy to the first molar and including the mandibular angle within the proximal segment.20,21 The pterygomasseteric sling was detached. The maxillary segment and distal segment were wired to the final splint to form the maxillomandibular complex, which guided the position of the distal segment. The bilateral medial cortex of the proximal segment was trimmed to reduce the bony interference between the proximal and distal segments. Once the proximal and distal segments had been properly positioned, a pair of 2-hole monocortical plates and screws were used to fix each sagittal split osteotomy of the mandible (Fig. 1). The jaws were then unwired to examine the occlusion before the final splint was removed. No additional surgery other than genioplasty or mandibular contouring was performed. Genioplasty was performed to improve the patient’s profile, proportion, or symmetry. Mandibular contouring was performed to improve their contour symmetry. No intraoral interarch elastics were used after surgery. After surgery, a liquid diet was generally prescribed in the first 2 weeks and a soft diet the following 2 weeks. Thereafter, a regular diet was permitted.
Cone-beam Computed Tomography
Cone-beam computed tomography (CBCT) of the head and neck was performed before treatment (T0), 1 week after surgery (T1), and after treatment (i.e., at the appointment for orthodontic debonding, at least 1 year after surgery; T2) by using an i-CAT 3D Dental Imaging System (Imaging Sciences International, Hatfield, PA, USA) with the following parameters: 120 kVp, a voxel size of 0.4 mm × 0.4 mm × 0.4 mm, a 40-s scan time, and a field of view of 20 cm × 20 cm. The patient’s head was positioned with the Frankfort horizontal plane parallel to the ground. The patient was instructed not to swallow during the scan and to maintain a centric occlusion bite.
Images were stored in the Digital Imaging and Communications in Medicine format and then transferred to a workstation (Avizo v7.0.0, VSG, Bordeaux, France) where they were rendered into volumetric images and segmented and analyzed by one investigator who was experienced in 3D analysis. Before the analysis, the 3D images were reoriented as follows: (1) the midsagittal plane (MSP) passed through the nasion, posterior nasal spine, and basion; (2) the axial plane was perpendicular to the MSP, parallel to the clear side of the porion and orbitale and passing through the nasion; and (3) the coronal plane was perpendicular to the MSP and axial plane and passed through the nasion. The 3D images were then reflected along the MSP until all the mentons were deviated to the left side (i.e., the deviated side). The cranial structures not affected by the surgery were used to superimpose (i.e., register) CBCT images taken at T0, T1, and T2 to position them at the same 3D coordinates (x, y, z) with the nasion as the origin. A positive value for the x, y, and z coordinates indicates the left, posterior, and superior side of the face, respectively.
Surgical And Postsurgical Movement Of Maxillary And Mandibular Segments
The position of the segments was determined using three landmarks obtained from CBCT images. Definitions and descriptions of the reference landmarks are presented in Table 1. These landmarks were located on each segment and formed the vertices of a virtual triangle, which contained information on the 3D position and orientation for each segment at all three time points (T0, T1, and T2).
Table 1
Definition of reference landmarks for virtual triangles
Landmark | Symbol | Definition |
Incisive foramen | IF | The most posterior midpoint of the incisive foramen |
Greater palatine foramen, deviated or opposite | GPF-d GPF-o | The most lateroposterior point of the greater palatine foramen on the deviated or opposite side |
Genial tubercle, posterior | GT | The most posterior midpoint of the genial tubercle |
Mental foramen, deviated or opposite | MF-d MF-o | The most anteroinferior point of the mental foramen on the deviated (d) or opposite (o) side |
Sigmoid notch, deviated or opposite | SN-d SN-o | The inferior midpoint of the upper border concavity of the ramus on the deviated or opposite side |
Anterior ramus, deviated or opposite | AR-d AR-o | The most anterior midpoint of the anterior border concavity of the ramus on the deviated or opposite side |
Posterior ramus, deviated or opposite | PR-d PR-o | The most posterior midpoint of the posterior border of the ramus on the deviated or opposite side |
The position of the maxillary segment was determined using the incisive foramen (IF), deviated greater palatine foramen, and opposite greater palatine foramen (Fig. 2, upper left). Surgical and postsurgical movements of the maxillary segment were assessed by calculating the changes in translations and rotations of the virtual triangles through the IF from T0 to T1 and from T1 to T2.
The mandibular position of the mandibular distal segment and mandibular proximal segments (deviated [d] and opposite [o]) was determined. The position of the mandibular distal segment was determined using the landmarks of the genial tubercle (GT), deviated mental foramen (MF-d), and opposite mental foramen (MF-o), which were identified on each mandibular distal segment (Fig. 2, upper right). Surgical and postsurgical movement of the distal segment was assessed by calculating the changes in translations and rotations of the virtual triangles through the GT. The position of the mandibular proximal segments was determined using each segment’s respective deviated or opposite landmarks for the sigmoid notch (SN-d/SN-o), anterior ramus (AR-d/AR-o), and posterior ramus (PR-d/PR-o), which were located on each mandibular proximal deviated segment (Fig. 2, lower left) or proximal opposite segment (Fig. 2, lower right). Surgical and postsurgical movement of the proximal segments was assessed by calculating the changes in translations and rotations of the virtual triangles through the SN-d and SN-o, respectively, for the deviated and opposite sides.
Movement in translation was left–right (along the x-axis; more to the left: +, more to the right: −); posterior–anterior (along the y-axis; more posteriorly: +, more anteriorly: −); and up–down (along the z-axis; more cranially: +, more caudally: −). Movement in the rotation was pitch (around the x-axis; clockwise rotation: +, counterclockwise rotation: −), roll (around the y-axis; clockwise rotation: +, counterclockwise rotation: −), and yaw (around the z-axis; counterclockwise rotation: +, clockwise rotation: −; Fig. 3).
Reliability
To assess intraexaminer error, the investigator performed CBCT measurements twice on 10 randomly selected patients, with the gap between the two measurements for each patient being 2 weeks. Intraexaminer reliability, evaluated using the intraclass correlation coefficient (ICC), was excellent (mean ICC = 0.996, 95% confidence interval: 0.966 to 0.999).
Statistical analysis
Statistical analyses were performed using the statistical software package SPSS version 23.0 for Windows (SPSS, Chicago, IL, USA). The Kolmogorov–Smirnov test was used to verify the normality of the data distributions. Descriptive statistics are expressed as the mean ± standard deviation (SD) for metric variables and as the frequency and percentage for nominal variables. The one-sample t test was used to test the movement during surgery (T0–T1) and after surgery (T1–T2, relapse). Stepwise linear regression analysis was performed to determine predictors of the mandibular stability of one given segment by considering the movement during surgery of all segments and postsurgical stability of all other segments as independent variables. To account for multiple comparisons, probabilities of less than 0.01 were considered significant.