Participants
This study involved patients who were treated with mandibular setback surgery using IVRO at the Department of Oral and Maxillofacial Surgery of the Affiliated Stomatology Hospital of Guangzhou Medical University from July 2015 to August 2018. Due to the retrospective nature of this study, it was granted an exemption in writing by the Affiliated Stomatology Hospital of Guangzhou Medical University IRB. Only patients who underwent facial 3D CBCT at the preoperative, 1-week postoperative, and 1-year postoperative stages were included in this study. A total of 27 patients (11 males and 16 females; mean age, 20.68 years; range, 18–33 years) were selected. All patients were free of congenital diseases and syndromes. Patients with a history of orthognathic surgery, cleft lip or palate, craniofacial syndromes, history of trauma, or facial asymmetry (where the midline of menton showed left-right deviation by 3.5 mm or more [6]) were excluded.
Data acquisition
3D facial CT images were obtained with a NewTom scanner (Imaging Science International, Hatfield, PA, Italy) using a 200–400 mm field of view, 120 kVp, and 47.7 mA, resulting in a 0.4-mm voxel size; preoperative, 1-week postoperative, and 1-year postoperative stages were designated as T0, T1, and T2, respectively. ProPlan CMF 3.0 (Materialize, Leuven, Belgium) was used to conduct 3D image reconstruction and 3D analysis of the images. After reconstruction of the 3D skull model, the Frankfort horizontal plane (FH) was used as the horizontal reference plane passing through the bilateral orbitales and right porion. The midsagittal plane was defined as the midline reference plane intersecting the nasion and sella and was perpendicular to the FH. Landmarks in this study (Fig. 1) were designated on the reconstructed 3D surface model and are detailed in Table 1, and these were selected in accordance with the procedure used in a previous study [7, 8]. Reference planes and measurements are presented in Tables 2 and 3, respectively.
Table 1
Landmarks used in this study
Landmarks
|
Description
|
N (Nasion)
|
The midpoint of the frontonasal structure — the intersection of the internasal and frontonasal sutures in the midsagittal plane
|
Se (Sella turcica)
|
The center of the hypophyseal fossa
|
Po (Porion)
|
Highest mid-point of the roof of the external auditory meatus
|
Or (Orbitale)
|
Lowest point of the infraorbital margin of the orbit
|
Cdsup (Condylion_superior)
|
Most superior point of the condyle head
|
Cdlat (Condylion_lateral)
|
Most lateral point of the condyle head
|
Cdpost (Condylion_posterior)
|
Most posterior point of the condyle head
|
Me (Menton)
|
Most inferior point of the symphyseal outline
|
F (Fossa of mandibular foramen)
|
The most inferior point on the fossa of the mandibular foramen
|
Jlat
|
The most lateral and deepest point of the curvature formed at the junction of the mandibular ramus and body
|
Jmed
|
The most medial and deepest point of the curvature formed at the junction of the mandibular ramus and body
|
Gopost (Gonion posterius)
|
The most posterior point on the mandibular angle
|
Gomid (Gonion midpoint)
|
The midpoint between Gopost and Goinf on the mandibular angle
|
Goinf (Gonion inferius)
|
The most inferior point on the mandibular angle
|
Table 2
Reference planes used in this study.
Planes
|
Description
|
FH plane (Frankfort horizontal plane)
|
The plane passing through both sides of the Or and right side of Po
|
MSP plane (Midsagittal reference plane)
|
The Plane through Se and N and perpendicular to FH
|
Ramal plane
|
The plane passing through Gomid, Jlat, and Jmed
|
Table 3
Measurements used in this study.
Measurements
|
Definition
|
Condylar unit length (mm)
|
Distance between Consup and F
|
Angular unit length (mm)
|
Distance between F and Gomid
|
Ramal height (mm)
|
Distance between Consup and Gomid
|
Mandibular body length (mm)
|
Distance between Gomid and Me
|
Frontal ramal inclination (°)
|
Angle formed by Cdlat – Gomid to MSP plane
|
Lateral ramal inclination (°)
|
Angle formed by Cdpost - Gopost to FH plane
|
Ramal volume diff (3×103 mm3)
|
Volume of mandibular ramus
|
Statistical analysis
To avoid investigator-related bias, 10 CT images were randomly selected and measured twice by one investigator in a 10-day interval using intra-class correlation. SPSS software for Windows (version 21.0; IBM Corp., Armonk, NY, USA) was used for statistical analyses. The average value and standard deviation for each measurement were calculated to detect the amount of change over time, and changes between T0 and T1, between T1 and T2, and between T2 and T1 were calculated using paired t tests. Differences were considered significant at P < 0.05.