With the exception of congenital anomalies, facial asymmetry is regarded as being caused by mandibular growth abnormalities in many cases. Conventionally, two-dimensional analysis using cephalometric X-rays is carried out to design the plan for orthognathic surgery. To improve the treatment outcome, however, it may be necessary to elucidate the factors responsible for asymmetry by performing three-dimensional analysis of the craniofacial morphology.
The objective of the present study was to perform three-dimensional measurements of the mandible using three-dimensional computed tomography (3D-CT) in patients with facial asymmetry requiring orthognathic surgery, and to identify the morphological characteristics of the mandible by comparing the deviated and non-deviated sides.
3D-CT images of 36 patients with facial asymmetry (≥ 4mm deviation of the mandibular midline) were analyzed using 3D planning software for orthognathic surgery. A total of 10 items were measured, including five angle measurements and five linear measurements with reference to the entire mandible, the mandibular body, or the mandibular ramus.
The deviated and non-deviated sides were measured, and values were compared using the paired Student's t-test. Multi-regression analysis (step-wise selection) was also performed to evaluate the deviation of the mandibular midline and the items measured.
In patients with facial asymmetry, a comparatively large mandibular ramus, mandibular body, and condylar process length were observed on the non-deviated side compared with the deviated side, with the mandibular ramus rotated anteromedially along the superior-inferior, left-right, and anterior-posterior axes. These differences between the deviated and non-deviated sides in the morphological characteristics of the mandible resulted in mandibular deviation and rotation. However, no left-right differences were observed in mandibular body height, mandibular body roll, or gonial angle. Our findings also suggested that deviation of the mandibular midline was significantly affected by mandibular ramus length, mandibular body length, and mandibular ramus pitch.
