Postoperative stability of maxillo-mandibular complex in facial asymmetry : a three-dimensional computed tomographic study

Abstract

Until now, most of the studies on postoperative stability have been concerned with assessing changes during the first year after surgery. However, unlike the SSRO (Sagittal Split Ramus Osteotomy) in which proximal and distal segments are fixed through rigid fixation, the mandible’s position is determined by occlusion, muscle, and the ligament’s postoperative physiologic adaptation in the case of IVRO (Intraoral Vertical Ramus Osteotomy) and bone remodeling can occur gradually over a long period of time due to the discontinuity of the overlapping of both segments. Therefore, studying skeletal stability through observation for a period of one year is insufficient.

Furthermore, in order to assess the improvement of asymmetry, it is important to accurately compare and analyze skeletal changes in the gonial angle on the right and left, and if surgery was performed with the use of the IVRO, it is necessary to observe changing aspects for a sufficient period during bone remodeling with respect to the gonial angle.

This study was carried out to assess skeletal changes and to examine stability over a long period after Le Fort I osteotomy for maxilla and IVRO for the mandible was carried out on 16 patients with Class III skeletal malocclusion and facial asymmetry. For this, photographs of three-dimensional computed tomography were taken directly before surgery, one year after surgery, and about four years after surgery. Then the images were reconstructed, and the maxilla and mandible were classified in terms of deviation and non-deviation with respect to the direction of menton deviation. Then the changes in length and angle were analyzed, and postoperative three-dimensional changes in maxilla and mandible were analyzed by using a direction vector on the maxilla and mandible planes. Accordingly, the following results were obtained.

1. As a result of comparing the side were there was deviation and the side were there was no deviation one year after surgery, it was verified that in all measurement the values did not show any statistically significant differences (p>0.05), so the asymmetry on the deviation and non-deviation sides was resolved through a surgery.

2. When comparing the pre-surgery condition and the condition one year after surgery, on the side were there was deviation, there were statistically significant decreases (p<0.05) in the frontal facial line angle, frontal ramal line angle, sagittal ramal length, lateral Go vertical distance, inferior Go vertical distance, and U6 vertical distance; and on the non-deviation side, there were statistically significant increases (P<0.05) in the sagittal ramal line angle, sagittal Mx. occlusal line angle, frontal facial line angle, frontal Mn. body line angle, sagittal Mn. body length, and lateral Go horizontal distance. And there were statistically significant decreases (p<0.05) in the sagittal ramal length, lateral Go vertical distance, inferior Go vertical distance, and U6 vertical distance.

With regard to the changes for the period from one year after surgery to four years after surgery, on the side were there was deviation, there were statistically significant increases (P<0.05) in the sagittal Mx. occlusal line angle, inferior Go vertical distance; and on the non-deviation side there were statistically significant increases (P<0.05) in the inferior Go vertical distance and U6 vertical distance.

3. As a result of analyzing the three-dimensional skeletal changes in maxilla and mandible one year after surgery and comparing these with the condition before surgery, in the case of the maxilla, there were statistically significant differences (p<0.05) in pitch; and in the case of the mandible, there were statistically significant differences (p<0.05) in yaw, roll and pitch. But, four years after surgery, there were no statistically significant differences revealed with regard to the yaw, roll and pitch in the maxilla and mandible.

In accordance with the above results, it was verified that for a patient with Class III skeletal asymmetry, the asymmetry of the maxilla and mandible was improved after surgery, and the results were maintained for up to four years without much change except for bone remodeling in inferior gonial area.