Mandibular autorotation in orthognathic surgery: a new method of locating the centre of mandibular rotation and determining its consequence in orthognathic surgery.

Rotation of the mandible is simulated through the centre of the condyle in planning orthognathic surgery. Previous studies have suggested that the initial mandibular movement is better characterized as multiple parts of the segments of a circle with the average centre located below and behind the centre of the condyle. This paper describes a method of locating the centre of mandibular rotation by computer-analysis of two lateral cephalograms with different degrees of opening. The method was used on 10 normal individuals showing an average centre of mandibular rotation 14.9 mm below and 5.0 mm behind the superior midsurface of the condyle for movements from occlusion to an opening of 10 mm. The implications of the located centre of rotation on orthognathic surgery was determined by simulating a closure of a 9.5 mm open bite. An error of up to 9.3 mm in the horizontal position of the maxilla would occur if the centre of the condyle was used. A retrospective study on 10 patients with increased anterior facial height was undertaken and a mean centre of rotation 8.4 mm behind and 25.1 mm below the centre of the condyle was found. Simulating the rotation of the mandible through the centre of the condyle would have resulted in a difference between the predicted and actual horizontal position of the maxilla ranging from 0.4-10.4 mm. It was concluded that using the centre of the condyle as the centre of mandibular rotation in the planning of superior maxillary movement with a Le Fort I osteotomy may cause considerable error in the horizontal position of the maxilla in most cases.