Condylar Position and Mandibular Rotation Axis in 3D Orthognathic Surgery

Objectives: To offer a simplified method for defining the rotation axis of mandible, and to compare the effect of the condylar position and rotation axis in virtual 3D orthognathic surgery between the class II and III malocclusion patients.
Methods: 3 patients with class II and 3 patients with class III malocclusion were included in this study. The deviation of three dimension (3D) condyle between the 5mm mouth opening position and retruded contact position (RCP) was compared. Subsequently, the 3D mandible in the RCP was rotated around the axis that was defined based on the approximate geometric center of bilateral condyle to open mouth 5 mm, then the deviation between the rotated 3D mandible and the primary 3D mandible in the 5mm mouth opening position was compared.
Results: The results showed that the condylar position changed greatly from small mouth opening (5mm) position to RCP in 3 patients with skeletal class II malocclusion, but exhibited no significant change in 3 patients with skeletal class III malocclusion (maximum positive deviation: 3.27mm vs 0.97mm; maximum negative deviation: -3.24mm vs -0.94mm; mean positive deviation: 1.02mm vs 0.35mm; mean positive deviation: -1.10mm vs -0.30mm). When rotating the 3D mandible from the RCP to open 5mm, the position of 3D mandible exhibited significant difference with the primary scanned 3D mandible that in the small mouth open (5mm) position, and also exhibited no significant change in patients with skeletal class III malocclusion (maximum positive deviation: 5.43mm vs 0.95mm; maximum negative deviation: --6.05mm vs -0.95mm; mean positive deviation: 1.22mm vs 0.28mm; mean positive deviation: -1.47mm vs -0.28 mm).
Conclusions: These findings suggested that adjusting the condylar position from mouth opening to the RCP, a position that is easy to obtain during actual surgery, is necessary before virtual 3D orthognathic surgery, and is especially useful for some patients with Class II relationships. We also offered a simplified method for defining the rotation axis of mandible during virtual 3D orthognathic surgery.