Condylar movement and mandibular rotation during jaw opening

Inaccurate description of mandibular rotation can have profound effects on orthognathic surgical treatment planning and surgical outcome, as well as affect the precision of appliances fabricated on articulators. Disagreement exists concerning movements of the condyle during jaw opening. Although mandibular function is often described as rotation around an instantaneous center located outside of the condyle, many believe that jaw opening occurs around an axis of rotation that remains fixed at the center of the condylar head. In this study, condylar movements and centers of mandibular rotation during jaw opening were examined in normal subjects with the Dolphin Sonic Digitizing System. All of the subjects demonstrated both translation and rotation of the condyle during initiation of jaw opening, and none had a center of mandibular rotation located at the condylar head. The findings support the theory of a constantly moving, instantaneous center of jaw rotation during opening that is different in every person. There were also differences in movement within the subjects between experimental trials. The uncertainty of predicting mandibular rotation for a given patient should be considered when planning surgical treatment and fabricating orthodontic appliances.