Inclination of the occlusal plane is associated with the direction of the masticatory movement path

Using lateral cephalograms and a jaw movement-recording system, the relationship between the masticatory movement path and dentofacial morphology was investigated in 17 subjects (9 males and 8 females, mean age 23.5 years) without a history of orthodontic treatment. The masticatory movement path was measured at the right and left lower first molar while the subjects chewed gum. The angle between the Frankfort horizontal plane and the masticatory axis (FH-masticatory angle), defined as the axis passing the opening and closing turning point on the sagittal masticatory path, was also measured. The correlation between the angular measurements derived from the lateral cephalogram and the FH-masticatory angle was then investigated. A positive correlation was observed in the FH-masticatory, occlusal plane (P < 0.05), and mandibular plane (P < 0.01) angles. Furthermore, it was found that the angle between the masticatory axis and the occlusal plane (69.1 +/- 4.2 degrees) remained constant even as the masticatory axis showed a tendency to incline forward as the mandibular plane angle became steeper; the rates of change of the FH-masticatory and the occlusal plane angles were approximately 1:1. This finding suggests that the masticatory movement path is closely associated with the occlusal plane.