Influences of Craniofacial Morphology on the Mandibular Rigid Body Motion

Objectives: The inter-incisal distance at the maximal mouth opening (mmo) posture has been widely used to represent the mandibular range of motion. It has been demonstrated that the mandible rotation can explain 65% to 76% of the variation of the mouth opening. Such difference might be due to how to calculate the mandibular rotation. The aim of this study was to analyze the mandible kinematics by the application of rigid-body mechanics. The influences of craniofacial morphology on the mandibular rigid body motions would be especially emphasized. Materials and methods: Twenty-three asymptomatic subjects (11 male, 12 female, mean age 26 years) have participated in this study. A high-precision IR-opto-electronical motion tracking system, Vicon 512 (Oxford Metrics, U.K.) was used to record the mandibular rigid body motions. Camera arrangement as well as marker array placement to represent the head and mandible were configured according to our previous optimization survey. Subjects were asked to perform spontaneous maximal mouth opening/closing movements during recording. The amount of mmo, mandible rotation, and condylar translation were derived directly from the 6-DOF recordings. Linear regression was then used to analyze the relationship among kinematical measurements and craniofacial variables, which were obtained from cephalometric tracing. Results: The mandible rotation alone can explain up to 76.6% of the variance of mouth opening. By adding mandibular length and ramus inclination into the regression model, 15.6% and 1.6% more variances can be explained, respectively. No any measured craniofacial morphological variables, including mandibular plane angle, Gonial angle, ramus inclination, anterior and posterior facial height, etc., are correlated with mandibular rotation. Conclusion: 93.8% of the variances of mouth opening can be explained by mandible rotation, mandibular length and ramus inclination. The amount of mandibular rotation seems not be influenced by craniofacial morphology.