Purpose: The purpose of this study is to investigate the relationship between the occlusal plane and the least motion axis (LMA) that has the minimum range of motion of the mandible in the sagittal plane.

Material and Methods: Forty-five volunteers (24males, 21females, 26.7 ±6.6 yrs) with asymptomatic TMJs participated in this study. An electro-magnetic jaw tracking device was employed to record the sagittal border jaw movement and habitual open-close movement, which started from the ICP, with a sampling rate of 100 Hz. The LMA was estimated from the sagittal border jaw movement using estimating algorithm of our own. In this study, the occlusal plane was defined as the plane pass through three reference points: incisal point (IN) and the central fossa points of right/left first molars (R6, L6) of the upper dentition. The horizontal plane containing the incisor point (IN) and the LMA could be defined as the IN-LMA plane. The closing sagittal angles relative to the occlusal plane (CA_OCC) and those relative to the IN-LMA plane (CA_LMA) were analyzed. The path of habitual open-close movement of the IN and M6 (the central points of R6 and L6) were projected on the sagittal plane, and the CA_OCC and the CA_LMA of the incisor and molar closing paths at 5.0 mm incisal closing level were calculated and compared.

Result: The mean values of the CA_OCC and the CA_LMA of the incisal path (mean±SD) were 79.47±8.40 degrees and 78.97±7.76 degrees, respectively. There was no significant difference between both planes (paired t-test: p > 0.05). The mean values of the CA_OCC and the CA_OCC of the molar path were 75.32±11.92 degrees and 74.82±10.94 degrees, respectively. There was also no significant difference between both planes (paired t-test: p > 0.05). In addition, there was a strong correlation between the CA_OCC and the CA_LMA. The Pearson's correlation coefficient for the incisor path was r=0.807 (p < 0.05), and was r=0.908 (p < 0.05) for the molar path. The slope of regression line was 0.99 for the incisor path, and 0.98 for the molar path. Consequently, the IN-LMA plane nearly corresponded with the upper occlusal plane.

Conclusion: As a result of this study, it was considered that the method using the LMA for quantitatively determining the occlusal plane is an appropriate means anatomically and functionally.