Reproducibility of mandibular helical axis parameters

Objective: The helical axis (HA) has been shown to be a complete and compact representation of jaw kinematics. Aim of this study was to inspect reproducibility over time of helical axis pathways of healthy subjects.

Method: Mouth opening/closing cycles of nineteen asymptomatic subjects (fifteen females and four males, mean age 24.9 years) were tracked and analyzed using the HA model. Subjects were randomly divided into two groups: in one the recordings were repeated after 15±9 days, in the other after 134±21 days. Following HA parameters were calculated for each opening and closing phase and for each session: Φmax, ΔΦ: mandibular maximum rotation and rotational excursion around the HA; Tmax and ΔT: mandibular maximum translation and translational excursion along the HA; dCPmax and ∑dCP_i/i: maximum and average of the distance between the HA and a point within the condyle; ∑Θx_i/i, ∑Θy_i/i, ∑Θz_i/i: averages of the angles between the HA and the X, Y and Z axes and ∑Θe_i/i: the mean global fluctuation of the HA spatial orientation. The parameter values were compared between opening and closing phases of one cycle and the two sessions by means of ANOVA for repeated measurements at a significance level of p<0.05.

Result: The parameters had an overall value of 29.1±6.7°, 29.2± 8.0°, 1.4±0.7 mm, 0.7±0.7 mm, 28.9±9.0 mm, 43.3±9.1 mm, 92.6±6.2°, 9.6±3.8°, 90.1±3.0° and 8.9±3.3° respectively and were comparable to the literature. No significant differences were found between the opening and closing phases and the two sessions for all parameters, except for dCPmax, for which the differences were irrelevant.

Conclusion: In healthy and asymptomatic temporomandibular joints, the mandibular helical axis parameters during mouth opening and closing do not vary significantly over a rather short period of time.