DESCRIPTION OF MANDIBULAR FINITE HELICAL AXIS PATHWAYS IN ASYMPTOMATIC SUBJECTS

Despite wide use of systems to record jaw motion with six degrees of f reedom, most studies have analyzed only the movement of a single mandi bular point. The finite helical axis (FHA) is a mathematical model whi ch can be used to describe comprehensively the movements of a rigid bo dy. The aim of this investigation was to describe the FHA of the mandi ble during habitual jaw movements. Thirty subjects (13 females, 17 mal es; mean age, 26 years; range, 18 to 34 years) without myoarthropathie s of the masticatory system participated in the study. Opening and clo sing movements, performed at l-Hz frequency, were recorded with the op toelectronic system Jaws-3D. Three opening and closing movements were recorded from the right side and three from the left side of the jaw. The movement data were low-pass-filtered for noise reduction prior to the computation of the finite helical axis by means of a software prog ram developed in our laboratory. The following parameters were calcula ted: the rotation of the FHA, its spatial orientation, and the transla tion along it, as well as its position and distance relative to an int racondylar point. In addition, methodological errors of the model were calculated. During opening and closing, the group mean FHA rotation w as 24.3 degrees+4.2 degrees. The group mean of the maximum total trans lation along the FHA was 0.9+/-0.7 mm. The group mean distance between the FHA and the intracondylar point was 48.9+/-9.9 mm. The FHA pathwa ys were smooth and varied between individuals. Furthermore, the finite helical axes were never localized within the condyle, and often were located outside of the mandible. The analysis of the FHA pathways yiel ds more information on whole mandibular movements than simply the move ments of a single condylar point.