Three-dimensional dynamical capabilities of the human masticatory muscles

Many habitual human jaw movements are non-symmetrical. Generally, it is obs erved that when the lower incisors move to one side the contralateral condy le moves forwards onto the articular eminence, whereas the ipsilateral cond yle stays in the mandibular fossa, moving slightly to the ipsilateral side. These jaw movements are the result of contractions of active masticatory m uscles and guided by the temporomandibular joints, their ligaments and pass ive elastic properties of the muscles. It is not known whether the movement s are primarily dependent on passive guidance, active muscle control or bot h. Therefore, the objective of this study was to analyse the interplay betw een these factors during non-symmetrical jaw movements. A six-degrees-of-freedom dynamical biomechanical model of the human mastica tory system was used. The movements were not restricted to a priori defined joint axes. Jaw movement simulations were performed by unilateral activity of the muscles. The ligaments or the passive elastic properties of the mus cles could be removed during these simulations. Laterodeviations conform to naturally observed ones could be generated by u nilateral muscle contractions. The movement of the lower incisors was hardl y affected by the absence of passive elastic muscle properties or temporoma ndibular ligaments. The latter, however, influenced the movement of the con dyles. The movements could be understood by analysing the combination of fo rces and torques with respect to the centre of gravity of the lower jaw. In addition, the loading of the condyles appeared to be an important determin ant for the movement. This analysis emphasizes that the movements of the ja w are primarily dependent on the orientation of the contributing muscles wi th respect to this centre of gravity and not on the temporomandibular ligam ents or passive elastic muscle properties. (C) 1999 Elsevier Science Ltd. A ll rights reserved.