The jaw open-close movements predicted by biomechanical modelling.

The aim of this study was to analyse unloaded jaw-opening and jaw-closing movements in humans. For this purpose a dynamical 6-degree-of-freedom mathematical model of the human masticatory system was developed. It incorporated morphology, muscle architecture and dynamical muscle properties. Various symmetrical jaw-opening and jaw-closing movements were simulated based upon different muscle activation schemes. It was found that the balance between swing and slide of the mandibular condyle at the onset of a jaw-opening movement was predominantly dependent on the level of activation of the digastric and inferior lateral pterygoid muscles. The level of activation of the temporalis muscle parts was of critical importance for the jaw-closing movements. The amount of jaw opening was limited by the passive forces of the jaw-closing muscles. In contrast, the influence of the passive forces of the jaw-opening muscles on the jaw-closing movement was neglectable. Throughout the movements the temporomandibular joints remained loaded. The average torques generated by the jaw-opening or jaw-closing muscles with respect to the centre of gravity of the lower jaw had similar orientations and can be considered to be responsible for joint stabilization. The average direction of their lines of action, however, was about opposite, and this can be considered as the major discriminant between a movement in opening or closing direction.