The role of passive muscle tensions in a three-dimensional dynamic model of the human jaw

The role of passive muscle tensions in human jaw function are largely unkno wn. It seems reasonable to assume that passive muscle-tension properties ar e optimized for the multiple physiological tasks the jaw performs in vivo. However, the inaccessibility of the jaw muscles is a major obstacle to meas uring their passive tensions, and understanding their effects. Computer mod elling offers an alternative method for doing this. Here, a three-dimension al, dynamic model was used to predict active and passive jaw-muscle tension s during simulated postural rest, jaw opening and chewing. The model includ ed a rigid mandible, two temporomandibular joints, multiple dental bite poi nts, and an artificial food bolus located between the right first molars. I t was driven by 18 Hill-type actuators representing nine pairs of jaw muscl es. All anatomical forms, positions and properties used in the model were b ased on previously published, average values. Two states were stimulated, o ne in which all optimal lengths for the length-tension curves in the closin g muscles were defined as their fibre-component lengths when the incisor te eth were 2 mm apart (S2), and another in which the optimal lengths were set for a 12.0 mm interincisal separation (S12). At rest, the jaw attained 3.6 mm interincisal separation in SZ, and 14.8 mm in S12. Activation of the in ferior lateral pterygoid (ILP) and digastric (DG) muscles in various combin ations always induced passive jaw-closer tensions, and compressive condylar loads. Maximum midline gape (from maximum bilateral co-activation of DG an d ILP) was 16.2 mm in S2, and 32.0 mm in S12. When both model states were d riven with muscle patterns typical for human mastication, recognizable unil ateral and vertical "chopping" chewing cycles were produced. Both states re vealed condylar loading in the opening and closing phases of mastication. D uring unilateral chewing, compressive force on the working-side condyle exc eeded that on the balancing side. In contrast, during the "chopping" cycle, loading on the balancing side was greater than that on the working side. I n S2, chewing was limited in both vertical and lateral directions. These re sults suggest that the assumptions used in S12 more closely approximated hu man behaviour than those in S2. Despite its limitations, modelling appears to provide a useful conceptual framework for developing hypotheses regardin g the role of muscle tensions during human jaw function. (C) 1999 Elsevier Science Ltd. All rights reserved.