Dynamic musculoskeletal biomechanics in the human jaw

The high prevalence of functional disorders in the human jaw emphasises the need to understand better its dynamic behaviour. In the present studies, dynamic mathematical models based on typical physical properties of the human jaw and skeletal muscles have been developed. In the first three studies, a model of the entire jaw was created and utilised to predict jaw elasticity and viscosity, and to simulate muscle-driven symmetrical and asvmmetrical jaw movements. Specifically these models were constructed without "ligaments" (temporomandibular capsule or other accessory jaw ligaments) to determine whether or not plausible motion could be simulated in their absence. In the fourth study, a specific model of the temporomandibular ligament and outer wall of the TMJ capsule was created to investigate further these passive structures' perceived role in constraining jaw movement. Variations in their structure have been implicated in jaw hypermobility and joint disorders. In the fifth study, a specific model of the masseter muscle was developed and consisted of six contiguous muscle compartments in which muscle fibre orientations and lengths differed. The functional implications of this complex internal structure was investigated when the model was stretched in opening and lateral jaw movements. In the first three studies, the jaw models required low elasticity and heavy damping to match in vivo conditions, and consequently low levels of muscle activity (tone) were needed to maintain a clinical jaw rest position. In the absence of "ligamentous" constraints about the jaw, plausible symmetric, and asymmetric movements were possible. In study four, putative capsular regions about the TMJ were suggested to remain taut throughout an entire ipsilateral jaw movement, however for contralateral, opening and protrusive jaw movements, the capsule remained slack during the middle third of the movement. In study five, although theoretically capable of generating high passive tensions, the masseter muscle model generated low overall passive tension to stretch. Dynamic modelling suggests hypothetical relationships between the jaw's structural and functional variables. Jaw muscle coactivation generates varied movements, and together with passive tensions affords a degree of jaw stability. Muscle and "ligaments" probably cooperate to constrain jaw movement, although muscle appears predominant in the midrange of jaw movements. The internal structure of the masseter muscle may allow maximum movement with minimum passive tension generation.Medicine, Faculty ofGraduat