Dynamic simulation of muscle and articular properties during human wide jaw opening

Human mandibular function is determined in part by masticatory muscle tensi ons and morphological restraints within the craniomandibular system. As onl y limited information about their interactions can be obtained in vivo, mat hematical modeling is a useful alternative. It allows simulation of causal relations between structure and function and the demonstration of hypotheti cal events in functional or dysfunctional systems. Here, the external force required to reach maximum jaw gape was determined in five relaxed particip ants, and this information used, with other musculoskeletal data, to constr uct a dynamic, muscle-driven, three-dimensional mathematical model of the c raniomandibular system. The model was programmed to express relations betwe en muscle tensions and articular morphology during wide jaw opening. It was found that a downward force of 5 N could produce wide gape in vivo. When t he model's passive jaw-closing muscle tensions were adjusted to permit this , the jaw's resting posture was lower than that normally observed in alert individuals, and low-level active tone was needed in the closer muscles to maintain a typical rest position. Plausible jaw opening to wide gape was po ssible when activity in the opener muscles increased incrementally over tim e. When the model was altered structurally by decreasing its angles of cond ylar guidance, jaw opening required less activity in these muscles. Plausib le asymmetrical jaw opening occurred with deactivation of the ipsilateral l ateral pterygoid actuator. The model's lateral deviation was limited by pas sive tensions in the ipsilateral medial pterygoid, which forced the jaw to return towards the midline as opening continued. For all motions, the tempo romandibular joint (TMJ) components were maintained in continual apposition and displayed stable pathways despite the absence of constraining ligament s. Compressive TMJ forces were presented in all the cases and increased to maximum at wide gape. Dynamic mathematical modeling appears a useful way to study such events, which as yet are unrecordable in the human craniomandib ular system. (C) 2000 Elsevier Science Ltd. All rights reserved.