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UBC Theses and Dissertations

Three dimensional computer modeling of human mandibular biomechanics Nelson, Gregory J.


Previous analyses of mandibular biomechanics have incorporated a wide variety of approaches and variables in attempts at describing the relationships between the forces generated by the muscle and the forces of resistance at the dentition and temporomandibular joints. The most difficult element to determine in man has been the role of the joint forces which require indirect analyses. A critical literature review points out the problems associated with previous analyses of mandibular mechanics and predictions of joint loading and the need for the incorporation of all relevant anatomical and physiological parameters in order to realistically quantify these relationships. A computerized mathematical model of human mandibular biomechanics for static functions is presented which allows the determination of forces occurring at the dentition and the joints due to the individual muscle force contributions. Utilizing the principles of static equilibrium the model provides for the determination of these forces for any individual for whom the necessary input parameters have been derived. Anatomically, this model requires the designation of the three dimensional coordinates of the origin and insertion points of nine pairs of masticatory muscles, any position of tooth contact, and the temporomandibular joint positions. Determination of the forces generated by the individual muscle groups, and therefore the overall muscle force resultant acting on the system, is given by the product of a number of physiological parameters. These include the physiological cross-section, the intrinsic force per unit of cross-sectional area, and the relative activation level of each muscle for the specific static function. Also required is the three dimensional orientation of tooth resistance force at the designated position of tooth contact, as well as that of the left joint force in the frontal plane. This information reduces the variables in the equilibrium equations to a determinate number which has a single unique solution for each of the tooth and two joint resistance forces. The magnitudes as well as three dimensional orientations of the resultant vectors of the muscles, the tooth resistance force and the two temporomandibular joints are thereby determined mathematically. Both bilaterally symmetrical and unilateral clenching functions as well as three intervals near the intercuspal position of chewing were tested with this model using data derived from literature sources from real subjects. This data was incorporated into a hypothetical average individual data file. Using this data, derivation of the magnitudes and orientations of muscle and tooth forces were made providing predictions as to the nature of temporomandibular joint loading for this individual. The extent of muscle force generated for static maximal clenching tasks modeled was a maximum of 1000 to 1200 N during intercuspal clenching. The orientation of muscle force with respect to the occlusal plane varied from about 90 degrees in the lateral plane, for more posterior molar functions, to 64 degrees for incisal functions. Maximal tooth resistance forces were around 500 to 600 N at the molars versus only 130 to 140 N at the incisors. Unilateral functions showed the working side joint to be more heavily loaded than the balancing side especially for a more posterior function (i.e. molar). Less muscle and therefore tooth force was produced unilaterally but with the benefit of even less residual joint force. Thus, unilateral functions appear to be much more efficient in terms of the distribution of forces between the dentition and joints. Variation in tooth orientation produced variations in both the orientation and magnitudes of the joint forces exhibiting a functional interrelationship of these forces. Based on the analysis in general, the joints were predicted to be capable of resisting up to 300 N of force per side directed anterosuperiorly at about 60 to 100 degrees in the lateral plane. More divergent forces at the joints were found to be of substantially lower magnitude in the lateral and frontal planes. These findings are in good agreement with other studies.

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