Calculation of Muscle and Joint Forces in the Masticatory System

The aim of this study was, among other questions, to disclose the direction and magnitude of the forces acting on the condyles of a mandible.

The masticatory system is highly redundant, i.e., there are 12 muscle and 2 joint forces in contrast to 6 equilibrium conditions. To reveal the occurring force patterns, in 10 healthy male subjects (31 ± 2.3 yrs.) simultaneous EMG- and force measurements have been performed [

1

] using surface as well as intramuscular electrodes and a force measuring device.

With the help of horizontal and frontal MRTs a 3D-model of the masticatory musculature was generated for each test person. On the basis of these data the lines of action of all muscles were calculated using a) the muscles’ geometry data, i.e., the centroids of origin and insertion areas [

2

], and b) FE-models based on an appropriate element which reflects the basic behavior of contractile tissue. The FE-models also take into account the muscle architecture (division into tendon and contractile tissue).

The force law relating the muscle force to the corresponding electrical activity was determined on the basis of vertical clenching tasks performed at different resultant bite force magnitudes. The experimental results can be well approximated by a second order polynomial.

With the knowledge of the lines of action and the force law, the muscle and joint forces can finally be calculated for the tested resultant bite forces.