Quantitative calculations of temporomandibular joint reaction forces--II. The importance of the direction of the jaw muscle forces.

The effect of error in determination of muscle force direction on quantitative calculation of the temporomandibular joint reaction force was investigated in a two-dimensional, two-muscle model. A computer program using the model incremented the direction of the temporalis and masseter muscle force vectors and generated a family of curves showing the relationship between muscle force direction and the direction and magnitude of the temporomandibular joint reaction force. The major effect of muscle force direction is its influence on the length of the muscle force moment arm. The analyses show that calculation of the joint reaction force is much more sensitive to errors in muscle force direction than to muscle force magnitude. Calculation of the magnitude of the joint reaction force is less sensitive to error than the calculation of joint reaction force direction. An error of 1 degree in muscle force direction could produce a 2 degrees error in joint force direction. No ideal location of the muscle force vectors was found. Vector positions that enhanced calculation of joint force magnitude increased the sensitivity to errors when calculating joint force direction. The difficulties of precisely determining muscle force directions are discussed. The present results indicate that a two-muscle model requires very precise determination of muscle force directions to reliably calculate the temporomandibular joint reaction forces.