A model study of muscle forces and joint-force direction in normal and dysplastic neonatal hips.

Orthopaedic treatment of congenital hip dysplasia does not always give the desired result. With the present model, prediction of the effects of various treatments on the force direction in the hip joint could help to improve and select treatment (the force direction is presumed to control the collum growth direction). The model contains three-dimensional mathematical descriptions of all muscles passing the hip joint, for various degrees of femoral dysplasia, and for various hip postures. Muscles run straight or curve round some skeletal parts. Muscle forces (all isometric) are calculated from muscle mass, density, pennation angle, mean fibre length, muscle elongation, and assumed activation levels. The latter serve as parameters for optimization. Resting lengths are taken from an assumed fetal posture, and from the observed neonatal posture. Differences between force directions before and after birth, as calculated with the model, agree with collum direction changes described by von Lanz and Mayet (1953).