—Overview— Estimation of Femoral Head Resultant Force with Three-Dimensional Rigid Body Spring Model

Abstract

Since 1981, when this author introduced the rigid body spring model (RBSM), it has been used for contact pressure analyses of the joint. This method has now been expanded three-dimensionally, and in this study it was applied for the estimation of the contraction forces of the muscles around the hip joint, and the resultant force vector of the femoral head. Every muscle is modeled as a spring whose stiffness is proportional to its cross-sectional area. The reaction forces for each spring, under various hip positions and loading conditions, are calculated for related muscle forces. The resultant force vector of the femoral head in the one-leg standing position is calculated as 2.93 times body weight in magnitude, and as 14.5° medially and 10.4° anteriorly in direction. The proximal part of the gluteus maximus muscle, the gluteus minimus muscle, and the gluteus medius muscle, play an important role in hip abduction. Varus osteotomy of 30° at the femoral neck decreases the magnitude of the resultant force vector to 0.8 times body weight, and increases medial inclination to 5° in direction. Co-contact of the hip muscles for shock absorption increases the magnitude of the resultant force, without directional change. In the subluxated hip, the magnitude of the resultant force is increased; however, the medial inclination of the force is also increased, and this reduces the hip instability. This method of resultant force estimation is expected to be useful in the biomechanical evaluation of the pre- and postoperative hip joint, and in facilitating improvements in joint prostheses.