Hip Joint Reaction Forces

During gait, a stride will take the hip through an average of 40° to 50° of motion (30°-40° of flexion and 5°-10° of extension) [19,46]. The force from weight bearing in the acetabulum during gait is biphasic with peaks in force occurring at heel strike and toe off. Areas of contact form two columns of force on the anterior and posterior rims, joining together in the superior aspect of the fossa [47]. As more force is applied to the hip, the areas enlarge as the femoral head settles deeper in the acetabulum. The areas of most frequent weight bearing are also associated with the stiffest and thickest articular cartilage [47].

The result of the forces transferred across the hip can be visualized radio-graphically in the femoral neck as Ward's triangle [48]. This triangle is outlined by cortical and tensile trabecular osseous formations in the femoral neck. Tensile forces are generated in the medial subtrochanteric cortex and applied into the weightbearing portion of the femoral head [48]. Cortical forces span from the foveal area of the femoral head through the superior femoral neck to the subtrochanteric cortex [48]. In hips with a neck shaft angle of greater than 125° (coxa valga), compressive trabeculae are more prominent due to the increased compressive forces accounted for by the deformity of the femur. In hips with a neck shaft angle of less than 125° (coxa varus), tensile trabeculae are more prominent due to the increased tensile stresses [49].

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