Abstract
1. The hip joint usually acts as the fulcrum of a lever system.
2. The centre of gravity of supported parts, which is usually medial to the hip joint, exerts a rotational effect upon the pelvis. If pelvic position is to be maintained this force must be counterbalanced by hip muscles.
3. The force transmitted by the hip joint is the sum of the supported body weight and the tension in the balancing muscles. This force often exceeds the total weight of the body.
4. In some circumstances the loss of one pound of body weight relieves the hip joint of three pounds pressure.
5. A long femoral neck is an advantage to hip function, but in arthroplasty this must not lead to mechanical failure such as breaking or loosening of the prosthesis, or fracture of the bone.
6. Medial displacement of the femoral head upon the pelvis may cause a great decrease in joint pressure, but medial displacement alone of the shaft upon the head and neck does not influence the mechanics of the joint.
7. The position of the centre of gravity of supported parts is easily altered by slight variations in spinal position. Great changes in hip joint pressure are caused by small coronal spinal movements, but the advantage to man of being able to walk with the eyes steady outweighs the mechanical disadvantage to which his hip is subjected.
8. A femoral abduction osteotomy improves the mechanics of a hip joint deformed in adduction.
9. A walking stick or a crutch is most helpful in relieving joint pressure and reducing the work done by hip muscles.
