Hip joint biomechanics are quite complex due to pelvic motion associated with it and range of movements it produces.
During normal gait, on heel-strike, the hip moves into 3o degree of flexion and at toe-off [when the foot is finally off the ground] about 10° of extension. The range of abduction to adduction is about 11°, and for internal-external rotation, the range is about 8°.
During different phases of gait cycle, different forces act on femoral head. Approximately two thirds of the hip force is produced by the abductors.
The directions of the resultant force on the joint are important to the function of total hips.
It is useful to consider the forces relative to axes based on the long axis of the femur.
In the coronal plane the forces acting make an angle of 15° to 27° to the long axis of the femur during stance phase of gait which results in axial compression, varus and mediolateral forces. In the saggital plane, anteroposterior forces on the femoral head,result in torsion.
The latter has significant role in the compressive failure of trabecular bone in uncemented stems and resulting in stem fractures.
Femoral offset often influences the mechanics of the hip. Femoral offset id often reduced in normal total hip replacement. This results in an increase in the required abductor force leading to a higher resultant joint force and sometimes a gait abnormality. An increase in offset would reduce the force but causes an increase in the bending moment on the stem.
Forces Acting on the Hip
The body weight can be taken as load applied to a lever arm extending from the body’s center of gravity to the center of the femoral head. The abductor mechanism acts on the lever arm extending from the lateral aspect of the greater trochanter to the center of the femoral head.
This force by abductors must be equal to the load applied by weight to hold the pelvis level when in a one-legged stance and a greater moment to tilt the pelvis to the same side when walking.
The ratio the length of the lever arm of the body weight to that of the abductor musculature is about 2.5 : 1. Therefore the force needed by abductor muscles must approximate 2.5 times the body weight to become equal to the force applied by body weight.
The estimated load on the femoral head in the stance phase of gait is equal to the sum of the forces created by the abductors and the body weight and is at least three times the body weight.
In earlier days of Chrnley, a concept was introduced to alter the length of lever arms. This was done by deepening the acetabulum and by reattaching the osteotomized greater trochanter laterally. this would result in decrease in the moment produced by the body, and the counterbalancing force that the abductor mechanism must exert is decreased.
The abductor lever arm may be shortened in arthritis, supratrochanteric shortening, external rotational deformities, and in many patients with developmental dysplasia of the hip.
In current practice of hip replacement altering the lever arm is not practiced. The stress is on preservation of the subchondral bone and to deepen the pelvis only as much as necessary to obtain bony coverage for the cup. Most total hip procedures are now done without osteotomy of the greater trochanter.
The basic goal is to preserve subchondral bone and to avoid problems related to reattachment of the greater trochanter.
In the saggital plane, the forces act to bend the stem posteriorly and become more pronounced when the hip is flexed. It has been found that the joint reaction force was lower when the hip center was placed in the anatomical location compared with a superior and lateral or posterior position.