Total Hip Replacement – Bone Implant Stresses and Factors Governing Them

The implant material, its shape, size and the method of affects the  of stress transfer to the bone. Implant loosening, and  fracture of the femur or the implant are risks that arise from stress transfers to the bone. At the same time stress also provides stimulus for maintaining bone mass.

Modulus of elasticity of the implant material affects the stress that it would cause. A decrease in the modulus of the stem, stem length and cross sectional area  causes a decrease in the stress  in the stem. But it would increase the stress in the proximal third of the cement mass, which transfers these stresses to the surrounding bone.

An Increase in the modulus of elasticity, the stem length, and the cross-sectional area of the stem increases the stress in the stem, but decreases the stress in the cement and proximal third of the femur.

Stress shielding occurs because the stress that was originally on the bone is borne by the implant after total hip replacement and the bone of the proximal femur is relieved of stress. As noted before a larger size stem would cause more stress shielding.

The area where the stress shielding occurs in greatest amount is the proximal medial cortex. The prosthesis with medial collar theoretically allows  axial loading of the bone in that area and thus reduces stress shielding. But the role of a collar in preventing loosening of a cemented femoral component has not been clearly established

Cementless stems are more physiological as compared to  fully cemented stems, depending on the stem size and the extent of porous coating.

The bone loss after total hip replacement is also dependent upon the bone mineral density of the patient. There is a strong association between the bone mineral density in the opposite femur and the percentage of bone mineral loss in the operated femur. Therefore the  patients with decreased bone mineral density are at higher risk of bone loss after the surgery.

Usually the  bone loss does not often progress after 2 years.

In acetabular side it has been found that cemented polyethylene cup causes development of peak stresses whereas  metal-backed cup with a polyethylene liner reduces the high areas of stress and distributes the stresses more evenly.

The preservation of subchondral bone, the use of a metal-backed cup or thick-walled polyethylene cup decrease the peak stress levels in the trabecular bone of the pelvis.