Stress distribution in the proximal femur after surface replacement: effects of prosthesis and surgical techniques.

The increasing number of reported femoral neck fractures following surface replacement indicates a potentially serious problem in this procedure. A comparison of the stress levels in the proximal femur before and after hip surface replacement was made using the finite element technique. A proposed surgical bone graft procedure utilizing a strut of cortical bone placed through the femoral head into the neck to help support the surface replacement was also studied as an adjunct for use in avascular necrosis. The findings indicate that stress magnitudes and distributions in the proximal femur are markedly altered after surface replacement when compared to a femur without a prosthesis. Stress levels in the femoral head with surface replacement were found to be significantly lowered suggesting future problems with stress shielding. Lateral neck stresses were found to be equal to or greater than stress levels in the femur without a prosthesis. Stress orientation in the femoral neck is also markedly altered. Surface replacement design and recommended surgical positioning were important variables in determining stress levels. Femoral neck stresses were found to be significantly lowered for all designs of surface replacement when the bone graft modification was utilized. The results of this study explain the major mechanisms of failure of the femoral component. The proposed bone graft modification is demonstrated by the finite element model to decrease the possibility of femoral neck fracture.