Abstract
Introduction
Hip resurfacing has become a popular procedure for young active patients with osteonecrosis of the femoral head. However, it is not yet clear exactly how much osteonecrosis would permit this procedure and how much would be a contraindication. The aim of the present study was to analyze the resurfaced femoral head using finite element models and, in particular, to examine the influence of the extent of osteonecrosis and metaphysical stem shaft angles within the femoral head.
Methods
We evaluated biomechanical changes at various extents of necrosis and implant alignments, using the finite element analysis method. We established three patterns of necrosis by depth from the surface of femoral head and five stem angles. Extension of necrosis as a quarter of femoral head diameter is type A, from a half is type B, and three-fourths is type C. We set five types of different stem angles from 125 to 145 degrees for the axis of femoral shaft. For these models, we evaluated biomechanical changes associated with the extent of necrosis and the stem alignment.
Results
Stress distribution near the bone-cement interface increased with expansion of the necrosis. The stress concentration was particularly evident in the Type C model which had wide cement mantle replacement on the resurfaced femoral head. The maximum stress on the prosthesis was decreased with stem angles ranging from 130 to 140 degrees. Stress concentration at the stem shaft was observed when implanted varus or valgus alignment. Stress concentration of cement was observed at the anterior and inferior edges in each model. Regardless of the extent of necrosis replaced by cement, the stress values at the inferior edge increased as the stem shaft angle became varus.
Conclusion
This study suggests that hip resurfacing for patients in whom osteonecrosis extends widely should be considered carefully; increased stress concentration near the bone-cement interface may occur when all the necrotic bone is replaced by cement. Furthermore, excessive varus or valgus implantation of the prosthesis has potentially adverse biomechanical effects for implants and the cement mantle.
