Effect of short stem alignment on initial fixation, stress transfer, and failure risk.

Abstract

Short stems are advantageous for revision as they preserve autogenous bone. At present, the method of short-stem installation is determined based on the surgeon's experience. To provide the guideline for installing a short stem, we aimed to investigate the alignment effect on the initial fixation of the stem, stress transfer, and the risk of failure numerically. Models in which the caput-collum-diaphyseal (CCD) angle and flexion angle were hypothetically changed based on the two clinical cases of hip osteoarthritis were analyzed using the non-linear finite element method. The medial settlement of the stem increased in the varus model and decreased in the valgus model. With varus alignment, the stresses acting on the femur were high in the distal to the femoral neck. In contrast, the stresses in the proximal to the femoral neck tend to be higher with valgus alignment, although the difference in the femur stress between varus and valgus alignment was slight. Both initial fixation and stress transmission are lower when the device was placed in the valgus model than in the actual surgical case. In order to obtain initial fixation and suppress stress shielding, it is essential to extend the contact area between the medial portion of the stem and the femur along the bone axis, and to ensure adequate contact between the lateral portion of the stem tip and the femur.