Analysis of residual stress in failed T-28 femoral stems

Retrieved stainless steel (Type 316L) T-28 femoral stems occasionally exhibit cracks on the medial surface. Since this region is loaded in compression during gait, a simple fatigue fracture explanation is not sufficient to account for these cracks. It has been suggested that they are related to residual stress. It is known that residual stress can contribute a large part of the strain energy near a crack tip. Four failed T-28 femoral stems have been analyzed after surgical retrieval. All of them had multiple visible cracks on the medial side in the region 2-5 cm distal to the calcar collar. All had failed by transverse fractures connecting with a crack on the medial side. These stems have been examined for residual stress by x-ray diffraction methods; significant levels of residual stress were found to be present in the sections tested. Stresses in the transverse direction (medial to lateral) were found to be strongly tensile near the lateral edge and changed to compressive near the medial edge. The area in the center of the sections was found to have nearly zero residual stress.