Analysis of microstructural aspects of a hip stem failure made of the REX 734 stainless steel

In this work we described the microstructure, mechanical and corrosion investigations of two total hip prostheses with cemented stems that were broken after relatively short period of implantation, ~ 3.5 years. The fractured stems were made of the REX 734 austenitic stainless steel, a modified version of the 316 L(V) grade. The REX 734 steel has, in comparison to the 316 L steel, lower content of nickel and higher amount of chromium, nitrogen and niobium. Broken metal endoprosthesis were removed from female patients with body mass index of 27.1 and 29.7. No discontinuity and defects on the prosthesis surface were observed. Radiographs obtained after failure revealed a lack of a cement fixation at the proximal part of the stem. Thus, a probable reason of the failure was a stem loosening at proximal part caused by differences of a tissue density of the proximal and distal part of femoral bone. Comprehensive microstructure observations revealed that in both cases the fatigue fracture was initiated at an anteromedial part of the stems. Failure initiation of both stems could be initiated by large niobium carbides detected in the microstructure of the stems. Despite coarse NbC precipitations, size up to 0.3 μm, the microstructure and corrosion properties of the broken stems made of REX 734 steel fulfil ISO (ASTM) standards.