Microstructure and surface investigations of TiAl6V4 and CoCr28Mo6 orthopaedic femoral stems

Abstract

Total hip arthroplasties (THA) achieve very good clinical results and show annually increasing numbers of implantation. Interactions of the bone with the implants surface are of major importance for a stabile fixation and longevity of the implant. Therefore, manufacturing of the material and the implants as well as their surface properties can have a decisive influence on the functionality of the implant. The aim of the present study is the investigation of two commercially available femur stems with analytical methods. One stem is made of a TiAl6V4 wrought alloy for cementless application and the other one is made of a CoCr28Mo6 cast alloy for cemented fixation. The change of the production-related microstructure within the implant, differences between surface and bulk properties and potential correlations between the production-related changes to predict failures are addressed. Longitudinal cross sections of tested stems were prepared metallographically, investigated using optical and scanning electron microscopy including EDS and EBSD and correlated with micro hardness depth profiles. Due to production and processing, a subsurface layer is formed in both alloys. The TiAl6V4 wrought alloy stem exhibits a homogenous recrystallization microstructure with fine grains of micrometre size. The subsurface layer of the stem is deformed in parts with embedded corundum particles within the depth of 10 µm. Corundum residues were detected on the entire stem surface and partially covered by the applied calcium phosphate spray coating. The CoCr28Mo6 cast alloy stem shows a dendritic microstructure with chromium- and molybdenum-rich interdendritic precipitations and a surface layer of smaller globular grains in the size of 50 µm to 200 µm. The face centred cubic (fcc) crystal structure was predominantly detected in the phase analysis. The brittle hexagonal close packed (hcp) phase was evident at the implants surface. Similar to the TiAl6V4 alloy stem, embedded residues of corundum particles were detected on the implant surface. This study shows different surface integrities for both stems in comparison to the base material. The observed residues from the manufacturing processes are generally well-embedded into the implant surface, however if released they could impair the functionality of the endoprosthesis as the particles might negatively affect the sterilization process or might reduce metal corrosion resistance.