Geometric Variations of Modular Head-Stem Taper Junctions of Total Hip Replacements

Taper degradation in Total Hip Replacements (THR) has been identified as a clinical concern, and the degradation occurring at these interfaces has received increased interest in recent years. Wear and corrosion products produced at the taper junction are associated with adverse local tissue responses, leading to early failure and revision surgery. Retrieval and in-vitro studies have found that variations in taper design affect degradation. However, there is a lack of consistent understanding within the literature of what makes a good taper interface. Previous studies assessed different design variations using their global parameters assuming a perfect cone such as: taper length, cone angle and diameters. This study assessed geometrical variations of as-manufactured head and stem tapers and any local deviations from their geometry. The purpose of this study was to provide a greater insight into possible engagement, a key performance influencing parameter predicted by Morse taper connection theory. This was achieved by taking measurements of twelve different commercially available male tapers and six female tapers using a coordinate measurement machine (CMM). The results suggested that engagement is specific to a particular head-stem couple. This is subject to both their micro-scale deviations, superimposed on their macro-scale differences. Differences in cone angles between female and male tapers from the same manufacturer was found to create a predominately proximal contact. However, distally mismatched couples are present in some metal-on-metal head-stem couples. On a local scale, different deviation patterns were observed from the geometry which appeared to be linked to the manufacturing process. Future work will look at using this measurement methodology to fully characterise an optimal modular taper junction for a THR prosthesis.