Method for Characterization of Material Loss from Modular Head-Stem Taper Surfaces of Hip Replacement Devices

Assessment of the head-stem taper junction requires the estimation of material loss from the taper surfaces of both femoral head and stem. This paper describes a method for the measurement and analysis of material loss from the modular taper junction of hip replacements, in particular femoral stem tapers for which the entire taper surface generally has been engaged. In such cases, no direct unengaged datum surface is readily identifiable to assess material loss. The highly anisotropic topology of some stem designs poses additional challenges to the measurement and analysis process. Estimation of material loss of retrieved femoral stems is further complicated by retrieval damage or surface deposits often present on the taper surface. The femoral head tapers typically exhibit areas of pristine surface attributed to the difference in taper length compared with the engaging stem. These areas can be selected as unworn when employed in the analysis process, provided they do not show surface damage or deposits. Measurement of the taper surfaces has been performed using a Talyrond (Ametek, Inc., US) out-of-roundness measurement instrument equipped with a 5-mu m diamond tip stylus. Vertical axial traces were employed to digitize the surface of the taper. Measurement data have been analyzed using a multistage process that has been adapted specifically for stem tapers. The underlying stem taper geometry is determined by means of a morphological filter applied to the high-aspect ratio microstructure. This paper presents a study of 40 retrieved large-head metal-on-metal hip replacements that have been analyzed to ascertain the material loss at the modular taper junction. The purpose of this study was to ascertain the viability of characterizing material loss from the stem taper junction and to provide insight into the overall material loss contribution.