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 where generally the entire taper surface has been engaged. In such cases no direct unworn datum 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 to 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 5µm diamond tip stylus. Vertical axial traces were employed to digitize the surface of the taper. Measurement data has been analyzed using a multi stage process that has been specifically adapted for stem tapers. The underlying stem taper geometry is determined by means of a morphological filter to remove the high aspect ratio microstructure. This paper presents a study of 40 retrieved LHMoM 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.