Background Fretting and corrosion at the head-stem taper junction has been cited as a potential clinical concern. Material loss measurement is a vital tool for quantifying changes due to in vitro, in vivo, and/or ex-vivo implant experience. Material loss measurement requires reconstruction of pre-implantation geometry to delineate damage. This is straightforward in principle for plain machined tapers, integrating between a fitted interpolated cone and the measured data. Mathematical filtration methods have been developed to remove this texture and facilitate measurement. The extent to which the assembly process could influence filtration accuracy is not currently known. Methods An engagement/ disengagement study was performed on 27 head/ stem pairs using 2, 4, and 8kN impact loads. Impact was delivered in three locations; axial, 10 o anterior, and 10 o anteroproximal. Pull-off force was measured, and stems were measured pre and post assembly using a Talyrond 365 roundness measurement machine. Results An increase in the plastic deformation of microgrooves and pull-off force was noted with increasing assembly loads. Off-axis impaction resulted in reduced pull-off strength and reduced uniform microgroove distortion. Volumetric change between pre and post assembly was below the noise threshold on stem and head surfaces. Discussion The measurement method was shown to be capable of capturing linear microgroove deformation of 1 µm peak-peak and volume loss above 0.1mm 3. Lower peak load was noted when impacting a seated head when compared with engagement of discrete heads and stems, under the same assembly conditions.
The presence of taper junctions in hip arthroplasty designs has been suggested to contribute to premature clinical failure, in part due to corrosion-assisted material loss. Characterization of the material loss at the taper junction is an important factor in determining not only taper performance but also the requirement for revision of primary stems. It is therefore clear that a robust metrological solution to this issue is required to understand the connection between damage to the stem surface and explicit criteria to initiate retrieval. Previously detailed studies have focused on the characterization of tapers with plain-machined surfaces, with most research concentrated on femoral head tapers. This study details a measurement and analysis method to characterize threaded-type microgroove trunnion surfaces that make up a large proportion of current implanted stem taper designs. Femoral stem designs from two manufacturers were measured using a Talyrond 365 (Ametek, Inc., Berwyn, PA) out-of-roundness measurement machine. Submicron measurement resolution was achieved, and the use of a 5-µm radius diamond stylus meant that any mechanical filtering effect was insignificant compared to conventional contact measurement techniques. The grooved topography of the trunnion surface presents challenges in ascertaining an “unworn” surface from which surface damage data are then separated. A novel filter was developed to delineate form and material loss. The filter is responsive such that the wavelength and amplitude of an individual component’s microgrooves can be removed from the analysis. Application of the filter to leveled data allowed bearing area analysis to be utilized to determine an accurate volume of material loss based on the residual form-removed data. A cohort of as-manufactured samples (n = 21) were measured and used to validate the filtering methodology. A component exhibiting induced surface wear (n = 1) was measured using the same instrumentation and processed using the filtering technique.
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