Abstract
INTRODUCTION
Precise determination of material loss is essential for failure analysis of retrieved hip cups. To determine wear, the measured geometry of the retrieval hast to be compared to its pristine geometry, which usually is not available. There are different approaches to generate reference geometries to approximate the pristine geometry that is commonly assumed as sphere. However, the geometry of press fit cup retrievals might not be spherical due to deformation caused by excessive press-fitting. The effect of three different reference geometries on the determined wear patterns and material loss of pristine and worn uncemented metal-on-metal hip cups was determined.
METHODS
The surfaces of two cups (ASR, DePuy, Leeds; one pristine, one a worn retrieval) were digitized using a coordinate measurement machine (CRYSTA-Apex S574, Mitutoyo; 3 µm accuracy). Both cups were measured undeformed and while being deformed between a clamp. Three different methods for generating reference geometries were investigated (PolyWorks|Inspector 2018, InnovMetric). Method 1: A sphere with the nominal internal cup dimensions was generated. Method 2: A sphere was fitted to the measured data points after removing those from worn areas (deviation > 3 µm is defined as wear) to eliminate the influence of manufacturing tolerances on the nominal diameter. Method 3: Measurements, which displayed visual deformation in the computed wear pattern based on the best fit sphere, were fitted with an ellipsoid. The direction of the deformation axes and the amount of deformation were used to scale the best fit ellipsoid. Linear wear was calculated from the distance of the respective reference geometry to the measured point cloud. Finally, material loss is defined as the difference in volume of the reference geometry and the measured geometry.
RESULTS
The method used for generating the reference geometry affected the determined wear greatly. Using the nominal manufacturing radius (larger than the best fit radius) for the worn cup falsely indicates deposit. This leads to approx. 39 % less wear volume compared to the best fit sphere analysis. Using an ellipsoid as reference geometry for both deformed cups improves the determination of the wear pattern and indicates areas of material loss better than a reference sphere. Additionally, the mistake in material loss determination is decreased, especially for the worn cup almost exactly to the wear volume analyzed with the best fit sphere before deformation.
DISCUSSION
For correct determination of material loss best fit geometries instead of nominal sizes have to be used to compensate the differences due to manufacturing tolerances. Furthermore, deformation always has to be eliminated to generate correct wear patterns and volumes. Using an ellipsoid as reference geometry improves the outcome. For generating an even more accurate reference geometry, the exact behavior of the cup during deformation must be understood. Limitations to this method are cups that do not provide pristine areas in order to generate an appropriate best fit geometry.
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