A Novel Method for Measuring Polyethylene Hip Liner Wear Using a Coordinate Measuring Machine and a Dual Stylus Probe

Ex vivo high-resolution measurement of highly crosslinked (HXL) polyethylene hip liner wear is necessary to characterize the in vivo performance of these polymers that exhibit increased wear resistance. Current studies focus on using a coordinate measuring machine (CMM) to acquire data representing the bearing surface( s) of HXL hip liners and use this data to determine linear and volumetric wear. However, these current techniques are subject to error in both data acquisition and data analysis. The purpose of this study was to identify these sources of error and present a novel method for HXL wear measurement that minimizes these contributions to error: our novel methods use a CMM to measure both the articular and backside surfaces of HXL hip liners for subsequent data analysis in Geomagic Control and MATLAB. Our method involves a vertical orientation of the hip liner to enable one CMM scan of both sides of the hip liner. This method minimizes identified sources of error and proves to be an effective approach for data acquisition of HXL hip liner wear. We also find that our data analysis technique of calculating changes in wall thicknesses is effective in accounting for errors associated with data analysis. Validation of this technique occurred via measurement of two never-implanted HXL hip liners of different sizes (28 mm and 32 mm). In comparing the 32 mm hip liner to its corresponding computer-aided design (CAD) model, we found that our data acquisition technique led to a 0.0019 mm discrepancy between the scanned liner and its CAD model in measured thickness at the pole. We calculated 0.0588 mm and 0.0800 of linear wear for the 28 mm and 32 mm hip liners, respectively, based on our data analysis algorithm. We hypothesize that these reported linear wear values of the never-implanted hip liners are due to machining tolerances of the hip liners themselves.