CALCULATION OF VOLUMETRIC MATERIAL LOSS FROM TAPER-TRUNNION JUNCTIONS: DEVELOPMENT AND VALIDATION OF A QUANTITATIVE METHOD

Introduction In total joint replacement devices, material loss from the taper junctions is a clinical concern. Previous studies of explanted orthopedic devices have relied on visual scoring methods to quantify the fretting-corrosion damage on the component interfaces. Previous research has shown that visual fretting-corrosion evaluation is correlated to the volume of material loss [1], but scoring is semi-qualitative and does not provide a quantitative measure of the amount of material removed from the surface. The purpose of this study was to develop and validate a quantitative method for measuring the volume of material lost from the surfaces of explanted devices at the taper-trunnion junction. Methods 10 new exemplar taper adapter sleeves (Ceramtec, Plochingen, Germany) were used for method validation. By using exemplar devices we were able to create clinically realistic taper damage in a controlled and repeatable manner using machining tools. Taper surfaces were measured before and after in vitro material removal using a roundness machine (Talyrond 585, Taylor Hobson, UK). Axial traces were measured on each taper surface using a diamond stylus. The mass of artificially removed material was also measured gravimetrically using a microgram balance (Sartorius, CPA225D, accuracy = ± 0.00003g). Surface profiles were analyzed using a custom MatLab script and Talymap software was used to provide 3D visualizations of the pattern of material loss. Calculated volumetric material loss was compared to the gravimetric value. A sensitivity analysis was conducted to determine the optimum number of traces to characterize the material loss from taper junctions. Results Our calculations of material loss predicted over 99% of the variation in gravimetric material loss (Figure 1, r2 = 0.9962). Examples of the pattern of material removal from explanted components resembled the patterns reported in explants (Figure 2). The sensitivity analysis showed that a minimum of 24 axial profiles are required for measurements to stay within 2% of the volume calculated with 144 traces for cases with an axisymmetric wear pattern. Discussion We have developed and validated a quantitative method for the material loss from taper junctions in orthopedic devices. Our sensitivity analysis showed that a minimum of 24 profiles are required to calculate volumetric material loss accurately, however a further sensitivity analysis is required to establish the minimum number of profiles required to accurately characterize “asymmetric” wear patterns. The measurement of 24 profiles takes approximately 20 minutes. The validation thus far has comprised material loss in an axisymmetric pattern. Work is underway to validate the evaluation of tapers with an asymmetric wear pattern. The axisymmetric and asymmetric patterns are realistic representations of wear patterns seen in explanted taper surfaces. This validated method of estimating material loss from taper junctions will be used in our ongoing research program to understand the mechanisms of fretting-corrosion in retrieved orthopaedic tapers.