Uncertainty evaluation of volumetric wear assessment from coordinate measurements of ceramic hip joint prostheses

Replaced hip joints are normally expected to last 15-20 years. However, although the procedure is nowadays successfully performed, up to 10% of procedures carried out are revision operations to replace joints, which have failed prematurely, mainly in case of polymeric or metallic bearing surfaces. In fact, particulate debris, generated by wear of the bearing surfaces, are "attacked" by the immune system which results in the death of surrounding bone tissue which in turn leads to loosening of the joint and pain to the patient. All-ceramic hip replacements were introduced to eliminate this mechanism and reduce wear. The most common method to evaluate wear volumes in hip joints is the gravimetric method, which has been proven to be straightforward and accurate. However, this method can only be applied during in vitro testing of new hip prostheses and not to assess retrieved components. Furthermore, even in case of in vitro testing, it can globally quantify the worn material, while it cannot provide local information on the worn surface geometry. Coordinate measuring machines (CMMs) can be used to evaluate both wear volume and wear distribution over the worn surface. However, the uncertainty of wear volume measurements from CMM data has not been thoroughly quantified yet. This work makes a step further, establishing an approach for uncertainty analysis and quantification. The paper focuses on evaluation of uncertainty components that affect wear volume measurements. In particular, nine ceramic femoral heads were analysed with both gravimetric and CMM methods in order to assess the uncertainty of wear volume measurements and compare these two methodologies. The results of this investigation showed that wear volume measurements from CMM data are in good agreement with gravimetric measurements, taking into account measurement uncertainty of both methods according to the "Guide to the expression of uncertainty in measurement (GUM)" and derived documents. Furthermore, the study uses the uncertainty analysis to optimize the probing strategy and suggests improvements to ISO 14242-2:2000