Volumetric wear assessment of retrieved metal-on-metal hip prostheses and the impact of measurement uncertainty

In hip replacement the use of hard bearing materials, which have been shown to have lower wear rates than traditional metal-UHMWPE couples, has been encouraged in an effort to extend component life-in-service. With a view to further extending this period of joint activity metal-on-metal hip resurfacings have been developed which allow for implantation into younger and more active patients. However, these bearings have recently been the subject of a MHRA device alert due to unexplained hip pain and reported soft tissue reactions and indeed are under investigation having been shown to exhibit high failure rates. This has highlighted the need for a traceable metrological approach to the quantification and characterization of in vivo wear as currently there is no standard dedicated to the measurement of such wear. This study is the first to develop a comprehensive method for measuring the wear geometry of retrieved bearings whilst also assessing and quantifying both the magnitude and effect of the three-dimensional measurement uncertainty on the measurement process as a whole. This study shows that the magnitude of the expanded measurement uncertainty is an important, and until now overlooked, factor and that a statement of uncertainty is vital in measuring retrieved components as a means of comparison between studies and as a measure of confidence in measurement data. Based on the results of this study, it is recommended that care be taken when performing wear measurements on retrieved bearings. Factors such as point pitch and scan line separation can have a great effect on the likely uncertainty of the obtained measurement result and indeed the uncertainty of the measurement can be of the same order as the wear being measured. (C) 2011 Elsevier B.V. All rights reserved.