Effect of Uncertainty on Wear Measurement of Metal-on-metal Total Hip Replacement Components

Introduction: Recent concerns over high failure rates in some metal-on-metal hip replacement bearings have highlighted the need for a standardised approach to wear analysis of such components at retrieval.
There is a need for a traceable metrological approach to the quantification and characterisation of in vivo wear as currently there are a number of approaches that have been adopted in literature and industry with a greater or lesser reference paid to good and reliable measurement practice.

Method: This paper shows the development of a comprehensive method for measuring the wear geometry of retrieved bearings. This is achieved using currently recognised co-ordinate metrology methods. Assessment and quantification of both the magnitude and effect of the three-dimensional measurement uncertainty on the measurement process as a whole is presented. The developed method is fully in accordance with established international standards.

Results and Discussion: Through initial measurements and simulated data, it has been shown that even for a CMM with a low maximum permissible error, the task-related volumetric uncertainty associated with measurement of spherical hip surfaces can be of the order of 1-2mm3. It is further shown in the data presented that uncontrolled measurement variables have a deleterious effect on the validity of the measurement result thus increasing the measurement uncertainty. With the precision required for accurate quantification of wear in new generation metal-on-metal bearing surfaces the magnitude of this uncertainty is significant.

Conclusion: An unambiguous 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. It is suggested that great care be taken when performing wear measurements on retrieved bearings.