Quantifying the bearing surface wear of retrieved hip replacements

Abstract

Accurate quantification of bearing material loss from retrieved metal-on-metal (MOM) hip replacements is key to understanding their failure. Geometric methods are currently the only means of estimating volumetric wear from retrieved implants and numerous contrasting approaches to obtain these measures have been published. Data collection strategies have been thoroughly discussed and refined to minimise the effect of error incurring factors; however, there is an opportunity to optimise the current methods of estimating the pre-wear geometry and, therefore, improve the accuracy of wear volume measurements. An automated analysis strategy to quantify volumetric wear is proposed in this study, which utilises the entire bearing surface to determine the implants pristine geometry. This involves the iterative removal of geometrically effected data points to optimise the fit and size of a perfect sphere. Once fitted, this reference geometry is compared with the measured data, in its entirety, to calculate the volume change representative of the quantity of material lost. Improving the reliability of this parameter could influence the care of a million patients that remain with MOM hip implants. Rigorous validation of this method will dominate future work, ensuring that the accuracy and reliability of this approach are sufficient to provide clinically meaningful data.

Quantifying material loss from the bearing surfaces of retrieved hip replacements: Method validation

“…Currently, a considerable amount of inter-centre variability exists between methods of analysing geometric data, to quantify material loss from the bearing surface of retrieved hip arthroplasties [9]. Specifically, their approach to identifying unworn data points and estimating the as-manufactured geometry.…”

“…As previously described in detail [9], the automated software initially best fits a perfect sphere to a measured point cloud, in its entirety. The RMSE (Root Mean Square Error) between these geometries is then calculated.…”

“…Previous studies have used a range of strategies, commonly user dependent, to approximate this unworn state [5][6][7][8][9]. A localised, pristine region is often the basis of such estimations.…”

“…To overcome these limitations, an automated analysis software was developed and introduced by the current authors in a previous publication [9]. Its implementation of an iterative process allows the systematic exclusion of worn regions, maximising the number of data points that inform extrapolation of the unworn geometry.…”

Does diametrical clearance influence the wear of Pinnacle hip implants?

“…1 , 15 These data were then analyzed utilizing a previously introduced, automated method (RR3D 16 ) to measure diametrical clearance and quantify volumetric wear. 17 In order to accurately quantify material loss from the bearing surface of hip arthroplasties, the as-manufactured geometry of the head and liner components must also be accurately reconstructed in the form of a perfect sphere, as illustrated in Figure 3 . 18 As a result, their original diametrical clearance can be calculated by subtracting the unworn diameters of both components, as shown in Figure 4 .…”