Method for characterising and quantifying volumetric edge-wear in ceramic-on-ceramic hip arthroplasty devices

The number of primary total hip arthroplasty (THA) surgeries has been increasing worldwide over the last decades reaching over 200 per 100,000 population with a high degree of variability between countries[1-3]. National Joint registry 2017 report mentions that in the UK during the last decade, 711,765 primary surgeries have been carried out in comparison to 80,042 revision surgeries to replace joints that have failed either prematurely or at the end of their useful life[1]. The life expectancy of a hip prosthesis is commonly expected to be 15-20 years. Of all current commonly used bearing surface combinations,the use of fourth generation ceramic-on-ceramic bearings have proved to be very efficient[4]and has grown in popularity for primary hip surgery in the decadepreceding 2017[1]. This is due to the low reported wear volumes associated with all ceramic bearings [5]as well as the fact that ceramic debris being bio-inert overcomes the commonly reported issues of systemic cobalt chromium ion concentration as reported in metal-on-metal bearings [6, 7]and issues of osteolysis induced by polyethylene wear debris in metal-on-UHMWPE [8, 9]. The interest in ceramic-on-ceramic is elevated also due to significant improvements in material properties and manufacturing process [10]. However, the ceramic-on-ceramic hip prosthesis are reported to squeak in-vivo [11, 12]which appears to be linked to edge-loading [13]. Also, it has been reported that an unusual stripe pattern of wear can occur in some retrieved acetabular cup liners [14]and it has further been postulated that this is caused by cup liner edge loading [15]. The combined measurement challenge of wear occurring at the edge of the acetabular liner of a low-wearing ceramic-on-ceramic prosthesis is therefore considerable. Various wear measurement methods have been developed to measure wear in hip prostheses[12, 16-18], yet current recognised industrial practice regarding in vitro measurement of wear for hip joint prostheses involves either gravimetric assessment or co-ordinate measurement [19]. Due to the considerable challenge in geometrical characterization of edge wear, current literature regarding the assessment of edge-wear in acetabular cup liners has been confined to in-vitro simulator studies and gravimetric measurement to assess wear volumes. Geometric characterisation of wear is essential in determining the contact conditions during gait and subsequent calculation of point and magnitude of the maximum stress condition. It is, therefore, vital that a robust and reliable method for geometric measurement and analysis of edge wear is created. Current methodologies for assessing wear on acetabular cups have focused on quantifying the amount of material loss on the bearing surface. The bearing surface is constituted by well-defined geometry and surface characteristics. As such methods are able to estimatethe unworn surface and determine the amount of material loss. However, the main limitation of these methods was that wear could only be estimated on the well-defined spherical geometry of bearing surface. Hence, when edge wear is present at the boundary between the bearing surface and outer cup geometry it is normally thresholded during analysis process when using current methods. This can potentially underestimate the amount of wear present on acetabular cups. This study provides details of the requirements and methodologies for the measurement and analysis of edge wear in ceramic-on-ceramic liners. Two methodologies have been developed based on measurements using a coordinate measuring machine and a roundness measuring machine. Both methods havebeen evaluated using ceramic liners tested in vitro under edge loading conditions, and the volume loss is compared to gravimetric measurements. The results show that both methods have the required resolution to measure volume loss of less that 1mm3and are thus capable of providing a volume loss estimation for ceramic-on-ceramic acetabular liners.