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

Characterizing edge-wear in ceramic-on-ceramic acetabular cups

The use of fourth generation ceramic as an orthopaedic biomaterial has proved to be a very efficient and has gained popularity for primary hip surgery in the last 8-10 years. Cumulative percentage probability of revision after 7 years for uncemented CoC is 3.09% and for hybrid CoC is 2.00%, this compares favourably with traditional metal-on-UHMWPE uncemented at 3.05% and hybrid at 2.35% (12th Annual Report - NJR, 2015). Such ceramic-on-ceramic hip prostheses are being implanted in ever younger, more active patients, and yet very few long-term large cohort retrieval studies are yet to be carried out due to the survivorship of the implants. It has been seen in previous studies that levels of wear in ceramic-on-ceramic bearing surface can be of the order of 0.2 mm3/million cycles (Al-Hajjar, Fisher, Tipper, Williams, & Jennings, 2013). This is incredibly low when compared to studies that characterize wear in other bearing surface combinations. It has also been reported that an unusual stripe pattern of wear can occur in some in-vivo retrieved cups (Macdonald & Bankes, 2014) and it has further been postulated that this is caused by cup edge loading (Walter, Insley, Walter, & Tuke, 2004). The combined measurement challenge of stripe wear occurring at the edge of a low-wear ceramic-on-ceramic device is considerable, a solution to which is presented here. Current literature on wear measurement of such cases has been confined to in-vitro simulator studies and use of gravimetric measurement which by definition has limitations due to the lack of spacial characterisation. This paper details a novel method for measuring edge-wear in CoC acetabular liners. The method has been employed in an in-vitro study where it has been benchmarked against gravimetric measurements. These liners were measured on a CMM to determine the volume of material loss. The measurements were conducted as a blinded post-wear study akin to measurement of retrieved components. The most challenging part of this novel method was to create a reference geometry that replicates the free form edge surface of the ‘unworn’ cup using the residual post-wear surface. This was especially challenging due to the uncontrolled geometry at the cup edge and intersection of geometric features at this point. To achieve this, the geometry surrounding the wear patch was used to create a localized reference feature that minimised the effect of global form errors caused by hand polishing in the edge area. Furthermore, the reference geometry is compared with the measured surface to determine the linear penetration and volumetric wear loss. Result of this novel method can be seen in Fig 1 and Fig 2. The findings have been compared to gravimetric results and a bar graph comparing two results can be seen in Fig 3. Overall the accuracy of the method for this cohort was 0.03-0.2 mm3 when compared to gravimetric reference measurements. This compares very favourably with previously published wear measurement methods and gives confidence in the ability to measure such small measurement volumes over complex geometry

Method for volumetric assessment of edge-wear in ceramic-on-ceramic acetabular liners

This paper details a novel method to characterize and quantify edge wear patterns in ceramic-on-ceramic acetabular liners using a roundness measurement machine to measure the post-wear surface. A 3D surface map is produced which encompasses the measured surface covering the wear patch, the uncontrolled edge geometry and form of the bearing surface. The data is analysed to quantify linear penetration and volume. The developed method was applied in a blind study to a set of six 36 mm ceramic- on-ceramic acetabular cup liners that were measured and analysed to characterise the edge wear. The in-vitro linear wear penetration ranged from 10 μm to 30 μm. The computed volumetric wear results obtained from the blind roundness measurement study were compared against the measured gravimetric results indicating a strong correlation between the results (0.9846). This study has also highlighted that measured liners exhibited an area of localised edge wear locates above the bearing surface as well as a smearing effect on the bearing surface caused by debris from edge wear. A study was carried out to test the repeatability of the measurement method and the inter-operator variability of the analysis. The results of the study show a standard deviation for the entire measurement and analysis process of 0.009 mm3 for first user and 0.003 mm3 for second user over twenty datasets. Hence the method displays high repeatability of the measurement and analysis process between users. This method allows for the delineation of form and wear through the determination of local geometry changes on what is essentially a freeform surface. The edge geometry is only partially controlled from a GD&T perspective and its geometry relative to the bearing surface varies from part-to-part. This method whilst being subjective allows for the determination of wear in this area with a high level of repeatability. However the limitation of this method is that it can only measure 5mm wide band of the liner due to the limited gauge travel range of 2mm