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 un-cemented CoC is 3.09% and for hybrid CoC is 2.00%, this compares favourably with traditional metal-on-UHMWPE un-cemented at 3.05% and hybrid at 2.35% ( 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 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. The findings have been compared to gravimetric results and a bar graph comparing two results can be seen in Fig 2. 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.
Metal-on-metal (MOM) total hip arthroplasty using large diameter femoral heads offer clinical advantages however the failure rates of these hips is unacceptably high. Retrieved hips have a wide range of wear rates of their bearing and taper surfaces and there is no agreement regarding the cause of failure. Detailed visual inspection is the first step in the forensic examination of failed hip components and may help explain the mechanisms of failure. The aim of this study was to determine if there was a correlation between the results of detailed inspections and the volumetric wear of the bearing and taper surfaces of retrieved hips. Detailed, non-destructive macroscopic and stereomicroscopic examinations of 89 retrieved MOM hip components were performed by a single experienced examiner using quantitative assessment to document the severity of 10 established damage features: Light scratches, Moderate scratches, Heavy scratches, Embedded particles, Discolouration, Haziness, Pitting, Visible wear zone, Corrosion, Fretting Each surface was considered in terms of zones comprising of quadrants (cup, head, and taper) and subquadrants (cup and head), Figure 1. Each zone was scored on a scale of 0 to 3 by determining the percentage of the surface area of the zone that exhibited the feature in question: a score of 0=0%, 1<25%, 25%<2<75%, 3>75%. The sum of the scores of each zone was used for the assessment of each damage feature. The volume of wear at the surfaces of each hip was measured with a Zeiss Prismo coordinate measuring machine (cup and head) and a Talyrond 365 roundness measurement instrument (taper), using previously reported methods1, 2. Simple linear regression models were used to asses the univariable associations between the inspection scores and wear volumes. Multiple linear regression models were subsequently used to asses the simultaneous contribution of the inspection scores, found significant in univariable analyses, on the wear outcome variables. All statistical analysis was performed using Stata/IC version 12.1 (StataCorp, USA) and throughout a p value < 0.05 was considered statistically significant.Introduction
Method
The changes in surface roughness occurring during the wear process at the bearing surface are of great importance in trying to understand the failure mechanism of large head metal-on-metal hip replacements. The aim of the study is to identify and characterise the areal surface parameter variation between the worn and unworn areas. Surface topography variations at the bearing surface have an effect on the lubrication regime [1]. In vitro tests of these components have suggested a “self-polishing” of the surface [2]. Traditionally A total of 50 retrieved metal-on-metal hip replacements were assessed using white light scanning interferometry (Talysurf CCI, Ametek, UK) to determine the difference in the areal surface topography parameters between the worn and unworn regions of the bearing surface. The worn area was identified by use of a previously described method [3] to produce a wear map of the bearing surface, this allows the identification of the regions of interest. A series of six measurements were taken on each component (figure 1) comprising of: 2 measurements at the equator of the head representing the unworn region, one measurement at the pole and just off centre from the pole and the rest of the measurements were taken inside and at the boundary of the wear area. Each measurement covered an area of 1 mm2 therefore it is crucial that the location of the measurement be established as accurately as possible. Data was analysed to determine the most relevant parameters that could be used to describe and highlight the changes in surface roughness that occur during the wear process.Background
Methods
Material loss at the head-stem taper junction may contribute to the high early failure rates of stemmed large head metal-on-metal (LH-MOM) hip replacements. We sought to quantify both wear and corrosion and by doing so determine the main mechanism of material loss at the taper. This was a retrospective study of 78 patients having undergone revision of a LH-MOM hip replacement. All relevant clinical data was recorded. Corrosion was assessed using light microscopy and scanning electron microscopy, and graded according to a well-published classification system. We then measured the volumetric wear of the bearing and taper surfaces. Evidence of at least mild taper corrosion was seen in 90% cases, with 46% severely corroded. SEM confirmed the presence of corrosion debris, pits and fretting damage. However, volumetric wear of the taper surfaces was significantly lower than that of the bearing surfaces (p = 0.015). Our study supports corrosion as the predominant mechanism of material loss at the taper junction of LH-MOM hip replacements. Although the volume of material loss is low, the ionic products may be more biologically active compared to the particulate debris arising from the bearing surfaces.
It has been speculated that high wear at the head-stem taper may contribute to the high failure rates reported for stemmed large head metal-on-metal (LH-MOM) hips. In this study of 53 retrieved LH-MOM hip replacements, we sought to determine the relative contributions of the bearing and taper surfaces to the total wear volume. Prior to revision, we recorded the relevant clinical variables, including whole blood cobalt and chromium levels. Volumetric wear of the bearing surfaces was measured using a coordinate measuring machine and of the taper surfaces using a roundness measuring machine. The mean taper wear volume was lower than the combined bearing surface wear volume (p = 0.015). On average the taper contributed 32.9% of the total wear volume, and in only 28% cases was the taper wear volume greater than the bearing surface wear volume. Despite contributing less to the total material loss than the bearing surfaces, the head-stem taper junction remains an important source of implant-derived wear debris. Furthermore, material loss at the taper is likely to involve corrosion and it is possible that the material released may be more biologically active than that from the bearing surface.
