A comparative surface topographical analysis of explanted total knee replacement prostheses: Oxidised zirconium vs cobalt chromium femoral components.

It has been proposed that an increased surface roughness of the femoral components of Total Knee Replacements (TKRs) may be a contributing factor to the accelerated wear of the polyethylene (PE) bearing and ultimately prosthesis failure. Oxidised Zirconium was introduced to the orthopaedic market in an attempt to reduce PE wear associated failures and increase the longevity of the prosthesis. In this study, non-contacting profilometry was used to measure the surface roughness of the femoral components of 6 retrieved TKRs (3 Oxidised Zirconium (OxZr) and 3 Cobalt Chromium alloy (CoCr) femoral components) and 2 as-manufactured femoral components (1 OxZr and 1 CoCr). A semi-quantitative method was used to analyse the damage on the retrieved PE components. The Sa values for the retrieved OxZr femoral components (Sa = 0.093 µm ± 0.014) and for the retrieved CoCr femoral components (Sa = 0.065 µm ± 0.005) were significantly greater (p < .05) than the roughness values for the as-manufactured femoral components (OxZr Sa = 0.061 µm ± 0.004 and CoCr Sa = 0.042 µm ± 0.003). No significant difference was seen between the surface roughness parameters of the retrieved OxZr and retrieved CoCr femoral components. There was no difference between the PE component damage scores for the retrieved OxZr TKRs compared to the retrieved CoCr TKRs. These results agree with other studies that both OxZr and CoCr femoral components roughen during time in vivo but the lack of difference between the surface roughness measurements of the two materials is in contrast to previous topographical reports. Further analysis of retrieved OxZr TKRs is recommended so that a fuller appreciation of their benefits and limitations be obtained.

Investigation of Taper Failure in a Contemporary Metal-on-Metal Hip Arthroplasty System Through Examination of Unused and Explanted Prostheses.

BACKGROUND: Large-diameter (≥36-mm) metal-on-metal (MoM) total hip replacements have been shown to fail at an unacceptably high rate. Globally, the DePuy Pinnacle prosthesis was the most widely used device of this type. There is evidence to suggest that one of the main reasons for the poor clinical performance of large-diameter MoM prostheses is the metal debris released from the head-stem taper junction-i.e., taper junction failure. The aim of this study was to investigate variation in the as-manufactured finish of the female taper surface and to determine its influence on material loss. We hypothesized that rougher surfaces with higher relative material peaks would be significantly associated with greater taper wear rates. METHODS: We analyzed 93 Articul/eze femoral head tapers with a 36-mm bearing diameter that had been used in combination with a Corail titanium uncemented stem. The influence of the surface topography of the as-manufactured female taper surface on taper wear was examined by means of a multiple regression model, taking into account other known variables. RESULTS: We identified great variation in the as-manufactured surface finish of the female taper surface, with a range of measured Ra values from 0.14 to 4.20 µm. The roughness of the female taper surface appeared to be the most important variable associated with taper wear (p < 0.001). The best-fitting regression model, including duration in vivo, head offset, reduced peak height (Rpk) value, stem shaft angle, and bearing surface wear rate, explained approximately 44% of the variation in taper wear rates. CONCLUSIONS: We concluded that the roughness of the female taper surface appears to be a significant factor in metal debris release from head-stem taper junctions. CLINICAL RELEVANCE: This study shows evidence that previously unappreciated variations in manufacturing processes may have a major impact on the clinical outcomes of patients.

Determining material loss from the femoral stem trunnion in hip arthroplasty using a coordinate measuring machine.

In contrast to the articulating and taper surfaces of failed total hip replacements, volumetric wear analysis of trunnions is not routinely performed. Metal wear particles from the trunnion may contribute not only to the failures of metal-on-metal total hip replacements but also to all hip replacements utilising metal trunnions. A validation study was performed with the material removed in stages from the trunnions of an Exeter V40 stem, a Corail stem and an Accolade stem to simulate different magnitudes of wear. The material loss from the trunnions was measured both volumetrically with a coordinate measuring machine and gravimetrically with a high-precision balance. A cohort of 28 ex vivo trunnions was also measured using the coordinate measuring machine. The maximum error between the two methods was found to be 0.13 mm(3). This result was comparable with the coordinate measuring machine method for the taper surface (0.2 mm(3)). The ex vivo trunnions had a median wear volume of 0.14mm(3) (range: 0.04-0.28 mm(3)). This is the first study to determine the accuracy of volumetric wear measurements of trunnions by comparing against gravimetric measurements. Volumetric wear analysis of trunnions may provide additional insights into failures of modular total hip prostheses and will be performed routinely at our centre.

Changes in surface topography at the TKA backside articulation following in vivo service: a retrieval analysis.

PURPOSE: With the advent of modular total knee arthroplasty (TKA) systems, backside wear at the articulation between the ultra-high-molecular-weight-polyethylene (UHMWPE) component undersurface and the tibial baseplate has received increasing attention as a source of clinically significant polyethylene wear debris. The aim of this study was to investigate the reciprocating interface at the TKA undersurface articulation using profilometry after in vivo service. Our null hypothesis was that there would be no discernible pattern or relationship between the metal tibial baseplate and UHMWPE surface profile. METHODS: A nanoscale analysis of thirty retrieved fixed-bearing TKA explants was performed. Surface roughness (Sa) and skewness (Ssk) were measured on both the UHMWPE component undersurface and the tibial baseplate of explants using a non-contacting profilometer (1 nm resolution). Four pristine unimplanted components of two different designs (Stryker Kinemax and DePuy PFC) were examined for control purposes. RESULTS: Mean explant baseplate surface roughness was 1.24 µm (0.04-3.01 µm). Mean explant UHMWPE undersurface roughness was 1.16 µm (0.23-2.44 µm). Each explant had an individual roughness pattern with unique baseplate and undersurface UHMWPE surface roughness that was different from, but closely related to, surface topography observed in control implants of the same manufacturer and design. Following in vivo service, UHMWPE undersurface showed changes towards a negative skewness, demonstrating that wear is occurring at the backside interface. CONCLUSION: In vivo loading of the TKA prosthesis leads to measurable changes in surface profile at the backside articulation, which appear to be dependent on several factors including implant design and in vivo duration. These findings are consistent with wear occurring at this surface. Findings of this study would support the use of a polished tibial tray over an unpolished design in total knee arthroplasty with the goal of reducing PE wear by means of providing a smoother backside countersurface for the UHMWPE component.

Retrieval analysis of alumina ceramic-on-ceramic bearing couples.

BACKGROUND AND PURPOSE: Ceramic-on-ceramic (CoC) bearings have been in use in total hip replacement (THR) for more than 40 years, with excellent long-term survivorship. Although there have been several simulator studies describing the performance of these joints, there have only been a few retrieval analyses. The aim of this study was to investigate the wear patterns, the surface properties, and friction and lubrication regimes of explanted first-generation alumina bearings. MATERIALS AND METHODS: We studied 9 explanted CoC bearings from Autophor THRs that were revised for aseptic loosening after a mean of 16 (range 7-19) years. The 3D surface roughness profiles of the femoral heads and acetabular cups (Srms, Sa, and Ssk) were measured to determine the microscopic wear. The bearings were imaged using an atomic-force microscope in contact mode, to produce a topographical map of the surfaces of the femoral heads. Friction tests were performed on the bearing couples to determine the lubrication regime under which they were operating during the walking cycle. The diametral clearances were also measured. RESULTS: 3 femoral heads showed stripe wear and the remaining 6 bearings showed minimal wear. The femoral heads with stripe wear had significantly higher surface roughness than the minimally worn bearings (0.645 vs. 0.289, p = 0.04). High diametral clearances, higher than expected friction, and mixed/boundary lubrication regimes prevailed in these retrieved bearings. INTERPRETATION: Despite the less than ideal tribological factors, these first-generation CoC bearings still showed minimal wear in the long term compared to previous retrieval analyses.

Does surface wettability influence the friction and wear of large-diameter CoCrMo alloy hip resurfacings?

The role of surface tension in the lubrication of metal-on-metal (CoCrMo alloy) hip resurfacings has been investigated to try to explain why all metal joints fail to be lubricated with simple water-based lubricants (sodium carboxymethyl cellulose), which have similar rheology to synovial fluid, but are lubricated with the same fluid with the addition of a proportion of bovine serum. As part of this study, surfactants, in the form of detergents, when added to carboxymethyl cellulose, have been shown to produce a predominantly fluid-film lubrication mechanism with friction even lower than the biological lubricant containing serum. Friction factors were reduced by 80% when a detergent was added to the lubricant. It is considered that the failure of the water-based fluids to generate fluid-film lubrication is due to the fact that 'boundary slip' takes place where the fluid does not fully attach to the bounding solid surfaces as assumed in Reynolds' equation, thereby drawing in less lubricant than predicted from hydrodynamic theory. The addition of surfactants either in the form of natural materials such as serum or in the form of detergent reduces surface tension and helps the water-based lubricant to attach more fully to the bounding surfaces resulting in more fluid entrainment and thicker fluid-film formation. This was confirmed by up to 70% lower wear being found when these joints were lubricated in a detergent solution rather than 25% bovine serum.

In vitro tests of substitute lubricants for wear testing orthopaedic biomaterials.

Bovine serum is the lubricant recommended by several international standards for the wear testing of orthopaedic biomaterials; however, there are issues over its use due to batch variation, degradation, cost and safety. For these reasons, alternative lubricants were investigated. A 50-station Super-CTPOD (circularly translating pin-on-disc) wear test rig was used, which applied multidirectional motion to ultra-high-molecular-weight polyethylene test pins rubbing against cobalt chromium discs. Thirteen possible alternative lubricants were tested. The use of soy protein as a lubricant gave statistically higher wear, while soya oil, olive oil, Channel Island milk, whole milk, whey, wheatgerm oil, 11 mg/mL egg white, albumin/globulin mix and albumin/globulin/chondroitin sulphate mix all gave statistically lower wear than bovine serum. The lubricants giving the closest wear results to bovine serum were 20 and 40 mg/mL egg white solutions. A light absorbance assay found that these egg white solutions suffered from a high degradation rate that increased with increasing protein content. While egg white solutions offer the best alternative lubricant to bovine serum due to the wear volumes produced, cost-effectiveness and safety of handling, protein degradation will still occur, leading to the need for regular lubricant replacement. Of the lubricants tested in this study, none were found to be superior to bovine serum.

Topographical analysis of the femoral components of ex vivo total knee replacements.

With greater numbers of primary knee replacements now performed in younger patients there is a demand for improved performance. Surface roughness of the femoral component has been proposed as a causative mechanism for premature prosthesis failure. Nineteen retrieved total knee replacements were analysed using a non-contacting profilometer to measure the femoral component surface roughness. The Hood technique was used to analyse the wear and surface damage of the matching ultra-high molecular weight polyethylene (UHMWPE) tibial components. All femoral components were shown to be up to 11× rougher after their time in vivo while 95 % showed a change in skewness, further indicating wear. This increase in roughness occurred relatively soon after implantation (within 1 year) and remained unchanged thereafter. Mostly, this roughness was more apparent on the lateral condyle than the medial. This increased femoral surface roughness likely led to damage of the UHMWPE tibial component and increased Hood scores.