Material Loss at the Head Taper Junction of the Metal-on-Metal Pinnacle Total Hip Replacement

Introduction The ASR XL (DePuy) total hip replacement (THR) is a notable example of a modern metal-on-metal (MOM) implant design that has demonstrated unacceptable survival rates, leading to its recall by the manufacturer; national joint registries have reported revision rates at 7 years of 40% when paired with the Corail stem [1]. The ASR XL THR has a considerably greater risk of revision than the ASR resurfacing hip, which used the same bearing design. This suggests that material loss at the head-stem junction may be responsible for the greater percentage of THR failures observed in this design. The Pinnacle MOM-THR (DePuy) however used the same Corail stem as the ASR XL THR but demonstrated better clinical results, with revision rates of less than 10% at 7 years [1]. The ASR XL and MOM Pinnacle are two designs that have been widely used in hip replacement surgery. The reasons for the differences in the failure rates of the two designs are not fully understood. Comparing the mechanisms of failure of both hips will help surgeons understand whether patients with MOM Pinnacle hips will experience the same types of problems as with those seen with the ASR XL. The aims of this retrieval study were to investigate the significance of differences between the ASR XL and MOM Pinnacle in relation to: (1) pre-revision whole blood Co/Cr ratios, (2) visual evidence of taper corrosion, (3) volumetric material loss at the bearing surfaces and (4) volumetric material loss at the taper surfaces. Methods This study involved a series of failed MOM hips consisting of the ASR XL (n=30) and Pinnacle (n=30), all that had been used with a Corail stem. The bearing material in each design was cobalt-chromium and the Corail stem is of a cementless titanium 12/14 design. The ASR XL and Pinnacle had a median head diameter of 47mm (39-55) and 36mm (36-40) respectively, and a median time to revision of 38.5 months (12-74) and 55 months (14-86) respectively. Pre-revision whole blood metal ion levels were collected for each Table 1 summarises patient and implant data for the hips in this study. The female taper surfaces of all 60 heads were examined macroscopically and microscopically to assess the severity of corrosion. Each surface was graded with a score of between 1 (no corrosion) and 4 (severe corrosion) using a well-published scoring system, which has been shown to be statistically reliable. A Zeiss Prismo (Carl Zeiss Ltd, Rugby, UK) coordinate measuring machine (CMM) was used to determine the volume of material loss at the cup and head bearing surfaces. Up to 300,000 data points were collected using a 2mm ruby stylus that was translated along 400 polar scan lines on the surface. The raw data was used to map regions of material loss by comparing with the unworn geometry of the bearing. A Talyrond 365 (Hobson, Leicester, UK) roundness measuring machine was used to measure the volumetric material loss at each of the head taper surfaces. Published protocols were used to take a series of 180 vertical traces along the taper surface using a 5μm diamond stylus; worn and unworn regions were mapped and used to calculate material loss. Neither the volumetric measurement data nor corrosion scores were normally distributed. Therefore non-parametric tests were performed to assess the statistical significance of differences between the two designs in relation to the parameters under investigation in this study. Results Both the whole blood Co ion levels and the Co/Cr ratios, Figure 1, of the ASR XL hips were significantly greater than the Pinnacles (p<0.05). There was no significant difference between the whole blood Cr ion levels between the two designs (p=0.0542). 18 of the ASR XL hips presented evidence of edge wearing of the cup, compared with 14 Pinnacle hips; this difference was not significant (p=0.438). The length of the stem trunnion contact engagement length with the taper was approximated as being 10.5mm for both designs. The median time to revision of the ASR XL hips was significantly less than the Pinnacle hips (p<0.01). There was visual evidence of corrosion in 93% (n=28) and 90% (n=27) of head tapers for the ASR XLs and Pinnacles respectively. Moderate to severe corrosion was observed in 67% (n=20) of ASR XLs compared to 60% (n=18) of Pinnacles. There was however no statistically significant difference between the scores of the two groups (p=0.927). Figure 2 presents the distribution of material loss rates for the bearing and taper surfaces of the two designs in this study. The median total bearing surface (combined cup and head) rate of material loss for the ASR XL and Pinnacle hips was 4.45mm3/year (0.32-22.85) and 4.03mm3/year (0.87-62.12) respectively. There was no significant difference between the two groups (p=0.928). The median material loss rate at the taper surfaces of the ASR XL and Pinnacle hips was 0.62mm3/year (0-4.20) and 0.30mm3/year (0-3.12); this difference was not significant (p=0.198). Discussion The work of this study presents comparisons of retrieval findings between the ASR XL and Pinnacle MOM-THRs; these hip designs were two of the most commonly implanted in patients worldwide. The significantly greater whole blood Co/Cr ratios found in the ASR XL group compared to the Pinnacle group are of interest. It is speculated that a Co/Cr ratio of greater than 1 may be an indicator of corrosion of an implant whereby more Cr ions are retained on the surface, whilst comparatively more Co ions are released into the blood. In the current study we found wear rates at the bearing surfaces of both designs to be comparable, suggesting that the significantly greater Co/Cr ratios in the ASR XL hips must be due to greater corrosion at the taper junction than the Pinnacles. Although the ASR XL hips had been implanted for a significantly shorter period of time, our visual assessment of the corrosion of the taper junctions found that corrosion scores were comparable between the two designs; indeed, a marginally greater number of ASR XL tapers had evidence of moderate to severe corrosion. This finding, coupled with the elevated Co/Cr ratios suggests that the ASR XL design is more susceptible to corrosion at the taper junction than the Pinnacle hip. We found that the median rate of material loss at the ASR XL taper was over twice that of the Pinnacle taper. Whilst not statistically significant, this difference may be due to a greater risk of corrosion at this interface in the ASR XL design. The differences in material loss and corrosion that were observed at the taper junctions may be explained by considering the larger head sizes of the ASR XL hips in comparison to the Pinnacles. It has previously been shown that increasing head size is correlated with greater visual evidence of corrosion and that increased frictional torque along the taper junction due a larger head diameter can increase the risk of fretting-corrosion. It is suggested therefore that the combination of the larger head sizes of the ASR XLs coupled with the comparatively short, rough surface of the Corail trunnion results in a cumulative effect leading to greater corrosion at the taper junction. Significance The results of the study suggest that the combination of (1) increased frictional torque in the larger ASR XLs and (2) the rough Corail trunnion surface, results in greater corrosion at the taper junction in comparison to the Pinnacle hips; this helps to explain the higher risk of revision in this hip design

Identifying The Pattern of Material Loss at the Head-Neck Junction Wear Helps Determine the Mechanism of Failure of Metal on Metal Total Hip Replacements

Material loss at the Head-Neck junction accounts for a third of the total volume material loss in contemporary metal-on-metal total hip replacements. It is speculated that the material loss is the result of corrosion and mechanical wear (fretting). High volumes of material loss have been reported, especially from the head taper. There is only one report on characterizing the pattern of material loss and this was in a very small number of cases (n=5). Our aim was to identify the different material loss patterns at the head taper and their corresponding mechanisms We retrospectively analysed a series of retrieved Large Head Metal on Metal Total Hip Replacements (155 cups, 155 femoral heads and 4 stems). We measured material loss on the bearing surfaces and the head-neck junction using well-published metrology methods. Furthermore we collected patient (age, gender and time of primary/revision operations), pre-revision (cobalt and chromium blood metal ion, oxford hip score, cup orientation and implant position) implant (cup and head size, manufacturer and corrosion severity) data. Finally we used surface analysis techniques (microscopy and spectroscopy) to identify fretting, imprinting and the material composition of debris. We devised a novel four-group classification and two blinded engineers classified the material loss patterns using wear maps derived from the metrology analysis We observed four distinct patterns of taper surface material loss at our retrieval centre and we set out to characterize these types and relate them to patient, implant and clinical variables. The four groups of material loss patterns were defined as: (1) Low wear (n= 63), (2) Open-end band (n=32), (3) Stripped material loss (n=54) and (4) Coup-Countercoup (n=6) (Figure). The Interobserver Reliability Kappa score was 0.78 (p<0.001) indicating substantial agreement between the two examiners. Analysis of variables between the groups identified significantly different head sizes (highest: Group 2, p=0.000), corrosion severity (highest: Group 2, p=0.004) and time to revision (highest: Group 3, p=0.040). We identified four different material loss patterns each with its own mechanism. Corrosion was identified as the principal mechanism in Groups 1 and 3. Group 1 head-neck junctions are thought to have a better seal with less fluid ingress in the junction. Group 3 head-neck junctions are attacked by corrosion either circumferentially, or unilaterally, along the whole engagement length. Mechanically assisted corrosion was the principal mechanism in Group 2. The higher friction torque opens up the open-end part of the junction and the ingressing fluid accelerates the corrosion. Extensive fretting was also observed under the scanning electron microscope. Intra-operative surgical damage was identified as the principal mechanism in Group 4, with only 6 components. The patterns and the mechanisms of material loss at the head-neck junction contribute to the understanding of large head metal-on-metal hip replacements. As a result, better implants can be designed in the future. Clinically, these findings suggest that head size and head taper-trunnion fit are the main factors that determine the longevity of the head-neck junction. On the other hand, patients selection does not influence the integrity of the junction

36mm Metal-on-Metal Hips have Similar Taper Material Loss Rates as Larger Diameter Hips from the Same Manufacturer

Introduction: There is a two-fold difference in failure rate between the two most commonly used metal-on-metal (MOM) bearing types in the US. We compared these two bearing types; one with a 36mm diameter with a modular cup and one with a diameter >36mm with a monoblock cup. Methods: This was a retrospective study involving 60 retrieved LD-MOM-THR hips of 2 different cobalt-chromium bearing designs (n=30 in each group) from a single manufacturer that had been paired with a single cementless titanium 12/14 stem design from the same manufacturer (Table 1). One group consisted of a monoblock cup whilst the other had a modular cup design with separate metal shell and liner components. Paired t-tests revealed that the two groups were statistically matched in relation to patient age, gender and time to revision. We used a well-published scoring method to visually assess the severity of corrosion at each head taper surface on a scale of 1 (none) to 4 (severe). We then used a roundness-measuring machine to measure the volume of material loss at the taper surfaces. The statistical significance of any differences between the two hip designs in relation to corrosion scores and material loss were then evaluated. Results: We found that 93% (n=28 in each group) of the tapers showed evidence of corrosion, however there was no significant difference between the two groups (p=0.61). The monoblock cup group had a median taper material loss rate of 0.397 mm3/year (0-4.198) and the material loss of the modular cup group was 0.216 mm3/year (0-3.117). There was no significant difference between the two groups (p=0.132). Discussion and Conclusion: We found corrosion and material loss rates for the tapers of 36mm hips to be comparable to larger diameter hips from the same manufacturer. This supports the classification of 36mm hips as large diameter

Clinical relevance of corrosion patterns attributed to inflammatory cell-induced corrosion: A retrieval study

In vitro studies have shown that human osteoclasts can corrode stainless steel and titanium leading to the production of metal ions responsible for inflammatory reactions. Moreover, traces of cellular activities on metal orthopaedic explants have recently been reported as inflammatory cell-induced (ICI) corrosion being the result of the cells sealing on the metal surfaces and releasing reactive oxygen species (ROS) through Fenton-like reactions. The extent and clinical relevance of this phenomenon has yet to be understood. We analysed a cohort of 100 CoCr alloy hips collected at our retrieval centre; we performed macroscopic and microscopic screening and used statistical analysis to correlate our findings with implant and clinical variables. We found that 59% of our implants had evidence of surface damage consistent with what has previously been described as cell-induced corrosion. There was a significant association between the patterns and aseptic loosening for the ASR modular (r = -0.488, p = 0.016) and the Durom modular (r = 0.454, p = 0.026). This is the largest implant retrieval study to examine the phenomena of so-called ICI corrosion and is the first to investigate its clinical relevance. We recommend further work to determine the role of cells in the damage patterns observed. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2015

A novel closed-loop electromechanical stimulator to enhance osseointegration with immediate loading of dental implant restorations

Abstract: The degree of osseomechanical integration of dental implants is acutely sensitive to their mechanical environment. Bone, both as a tissue and structure, adapts its mass and architecture in response to loading conditions. Therefore, application of predefined controlled loads may be considered as a treatment option to promote early maturation of bone/implant interface prior to or in conjunction with crown/prosthesis attachment. Although many studies have established that the magnitude, rate of the applied strain, and frequency have significant effects on the osteogenic response, the actual specific relationships between strain parameters and frequency have not yet been fully defined. The purpose of this study was to develop a stimulator to apply defined mechanical stimuli to individual dental implants in vivo immediately after implantation, exploring the hypothesis that immediate controlled loading could enhance implant integration. An electromechanical device was developed, based on load values obtained using a two-dimensional finite element analysis of the bone/implant interface generating 1000 to 4000 me and operated at 30 and 3 Hz respectively. The device was then tested in a cadaveric pig mandible, and periosteal bone surface strains were recorded for potential future comparison with a three-dimensional finite element model to determine loading regimens to optimize interface strains and iterate the device for clinical use

Understanding Why Dual-Taper Hips Fail

Our multi-disciplinary team of surgeons and engineers will present the mechanisms of failure of 100 retrieved dual taper hip arthroplasties, identifying surgical, implant and patient risk factor

A novel ceramic coating for reduced metal ion release in metal-on-metal hip surgery

Abstract: An ovine total hip arthroplasty model was developed to evaluate metal ion release, wear, the biological response and adverse tissue reaction to metal-on-metal (MoM) bearing materials. The performance of an advanced superlattice ceramic coating (SLC) was evaluated as a bearing surface and experimental groups divided into; (1) MoM articulating surfaces coated with a SLC coating (SLC-MoM), (2) uncoated MoM surfaces (MoM), and (3) metal on polyethylene (MoP) surfaces. Implants remained in vivo for 13 months and blood chromium (Cr) and cobalt (Co) metal ion levels were measured pre and postoperatively. Synovial tissue was graded using an ALVAL scoring system. When compared with the MoM group, sheep with SLC-MoM implants showed significantly lower levels of chromium and cobalt metal ions within blood over the 13-month period. Evidence of gray tissue staining was observed in the synovium of implants in the MOM group. A significantly lower ALVAL score was measured in the SLCMoM group (3.88) when compared with MoM components (6.67) (p = 0.010). ALVAL results showed no significant difference when SLC-MOM components were compared to MoP (5.25). This model was able to distinguish wear and the effect of released debris between different bearing combinations and demonstrated the effect of a SLC coating when applied onto the bearing surface