These notes represent a summary of each presentation at the meeting, Metal on- Metal Hip Replacements: Solving The Uncertainties. The meeting was: sponsored by Orthopaedic Research UK; hosted by Alister Hart (orthopaedic surgeon, Imperial College London), Prof Gordon Blunn (Bioengineer at RNOH, University College London) and Prof Julia Shelton (Tribologist, Queen Mary University of London); and held at the Royal College of Surgeons on Wednesday 14 th December 2011. Epidemiology of the problem K.Tucker (&M. Pickford). One of the main messages was that non-asr MOM hip implants were being watched closely by the National Joint Registry. Session 1 Mechanisms of Failure of MOM Hips Contribution of the stem taper to failure. Michael Morlock. Large heads exhibit high friction, especially MOM in adverse situations (dry contact area, eg due to rim loading). Titanium loss from implants is measurable and should be measured in critical cases Appropriate force of impaction of heads onto the taper during THR (~ 4kN) Tapers of different manufacturers are different, don't mix and match The role of corrosion. Anne Neville. Ann clearly defined terms used in the field of corrosion: Electrochemical process: transferring from metallic to ionic species Material loss in hip implants is a combination of wear and corrosion including corrosion of wear particles the contributions of each is not well understood yet Corrosion is to be expected in implants placed in a saline solution with micromotion Conditions in the head-taper crevice are different to the free surface; local acidification occurs Passivity removed by fretting (micro motion) and/or rubbing at the femoral head/cup interface Galvanic corrosion: normally thought to be between different metals, but with 2 metal components of the same type and passivity removed on 1 surface galvanic corrosion can occur Other points: Hip sim: 50% of material loss can be due to corrosion depending on the tribological conditions. The contribution of corrosion changes as a function of conditions and time. Antibiotic in bone cement has an effect on initiation and propagation of crevice corrosion Potential for corrosion is affected by what the component is coupled to 1
Ti, CoCr both rely on passivity for protection importantly what happens when the passive layer is removed by a tribological process? Pseudotumour after MoM hip resurfacing. Richie Gill. Definition of pseudotumour term is a fluid filled or solid inflammatory lesion surrounding a MOM hip, that is not infective. Includes fluid collections. There is evidence linking implant size and design to wear, and that pseudotumours are associated with increased amounts of wear. Amount of wear experienced is greatly influenced by lubrication and edgeloading (primary and secondary, increases wear probably due to disruption of lubrication). Reducing head size (i.e. implanting component smaller than native head) during resurfacing increases the risk of secondary edge-loading and subsequent higher wear. Demonstrated toxicity of sub-micron sized particles containing cobalt to macrophages. Proposed mechanism linking formation of pseudotumours with MOM wear particles of specific size range. Patients with high wear. David Langton. Cobalt should be used, ignore Cr Cobalt correlates v well with volumetric wear High sens and spec for abnormal wear with 5ppb cut off level 10ppb is a 100% specific for abnormal wear >20ppb : all had gross metallosis No patient with adverse tissue reaction and normal wear THR very different to resurfacings: Good correlation between taper wear and cobalt Taper wear contributes more to cobalt Taper debris is different: contains more cobalt 2-5ppb : no clinical decision 20ppb revise Wear measurement of MoM hips. Liam Blunt / Paul Bills. Tactile and coordinate measurement: must be shown to be fully traceable. Therefore can be shown to give validated results (weakness of optical systems) Errors in publications are larger than stated: Validation should target the areas that are potentially weak (and signif): eg. edge wear Taper wear measurement: Vertical traces offer the best measurement practice and reduce measurement error Size of probe for measurement is key: CMM ruby will not trace the troughs of the surface wear scars and may therefore underestimate the wear volumes 2
Session 2 Managing Clinical Problems When to revise a MoM hip. Tony Nargol Cup inclination angle is important but not the only reason for failure Modular versus resurfacing. Ben Bolland. Resurfacing Vs Modular MOM THR (Summary) MOM THR s appear to have a worse outcome than hip resurfacings irrespective of brand with o Higher early revision rate o Higher incidence of Adverse reaction to metal debris o More aggressive and fulminant bone and soft tissue involvement. The BHR is currently the best performing resurfacing device at a national device. Regional series have highlighted: o Men continue to function well. Worst outcome in females with small head sizes. o Lower incidence of ARMD with less aggressive soft tissue and bone involvement at both macroscopic and histological levels Diagnosis (Ref: Southampton series: LHMOMTHR (CPT stem, BHR head/cup) o Factors correlating with higher failure rates in hip resurfacing devices (female sex, cup size and position) do not appear to be as relevant with Modular MOMTHR s. o Metal ion levels in isolation are not specific of failure. A value that corresponds to a high level is yet to be established. o Factors that increase the moment arm / torque through the implant interfaces (higher offset, stem size) are associated with a higher failure rate Mechanism of failure: Retrieval analysis has demonstrated: o Normal wear at the articulating surface with increased wear at the trunnion surface o Corrosion has been evident on the stem in some series. The clinical relevance of chromosomal abnormalities. Tim Briggs. We have previously demonstrated a significant increase in blood and urinary levels of Co and Cr, and chromosomal aberrations in T lymphocytes of whole blood in a group of patients with their original MoM articulation in situ for over 27 years compared to age and sex matched groups. This included structural abnormalities including translocations and breaks. However the chromosomal aberrations that we observed in our MoM group appeared to be reversible when the MoM articulation was revised to MoP. In a prospective randomised study comparing MoM articulation against a MoP bearing there are chromosomal differences in the T lymphocytes between the two groups at two years. 3
No patients in the study had any translocation, gain or break in chromosomes preoperatively. The total number of patients showing any type of chromosomal aberration at two years in the MoP group was 21 (95%) and in the MoM group was 27 (100%). Breaks were significantly higher in the MoM group at 1 and 2-years post-operatively (P = 0.02). No other significant differences were observed between the articulation groups. A mild statistical association between the metal ion levels and chromosome aberrations was detected. At 2 years, the strongest correlation was between cobalt levels and breaks. The clinical consequences of the chromosomal changes seen in this study are unknown. We also do not know if the same changes are present in other cells in the body. One of the hallmarks of malignancy is an increase in chromosomal aberrations. Although similar structural aberrations to the ones observed in this study (translocations and breaks) are known to be associated with certain malignancies, the possible long-term risk of cancer following MoM total hip arthroplasty remains confusing and inconclusive There is no doubt that further prospective studies investigating chromosomal damage in different tissues and in reproductive cells following THA are required. Long term studies are required. The systemic effects or risks of cancer. Patrick Case The long term effects of exposure to cobalt and Cchromium from metal on metal hip replacements are unknown. The levels of exposure are greater when there is adverse reaction to metal debris. The committee of mutagenicity concluded in their 2006 statement The Committee agreed there was good evidence for an association between CoCr-on-CoCr and CoCr or TiAlV on polyethylene (PE) hip replacements and increased genotoxicity in patients. The evidence for the increased genotoxicity observed and the increased blood levels of chromium and cobalt, in patients with Co- Cr-on-Co-Cr hip replacements or Co-Cr on polyethylene hip replacements, gave rise to concern because this may present a potential risk of carcinogenicity in humans. However, it was not possible to make any definite conclusions as to which metal ions, or interactions between metal ions or particulate metals might be responsible for the observed genotoxicity. Measurement of metals in patients with resurfacing implants show a particular increase in the urine. In large epidemiological studies it appears that the overall rate of cancer is not changed in comparison with the general population. However there appears to be a change in distribution. Immediately or shortly after operation the rate of respiratory or colorectal cancer is lower. One possibility is that this might reflect the confounding influences of a relative lack of smoking and the ingestion of non steroidal anti-inflammatory drugs. There does appear to be a late increase of cancer of the bladder, kidney, mouth and of malignant melanoma. If true it remains to be established whether this is related to metal exposure 4
Session 3 Variability in Outcome of MoM hips The role of metallurgy?. Alfons Fischer. In part, the failures of MoM hips are a result of a very shallow understanding of the wear mechanisms in MoM joints as well as their relation to the microstructure. Microstructures of the used HC-CoCrMo alloys depend distinctly on the entire production. In addition these incipient microstructures change markedly under contact stresses into a nanostructured metallo-organic compound consisting of nanocrystals and decomposed proteins called tribomaterial. This contribution depicts the large variety of microstructures from standardized HC- CoCrMo retrievals. These alloys are mainly used because of the supposed wearreducing effect of carbides. The investigated retrievals reveal different types of hard phases being carbides and/or intermetallic phases. Some are < 10 µm and homogenously distributed, while others appear as thin and brittle net-like films at grain boundaries. Despite the high carbon content the gross volumefraction of such hard phases scatters between 0 and 6.4 vol. %. Coarser or non-compact shape types often show microcracks already below the articulating surfaces and, therefore, can increase wear. Complications of hip resurfacing hip replacement. Derek McMinn The evidence in favour of the BHR is strong. In my personal series greater than 1 yr FU 3,205 BHR's 97% survival at 14 years. The Oswestry outcome centre FU of 99 early BHR surgeons patients in 38 countries, 3,036 hips @ min.10 yr FU, 95.7% survival. Australian register BHR 93.7% survival at 10 years. In my LHMoM THR's BHR cups, SS or Ti alloy stems n=383 98% survival at 10 years with no taper failures. Supported by NJR data BHR on S&N THR stem in men ( large size allegedly more prone to taper failure ) n=1,298 96.5% survival at 6+ yrs. BHR in women not so good, my personal series 877 women 94% survival at 14 years. NJR BHR in women 1% failure per year at 6+ yrs BHR on CoCr THR stem not good- Latham series n= 121 72% survival at 8 years- Taper failures. Our analysis of NJR data - flexible parametric adjustment for age, gender, complexity, ASA grade- all OA. In men + women uncemented THR small but significantly higher implant revision rate. However small but significantly higher death rate with cemented THR. In men only adjusted analysis, BHR had markedly lower death rate than uncemented or cemented THR and a revision rate intermediate between uncemented and cemented THR. The variability of wear. Marcus Wimmer. Wear is multifactorial and influenced by the implant manufacturer, surgeon, and patient. Documented risk factors for high MoM wear are found in the bearing design (small head size, small coverage arc, tight tolerances), modularity, impingement (small head/neck ratio), component positioning (high cup inclination and/or 5
anteversion), inappropriate restoration of mechanical axes (e.g. insufficient offset), female gender, and dysplasia as preoperative diagnosis. However, even in a well controlled environment (such as during hip wear testing) highly variable wear rates spanning more than 2 magnitudes are achieved. This highlights the role of the manufacturer and importance of proper implant design. In the past, design goals followed the establishment of full fluid film lubrication. However, this study suggests that the main regime during daily living is boundary or mixed lubrication. Since a direct metal-metal contact yields adhesion and thus severe wear, the importance of a separating tribofilm (or layer) is highlighted. In the case of MoM joints the tribofilm stems from the surrounding pseudosynovial fluid and wear debris. It is partly graphitic, explaining its extraordinary lubricous properties. It is formed through the synergistic interaction of wear and corrosion, however, little is known about its generation and stability under tribological stress. Future design goals should take this into account. 6
Session 4 Interpretation of Clinical Investigations Radiographs. Raghu Raman. 1. plain radiographs are useful for cup position analysis but CT should be considered for version analysis if this is deemed clinically important, this is partly because the large metal heads obscures the cup edges 2. plain radiographs help plan revision surgery, particularly by assessing acetabular bone stock (beware silent osteolysis and overreamed cups) 3. plain radiographs do not pick up the soft tissue destruction that can dramatically increase the surgical difficulty / complication rate 4. plain radiographs should be used in conjunction with other tests such as blood metal ion levels and mars mri MRI. Keshthra Saatchithananda (consultant radiologist). MRI is superior to USS because it allows: 1. comparison of a series of images (past or future) 2. other radiologists to validate findings 3. diagnosis of non-hip related problems (prostatic metastases, ovarian pathology) 4. detailed assessment of the signal return (and therefore function) of the hip abductors. Blood metal ion levels. Stephen A Jones. The interpretation of metal ion levels in patients with a MoM arthroplasty is a new challenge for hip surgeons. Following the MHRA advise and a specific product recall it has become a commonplace investigation. However, limited information exists about its application in clinical practice. It is important that whole blood and serum values measured are not used interchangeability, as this will introduce significant error. In patients with levels of Cobalt and Chromium greater than 7ppb the MHRA has suggested closer surveillance. The MHRA did not recommend metal ion level measurement as a screening test and the level chosen was based on outlier definition from published data. Cardiff data on over 200 MARS MRI cases was correlated with metal ion data. This confirms that metal ion levels in isolation are neither sensitive nor specific and have a poor predictive value for adverse reaction in MoM arthroplasty. Metal ion levels should not a used in isolation for patient assessment. It is recommended that the important factors to consider are patient s symptoms, in particular pain, swelling and limb especially if there has been a change in these. Furthermore, this emphasises that cross-sectional imaging by MARS MRI or Ultrasound is currently the most reliable method of surveillance in particular at risk groups. This provides a direct assessment of biological adverse reaction that ultimately has greatest impact on revision outcome. 7
The Belfast Follow up protocol for the ASR. David Beverland. In Belfast I did 153 ASRs of which 121 were ASR XLs. Of the 32 resurfacings 25 were in male patients with a 100% survivorship at 7 years. In my female ASR resurfacings and all ASR XLs my revision rate is over 30% with a minimum follow-up of almost 6 years. I stopped using the ASR in May 2007 because of a significant increase in pain. However there was no significant difference in Oxford scores. I had a sudden increase in failure rate in December 2008 and so in April 2009 we began to review all of my ASR patients. Every patient was seen by a member of the outcomes team and all had metal ion levels done. Any patient with pain, a cup inclination >50⁰ or metal ions >7ug/l also had an MRI (70 patients at time of first review). After that the ASR was recalled in August 2010. By that time almost 10,000 ASRs had been implanted in the UK of which just over half were ASR XLs. Best estimates are that to date only half of ASR patients in the UK have been reviewed! I have had 17 aseptically loose Corail stems out of 120 (14.2%). In the rest of my series I have done >3,900 Corail stems with a Pinnacle cup with a total of 4 aseptically loose stems = 0.108% - >130 times greater with the ASR! I have had 20 aseptically loose cups out of 121 ASR cups = 16.5%. In the rest of my series I have done 6,700 Duraloc / Pinnacle cups with a total of 5 aseptically loose all 5 of those fell out in the first few days = 0.07% - >235 times greater with ASR! We have also had a number of taper problems. I feel all large diameter M on M heads produce excess torque or toggle which can create problems in 3 locations. 1. Cup bone interface leading to a loose cup. 2. Trunion - taper interface leading to taper wear giving high metal ions with associated soft tissue problems. 3. Stem bone interface leading to a loose stem. Summary of the Belfast Follow up protocol For me 32.2% revision rate @ 5years in time I believe all other surgeons with a lower revision rate will have the same experience with the ASR XL Edge wear is a major factor influenced by cup version and inclination Torque also a major factor with all hard on hard - the taper is a weak link Therefore with hard on hard caution >36mm - avoid large diameter M on M THAs Review all M on M face to face not postal with x-rays and metal ion levels Low threshold for further imaging either USS or MRI 8