Comparative assessment of implantable hip devices with different bearing surfaces: systematic appraisal of evidence
BMJ 2011; 343 doi: https://doi.org/10.1136/bmj.d7434 (Published 29 November 2011) Cite this as: BMJ 2011;343:d7434
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An important article for all aspiring surgeons showing new is not necessarily better. I do hope the Daily Tabloids will also pick up on this so the number of patients requesting particular implants or bearing surfaces reduces. Surely the most important part of hip arthroplasty surgery is not the bearing surface but the surgeon who performs it. Most importantly surgeons with high volume and low infection rates.
It is well known that there is a difficult learning curve with hip resurfacing. Making conclusions based on registry data is important particularly with respect to post implantation surveillance, however, they include both good and bad surgeons and their results. These implants should not be thrown away but limited to specific centres of excellence, they are certainly not for the occasional surgeon.
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A few people report that they have developed electrosensitivity from metal replacement joints. Many of the symptoms of cobalt poisoning, associated in some cases with metallic implants, are also common to the non-specific symptoms of electromagnetic allergies. These include tinnitus, vertigo, headaches, cognitive problems, irritability, fatigue, depression, hypothyroidism, peripheral neuropathy and cardiomyopathy. Cobalt is ferromagnetic and thus patients with cobaltism moving in an environmental electric field can presumably induce internal electric currents. These would be in addition to electrical induction via the metallic implant itself, an issue already identified with some dental amalgams.
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My clinic’s experience with metal-metal hips is emblematic of the difficulties surgeons have in evaluating the risks and benefits of new arthroprosthetic technology. We assume that a regulatory pre-market approval process and a post market monitoring process insure the efficacy and safety of newer devices.
Our initial metal-metal hip experience was favorable. We falsely attributed this success to the metal-metal bearing. Several years into our experience an uncharacteristically high percentage of our patients had progressive unresolved or new hip pain. Some of these patients had gross metal debris about the hip with tissue damage. Two of these patients had remarkably high cobalt levels, cognitive decline, cranial neuropathy and early cardiomyopathy. These cases of “Arthroprosthetic Cobaltism” were published.(Tower 2010)
We have just identified our fourth case of arthroprosthetic cobaltism. A pre-revision serum cobalt of > 200 mcg/L (normal is < 1) was found by the patients primary provider during a work up for progressive central nervous system, cardiac and constitutional symptoms. An elevated TSH was also found. The patient’s metal-metal hip had been in for about two years and over one year hip pain and noise were noted.
Patients with metal-metal hips may present to their primary provider with combinations of symptoms related to the prosthetic hip, nervous system, heart, and thyroid gland.
Tower, S. S. (2010). "Arthroprosthetic cobaltism: neurological and cardiac manifestations in two patients with metal-on-metal arthroplasty: a case report." The Journal of bone and joint surgery. American volume 92(17): 2847-2851.
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I must commend the authors on the daunting task of taking multiple papers from various sources using different methodology and levels of evidence and compiling a meaningful data set and results with conclusions that affect the daily practice of joint surgeons. As a joint replacement surgeon, review of the literature often results in confusion as to what is "best practice" and the data of the various types of alternative joint bearing surfaces often claiming superiority with cost of implants having become a major issue with the alternative bearings often costing significantly more than the "tried and true" metal-poly bearing. These implants may have a place for certain patients of high weight and/or activity levels who are young where metal-poly has not fared well in the past.
This paper clearly shows no superiority in functional outcomes of the alternative bearings, however, when looked at on a larger scale. Although the metal-metal bearings significantly scored slightly lower than metal-poly, the difference is clinically meaningless. Clearly the advantage of lower dislocation rates with the metal-metal bearings which often use large heads does not offset the higher revision rates and lack of clinical superiority to metal-poly. Indeed, with the use of high crosslinked poly many surgeons have opted for larger heads to decrease the dislocation rate since the wear characteristics of modern poly bearings are proving to be quite good.
Further study of alternative bearings is required on a large scale to draw definitive conclusions and although metal-poly bearings are not perfect and have their own set of limitations, it appears for the time being that metal-poly still sets the standard by which all other bearing surfaces must be measured against. Metal-metal, ceramic-ceramic and ceramic-poly have shown little or no improvement in performance with concomittant increase in cost and potential complications that bears further scrutiny before they can be considered a standard.
Metal-poly has proven to be relatively durable, more forgiving in implant positioning and therefore somewhat less technique dependent, with failure modes that are well known, studied and with evolution of the current polyethylene technologies, minimized compared to historical controls. Enough said.
Competing interests: No competing interests
Re: Comparative assessment of implantable hip devices with different bearing surfaces: systematic appraisal of evidence
We thank Dr. Alexiades for a response to our paper titled ‘Comparative assessment of implantable hip devices with different bearing surfaces: systematic appraisal of evidence’(1) The author certainly appreciates the power of evidence review and combining the evidence to address comparative effectiveness and safety of alternative vs ‘tried and true’ or ‘traditional’ bearings. We wanted to note that we focused our evidence appraisal on convention hip replacement and hip resurfacing has been excluded.
We agree with the author that the difference between metal-on-metal and metal-on-polyethylene bearings in terms of functional outcomes is not substantial and might be clinically small. However, we should note that current instruments have ‘ceiling effects’ as majority of patients have very substantial and near perfect scores after surgery. Hence a small difference might be a sign of much larger effect if more discriminatory instruments were available. Certainly we have limited knowledge of what is the minimally important difference for device comparison in orthopedics. The Harris Hip Score and other well known instruments were not designed to measure differences between devices (comparative effectiveness) in terms of change in functioning after surgery. Our paper provides a strong negative answer to claims that ‘alternative’ newly re-introduced metal-on-metal bearings are associated with better functioning when compared to metal-on-polyethylene. This is an important conclusion based on all comparative evidence published up to date. It is still very important to design instruments or methods to overcome the limitations of current functioning measurements and discuss minimally important differences (both change from baseline and comparative purposes) in orthopedic surgery.
We agree that cross-linked polyethylene might have advantages over older polyethylene and allows the use of larger heads sizes. A recent evidence review at least partially substantiated this hypothesis based on biomechanical characteristics and classified various polyethylenes into two categories of cross-linked and conventional polyethylene(2). Clinical registries can substantially advance this evidence by evaluating long-term revision occurrence after use of cross-linked and conventional polyethylene. For example, the Australian registry in its most recent report has highlighted the good outcome of large head size combined with cross linked polyethylene(3)
Finally, the comparative evaluation of all bearing surface is one of the main goals of on-going largest up to date multinational investigation led by International Consortium of Orthopedic Registries (ICOR). Inclusion of various bearing categories and any other variables for risk adjustment or subgroup effects will be subject to reaching consensus by leading registry researchers. While product level evaluation is desirable and necessary the number of bearing categories will be set pragmatically to investigate the ‘class’ effects such as metal-on-metal and cross-linked polyethylene and then ‘outlier’ better or worse performing products. If there are remaining claims about specific products that might have better or worse effects within a particular class then the burden to prove should be on manufacturers to substantiate those claims, particularly if these products are based on ‘non-inferiority’ path for regulatory approval. We hope that these studies will be large enough, well designed and comparative to convince the clinicians, researchers and regulators.
We also thank Dr. Tower for comments related high metal ion levels in patients implanted metal-on-metal implants. As we noted in our paper we did not summarize the evidence related to metal sensitivity or toxicity. Our findings are related to clinical outcomes after joint replacement. However, our results related to metal-on-metal implants are augmented by reports that demonstrate remarkably high metal ion levels and related toxic outcomes. The comments by Dr. Tower raise an important public health issue: how to we advance implant safety and effectiveness evaluation and prevent disasters related to faulty orthopedic implants? In addition, it is important to understand the importance of high metal ion levels in patients with no symptoms and functional implants. This certainly should be the focus of future investigations.
1. Sedrakyan A. Comparative assessment of implantable hip devices with different bearing surfaces: systematic appraisal of evidence. BMJ. 2011.
2. Kuzyk PR. Cross-linked versus conventional polyethylene for total hip replacement: a meta-analysis of randomised controlled trials. J Bone Joint Surg Br. 2011 May;93(5):593-600.
3. Australian National Registry. http://www.dmac.adelaide.edu.au/aoanjrr/index.jsp accessed December 30, 2011
Competing interests: No competing interests