The indications for hip replacement, especially in young patients with end-stage degenerative arthritis of the hip, have increased during the past 20 years. Because of the accelerated risk of polyethylene wear and subsequent prosthetic loosening due to osteolysis, new interest has focused on the metal-on-metal articulation couple. Failures of first-generation metal-on-metal total hip arthroplasties (THAs) are attributed to suboptimal surgical technique, excessive or negative clearance, poor fixation, and neck–socket impingement.1 Improvements in metallurgic and tribological properties (ie, sphericity and radial clearance) of metal-on-metal bearing couples has led to a renaissance of metal-on-metal resurfacing arthroplasties and THAs. Second-generation metal-on-metal wear rates are noted to be 20 to 100 times lower than metal-on-polyethylene wear rates.2
Good clinical and radiological results have been reported after metal-on-metal resurfacing arthroplasties in patients younger than 55 years, with survival rates of 99% and 98% at 10- and 13-year follow-up, respectively.3 Also, good results have been described after a minimum 6-year follow-up (mean, 5.7 years) of cementless second-generation metal-on-metal THAs in prospective, randomized, controlled clinical trials, with a survival rate of 100%.4–6 Few prospective, randomized, controlled studies compare conventional metal-on-polyethylene THAs with second-generation metal-on-metal THAs. Zijlstra et al7 reported a 10-year survival rate of 95.5% for a metal-on-metal group and a 10-year survival rate of 96.8% for a metal-on-polyethylene group, with no difference in clinical or radiological outcomes. Small bearing couples of 28-mm femoral heads were used.
Interpretation of the results of different prosthetic devices, as well as metal-on-metal resurfacing arthroplasties, is problematic because of differences in design, alloy, radial clearance, risk of head–neck impingement, edge wear, and other metallurgic properties. Besides better metallurgical and tribological properties, orthopedic companies also claim clinical advantages of large-head metal-on-metal THAs over conventional small-head metal-on-polyethylene THAs, including a larger range of motion and a decreased risk of dislocation. However, in a randomized clinical trial, no clinically relevant difference in range of motion was observed between conventional 28-mm metal-on-polyethylene THAs compared with large-head metal-on-metal THAs.8
Until now, few prospective, randomized, controlled trials have documented the long-term clinical and radiological results of metal-on-metal resurfacing arthroplasties and metal-on-metal THAs, including survival analysis. No proven clinical advantage of metal-on-metal over metal-on-polyethylene THAs currently exists. Future research should focus on long-term follow-up of the new generation of hip replacements.
Recently, concern has emerged about the development of pseudotumors in large-head metal-on-metal THAs and resurfacing arthroplasties. These pseudotumors, also known as aseptic, lymphocyte-dominated, vasculitis-associated lesions or adverse reactions to metal debris, are caused by a local metal allergy or sensitivity and a type IV, local, delayed-type hypersensitivity caused by T lymphocytes. Even with low wear rates, this type of allergy can develop. Whether aseptic, lymphocyte-dominated, vasculitis-associated lesions are specific to metal-on-metal implants is under investigation.
More lymphocytes and plasma cells and fewer macrophages are found in tissue reactions in metal-on-metal THAs compared with metal-on-polyethylene THAs. This histological difference may be attributed to the difference in size between polyethylene (several micrometers) and metal particles (submicron- and nanometer-size particles). The reported incidence of symptomatic pseudotumors varies.9,10 Pseudotumors can be caused by the local periprosthetic deposition of cobalt and chromium particles, sometimes resulting in soft tissue masses in the groin area; pain, swelling, and discomfort in the leg; or compression of the neurovascular bundle. Whether the production of metal particles is caused by wear of the bearing couple itself, such as edge wear in large-head metal-on-metal THAs and metal-on-metal resurfacing arthroplasties, or at the taper–head connection, such as in large-head metal-on-metal THAs, is under investigation. Small-diameter components used for women and adolescents and a high inclination of the acetabular component could be significant risk factors in pseudotumor formation.
Due to periprosthetic pseudotumor formation and a high failure rate, the Articular Surface Replacement (DePuy, Johnson & Johnson, Leeds, United Kingdom) was withdrawn from the market. Results from the registries from Australia, England, Wales, and New Zealand demonstrated an overall increased rate of revision arthroplasty in metal-on-metal THAs and resurfacing arthroplasties.11 In my experience with a recall of 143 resurfacing Birmingham Hip (Smith & Nephew, Memphis, Tennessee) arthroplasties in 125 patients, computed tomography scans showed a pseudotumor in 40 (28%) hips. In 11 of the 40 cases, patients had pain, swelling, and discomfort in the groin area and either had already undergone or are currently scheduled for revision. Patients with pseudotumors had significantly increased serum chromium and cobalt ion levels.
Contrary to the micrometer-sized polyethylene particles in conventional articulations, metal particles generated in metal-on-metal bearings are nanometer sized.12 These ultra-small particles cause less granulomatous inflammation but are produced in greater numbers.13 This can lead to a greater release of osteolytic enzymes compared with large polyethylene particles, which results in granulomatous giant cell formation with less release of osteolytic mediators. Nanometer-sized metal particles can corrode in synovial fluid, forming cobalt and chromium ions. After particle corrosion, the serum level of chromium and cobalt ions is elevated. The clinical consequence of elevated chromium and cobalt ions is also under investigation. Measurement of serum cobalt and chromium concentrations has been advocated as a monitoring tool for high-wear-induced failure of metal-on-metal bearings. In revised metal-on-metal THAs, systemic cobalt and chromium ion levels strongly correlated with hip synovial fluid cobalt and chromium levels. The latter were approximately 40 to 50 times higher.14 In addition, both serum and synovial ion levels correlated strongly with femoral component wear, and serum levels >17 to 19 μg/L were more often associated with intraoperative metallosis.
Until now, the systemic effects of cobalt and chromium ions were unknown. Nevertheless, concerns remain regarding the long-term biological effects of metal wear debris and metal ions relating to chromosomal damage, possible carcinogenesis effects on fetuses in women of child-bearing age, metal allergy and sensitivity, and metal-induced toxicity, including bone loss. No reports exist of implant-induced cancer, and the risk of cancer does not appear to be higher after metal-on-metal THA. Moreover, no proven teratogenicity of cobalt and chromium has been found in humans.
In April 2010, the British Orthopaedic Association issued a medical device alert to all hospitals and doctors in the United Kingdom because of reports of adverse reactions to the metal wear particles in metal-on-metal artificial hips.15 Also, the Dutch Orthopaedic Association recently advised not to implant large-head resurfacing THAs and metal-on-metal THAs until more is known about safety and until long-term follow-up results are available. It was strongly advised to use metal-on-metal THAs only in prospective, randomized clinical trials, provided that researchers had the informed consent of the patients and the approval of the local medical ethical committee and the study was placed in the national trial registry. Currently, all of my patients who received a metal-on-metal THA have been recalled for a clinical examination and screening for the possibility of a pseudotumor by radiological examination, serum cobalt and chromium ion analysis, and computed tomography scan for periprosthetic soft tissue reaction.
Revision of metal-on-metal resurfacing arthroplasties and metal-on-metal THAs must be performed in case of a symptomatic pseudotumor. According to the British Orthopaedic Association, revision should be considered in the presence of pain, high cup inclination angle, and elevated metal ion levels.15 It can be argued that, in the presence of pain and high inclination angles, revision should be considered anyway. In cases of severe tissue reaction and large pseudotumors, revision surgery can be complex, with extended soft tissue damage. So far, the results of revisions of failed metal-on-metal resurfacing arthroplasties and metal-on-metal THAs have been good in the short term,16 but results after long-term follow-up are not available, especially in hips that were revised because of pseudotumor formation.
Long-term results of conventional metal-on-polyethylene THAs are excellent.17 Until now, second-generation metal-on-metal THAs and metal-on-metal resurfacing arthroplasties have had varying mid- and long-term results. Until long-term results of these new types of hip implants are available and more is known about the local and systemic effects of this generation of metal wear particles and ions, the gold standard will be the metal-on-polyethylene articulating couple. According to the advice of the Dutch Orthopaedic Society, I would also recommend delaying the implantation of metal-on-metal THAs and resurfacing hip arthroplasties until more is known about the safety and long-term results of these implants.
Given the recent European experiences with metal-on-metal THAs and metal-on-metal resurfacing arthroplasties and the results from the registries of Australia, England, Wales, and New Zealand, new types of implants should be introduced in unbiased, clinical, double-blind, prospective trials with the informed consent of patients and according to the rules of the local ethical committee and registration in the national trial registry. A need exists for well-designed, prospective, randomized studies and high-level systematic reviews, as well as national joint registries to assess innovations in joint replacement. Only in this way can the potential advantages and disadvantages of introducing new techniques be assessed with the best possible scientific evidence.
- Plitz W. Metal/metal coupling in total hip arthroplasty. Progress or setback [in German]?Orthopade. 2007; 36(3):212–219. doi:10.1007/s00132-007-1062-y [CrossRef]
- McKellop H, Park SH, Chiesa R, et al. In vivo wear of three types of metal on metal hip prostheses during two decades of use. Clin Orthop Rel Res. 1996; (329 suppl):S128–S140.
- McMinn DJ, Daniel J, Ziaee H, Pradhan C. Indications and results of hip resurfacing [published online ahead of print November 16, 2010]. Int Orthop. 2011; 35(2):231–237. doi:10.1007/s00264-010-1148-8 [CrossRef]
- Amstutz HC, Campbell P, McKellop H. Metal on metal total hip replacement workshop consensus document. Clin Orthop Rel Res. 1996; (329 suppl):S297–S303.
- Lombardi AV Jr, Mallory TH, Alexiades MM, et al. Short-term results of the M2A taper metal-on-metal articulation. J Arthroplasty. 2001; 16(8 suppl 1):122–128. doi:10.1054/arth.2001.29307 [CrossRef]
- Lombardi AV Jr, Mallory TH, Cuckler JM, Williams J, Berend KR, Smith TM. Mid-term results of a polyethylene free-metal-on-metal articulation. J Arthroplasty. 2004; 19(7 suppl 2):42–47. doi:10.1016/j.arth.2004.06.016 [CrossRef]
- Zijlstra WP, van Raay JJ, Bulstra SK, Deutman R. No superiority of cemented metal-on-metal over metal-on-polyethylene Total hip arthroplasty in a randomized trial at 10-year follow-up. Orthopedics. 2010; 33:154–161. doi:
- Zijlstra WP, van den Akker-Scheek I, Zee MJ, van Raay JJ. No clinical difference between large metal-on-metal total hip arthroplasty and 28-mm-head total hip arthroplasty [published online ahead of print March 4, 2011]?Int Orthop. 2011; 35(12):1771–1776. doi:10.1007/s00264-011-1233-7 [CrossRef]
- Ollivere B, Darrah C, Barker T, Nolan J, Porteous MJ. Early clinical failure of the Birmingham metal-on-metal hip resurfacing is associated with metallosis and soft-tissue necrosis. J Bone Joint Surg Br. 2009; 91(8):1025–1030. doi:10.1302/0301-620X.91B8.21701 [CrossRef]
- Canadian Hip Resurfacing Study Group. A survey on the prevalence of pseudotumors with metal-on-metal hip resurfacing in Canadian academic centers. J Bone Joint Surg Am. 2011; 93(suppl 2):1118–1121.
- Graves SE, Rothwell A, Tucker K, Jacobs JJ, Sedrahyan A. A multinational assessment of metal-on-metal bearings in hip replacement. J Bone Joint Surg Am. 2011; 93(suppl 3):43–47. doi:10.2106/JBJS.K.01220 [CrossRef]
- Keegan GM, Learmonth ID, Case CP. A systematic comparison of the actual, potential, and theoretical health effects of cobalt and chromium exposures from industry and surgical implants. Crit Reviews Toxicol. 2008; 38(8):645–674. doi:10.1080/10408440701845534 [CrossRef]
- Doorn PF, Campbell PA, Amstutz HC. Metal versus polyethylene wear particles in total hip replacements. A review. Clin Orthop Relat Res. 1996; (329 suppl):S206–S216. doi:10.1097/00003086-199608001-00018 [CrossRef]
- De Smet K, De Haan R, Calistri A, et al. Metal ion measurement as a diagnostic tool to identify problems with metal-on-metal hip resurfacing. J Bone Joint Surg Am. 2008; 90(suppl 4):202–208. doi:10.2106/JBJS.H.00672 [CrossRef]
- Gerritsma-Blecker CLE, Deutman R, Mulder TJ, et al. The Stanmore total hip replacement. A 22-year follow-up. J Bone Joint Surg Br. 2000; 82(1):97–102. doi:10.1302/0301-620X.82B1.10058 [CrossRef]
- De Smet KA, Van Der Straeten C, van Orsouw M, Doubi R, Backers K, Grammatopoulos G. Revisions of metal-on-metal hip resurfacing: lessons learned and improved outcome. Orthop Clin North Am. 2011; 42(2):259–69,ix. doi:10.1016/j.ocl.2011.01.003 [CrossRef]
- Gerritsma-Bleeker CL, Deutman R, Mulder TJ, Steinberg JD. The Stan-more total hip replacement. A 22-year follow-up. J Bone Joint Surg Br. 2000; 82(1):97–102. doi:10.1302/0301-620X.82B1.10058 [CrossRef]