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Metal on Metal Total Hip Arthroplasty Convertir en PDF Version imprimable Suggérer par mail

Steven J.M. MacDonald, M.D., FRCSC
Associate Professor of Orthopaedic Surgery
University of Western Ontario
Chief of Orthopaedics
London Health Sciences Centre, University Campus
London, ON

Total hip arthroplasty is a highly efficacious, cost-effective intervention with measurable disease specific and patient specific outcome tools. In the majority of patients, the standard coupling of a cobalt chrome femoral head and a polyethylene liner will last a lifetime. Concerns however, regarding wear of ultrahigh molecular weight polyethylene with subsequent osteolysis have dominated the literature of total joint arthroplasties for the past 20 years. To reduce wear debris generation, osteolysis, and the need for revision, the metal on metal articulation has been proposed as an alternative bearing couple.  

Metal on metal articulations enjoyed widespread clinical use in the 1960's and early 1970's with the McKee-Farrar prosthesis being the most common. It was first done in North America by Patterson and Brown in Vancouver.1 Metal on metal total hip replacements gradually fell out favor and metal on polyethylene articulations have predominated clinical practice to date. There were several factors that lead to the decline in popularity of metal on metal hip replacements. Most importantly, the early results published by Charnley, with a standard metal polyethylene bearing, were excellent and superior to the results being achieved with the McKee-Farrar prosthesis. Secondly, there were multiple problems with early metal on metal designs. Impingement, inadequate clearances, poor manufacturing tolerances, poor material selection, poor stem design, and poor implantation technique all lead to early failures. However, the metal on metal implants clinically demonstrated very low wear rates of the bearing surfaces (Figure 1). Addressing these previous design flaws, the metal on metal hip arthroplasty was reintroduced in the late 1980's with the Metasul (Sulzer Medica), and to date over 160,000 have been implanted worldwide. Many other metal on metal implants have been released recently, incorporating current understanding and design philosophies.
The most significant advantage to the use of a metal on metal couple in total hip arthroplasty is the clearly documented reduction in wear of the bearing surface. Metal on metal bearings, when examined in vitro on hip simulators, undergo an initial run-in period of higher wear rates but then reach much lower steady state levels of wear.2 A conventional polyethylene liner is thought to wear at the rate of .1mm per year. Metal on metal articulations however have a wear rate of up to 100 times less than that.3 In a recently published review of 231 retrieved current generation metal on metal articulations4, the linear wear rate of the metal on metal bearing was 40 times lower than that demonstrated by conventional metal on polyethylene bearings. As well, the volumetric wear rate of the metal on metal articulations was approximately 200 times lower than previous metal on polyethylene hips.

Figure 1. A retrieved Ring prosthesis. The implant failed secondary to aseptic loosening, however the bearing surface demonstrates minimal wear at 26 years.
There are, however, disadvantages to the metal on metal design. Cost issues, metal hypersensitivity, lack of long-term clinical results for current designs and the release of metal ions have all been cited previously. The elevation of metal ions is seen in both the blood and urine of patients with a metal on metal hip arthroplasty. Previous reports have clearly demonstrated this in both older and newer designs.5,6 In a prospective randomized clinical trial comparing metal on metal to metal on polyethylene bearings7, it was demonstrated that patients with a metal on metal implant had on average a 7.9-fold increase in erythrocte cobalt, a 2.3-fold increase in erythrocyte chromium, a 1.7-fold increase in erythrocyte titanium, a 35.1-fold increase in urine cobalt, a 17.4-fold increase in urine chromium and a 2.6-fold increase in urine titanium at two years follow-up. Patients with a metal on polyethylene bearing had no change in eryhrocyte titanium, urine cobalt, or urine chromium and a 1.5-fold increase in erythrocyte cobalt, a 2.2-fold increase in erythrocyte chromium, and a 4.2-fold increase in urine titanium. The significance, if any, of elevated metal ions however remains to be determined. Previous authors8 have not seen any increased risks specifically to patients, however long-term evaluations are not available.

The clinical results of current generation metal on metal total hip replacements are encouraging, but only short-term evaluations are available to date.9,10,11 With improvements in design and fixation that have been incorporated into current models, early failures secondary to loosening and requirements for revision would appear to be similar to metal on polyethylene articulations.

There are still questions that remain unanswered. Does the modularity of the metal liner insert contribute to metal ion elevation? What is the ideal carbon content of the bearing surfaces? Should only larger diameter femoral heads be used given their advantages with increased stability and lower wear rates due to significantly improved fluid film lubrication? Are there any long-term clinical effects of elevated metal ions? Further evaluations and clinical trials are required to help solve these issues. At the present time however, a metal on metal articulation would appear to be a viable alternate bearing surface for a patient requiring a total hip arthroplasty.

References

  1. Patterson F.P., Brown C.S.: The McKee-Farrar total hip replacement: Preliminary results and complications of 368 operations performed in five general hospitals. J Bone Joint Surg 54A:257-275, 1972.

  2. Streicher R.M., Schon R., Semlitsch M.F.: Tribologic behavior of metal/metal combinations for artificial hip joints. Biomed Tech 35:107-111, 1990.

  3. Willert H.G., Buchhorn G.H., Gobel D., Koster G., Schaffner S., Schenk R., Semlitsch M.: Wear Behavior and Histoplathology of Classic Cemented Metal on Metal Hip Endorprostheses. Clinical Orthop 329S:S160-S186, 1996.

  4. Rieker C., Kottig P.: In vivo tribological performance of 231 metal-on-metal hip articulations: Hip International 12(2): 73-76, 2002.

  5. Jacobs J.J., Skipor A.K., Doorn P.F.: Cobalt and chromium concentrations in patients with metal on metal total hip replacements. Clin Orthop 329(Suppl): S256-S263, 1996.

  6. Brodner W., Bitzan P., Meisinger V.: Elevated serum cobalt with metal on metal articulating surfaces. J Bone Joint Surg 79B:316-321, 1997.

  7. MacDonald S.J., McCalden R.W., Chess D.G., Bourne R.B., Rorabeck C.H., Cleland D., Leung F.: Metal-on metal versus polyethylene in hip arthroplasty: A randomized clinical trial. Clin Orthop, In press.

  8. Visuri T., Pukkala E., Paavolainen P., Pulkkinen P., Riska E.B.: Cancer risk after metal on metal and polyethylene on metal total hip arthroplasty. Clin Orthop 329(Suppl): S280-S289, 1996.

  9. Lombardi A.V., Mallory T.H., Alexiades M.M., Cuckler J.M., Faris P.M., Jaffe K.A., Keating E.M., Nelson C.L., Ranawat C.S., Williams J., Wixson R., Hartman J.F., Capps S.G., Kefauver C.A.: Short-term results of the M2a-taper metal-on-metal articulation. J Arthroplasty 16:122-128, 2001.

  10. Wagner M., Wagner H.: Medium-term results of a modern metal-on-metal system in total hip replacement. Clin Orthop 379:123-133, 2000.

  11. Dorr L.D., Wan Z., Longjohn D.B., Dubois B., Murken R.: Total hip arthroplasty with the use of the metasul metal-on-metal articulation. J Bone Joint Surg 82A:789-798, 2000.

 

 
 
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