Safety of Metal on Metal Hip Implants
By Marcus M. Reidenberg, MD, FACP
Weill Cornell CERT
Summary by Kathleen Mazor, EdD
HMO Research Network CERT
Reports of adverse events after metal on metal hip implants (1, 2) have raised concern about the safety of this device (3). One should consider this question in the context of overall risks and benefits of hip replacement surgery.
The benefits of replacing severe osteoarthritic joints with artificial ones are increased mobility and less or no pain in the hip. Many studies have been done assessing these benefits. An older study found that 90% of patients had moderate or severe hip pain before replacement while only 23% had this much pain 6 months after surgery. Only 19% could climb stairs with little difficulty before surgery but 75% could do it after the new hip was in place (4). More recent studies have confirmed that most, but not all, people have substantial decrease in hip pain and increase in mobility after hip replacement surgery (5, 6). Research is ongoing to learn more about factors that effect the outcome of hip replacement so that preoperative predictions of benefit will be more accurate.
The major risks of hip replacement are those that occur during or shortly after the surgery such as infection and those that are delayed such as loosening of the implant. The concern over metal on metal hips relate to a delayed adverse effect of soft tissue inflammation and destruction (1, 7). This effect is due to tiny microscopic bits of metal being ground off the surface of the hip joint by the metal on one side of the joint rubbing on the metal on the other side of the joint every time the leg moves. A small fraction of sensitive people who have these metal on metal joints are very sensitive to these microscopic metal particles. The tiny particles cause a reaction in the body leading to loss of bone around the metal and a loosening of the joint. This is named aseptic osteolysis and can occur with any type of hip joint replacement. Some people develop an accumulation of inflammatory tissue around the joint in addition to the aseptic osteolysis. The present thought is that this inflammatory tissue results from an allergic reaction to the metal in the shed particles (7). Treatment is reoperation (3). Many factors effect the likelihood that loosening of the joint or inflammatory tissue accumulation will progress to the need for re-operation. These include the design of the replacement joint, the nature of the joint and the metals used to make it, the way the metal parts are placed in the hip, and the sensitivity of the patient to the material now inside the body.
One must also be aware that aseptic osteolysis can occur after all the other kinds of hip replacements, not just metal on metal. Any of these types of artificial hips that have osteolysis can require additional surgery. Another late adverse event for any type of artificial joint is infection of the hip. This comes from bacteria entering the blood stream and settling next to the hip. This is why patients with artificial joints are usually advised to take antibiotics at the time of dental procedures and other procedures that are associated with bacteria entering the blood stream.
A theoretical issue with the metal joints is that enough of the cobalt or chromium metal will be absorbed systemically to cause systemic effect. There is some experience with heavy industrial exposure to these metals but this is so different from the nature of the exposure from a metal alloy joint that extrapolating from industrial exposure may not be valid. The studies reported to date do not find any adverse systemic effects from these joints. What is clear is that the microscopic metal particles are the cause of the inflammatory tissue accumulation (7).A summary of research papers comparing outcomes of various types of artificial hips used in hip replacement surgery found little difference among them (8). A new report of data from the British National Joint Registry following over 400,000 joint replacement operations found that metal-on-metal joints failed and required revision surgery at a much faster rate than other types of artificial joints (9). This illustrates the importance of registries of all patients receiving implanted medical devices that are supposed to last a long time, usually lifelong, in the patients. Comprehensive registries appear to be the only practical way to get long term outcome data so essential for comparing effects of various devices designed to be used for the same thing, such as different kinds of artificial hips (10).
- Mahendra G, Pandit H, Kliskey K, Murray D, Gill HS, Athanasou N. Necrotic and inflammatory changes in metal-on-metal resurfacing hip arthroplasties. Acta Orthop. 2009 Dec;80(6):653-9.
- Crawford R, Ranawat CS, Rothman RH. Metal on metal: is it worth the risk? J Arthroplasty. 2010 Jan;25(1):1-2.
- Meier, B. “As Use of Metal-on-Metal Hip Implants Grows, Studies Raise Concerns.”New York Times 3 March 2010.
- Dawson J, Fitzpatrick R, Carr A, Murray D. Questionnaire on the perceptions of patients about total hip replacement. J Bone Joint Surg Br. 1996 Mar;78(2):185-90.
- Marx RG, Jones EC, Atwan NC, Closkey RF, Salvati EA, Sculco TP. Measuring improvement following total hip and knee arthroplasty using patient-based measures of outcome. J Bone Joint Surg Am. 2005 Sep;87(9):1999-2005.
- Ng CY, Ballantyne JA, Brenkel IJ. Quality of life and functional outcome after primary total hip replacement. A five-year follow-up. J Bone Joint Surg Br. 2007 Jul;89(7):868-73.
- Looney RJ, Schwarz EM, Boyd A, O'Keefe RJ. Periprosthetic osteolysis: an immunologist's update. Curr Opin Rheumatol. 2006 Jan;18(1):80-7.
- Sedrakyan A, Normand S-LT, DAboc S, Jacobs S, Graves S, Marinac-Dabic D. Comparative assessment of implantable hip devices with different bearing surfaces: systematic appraisal of evidence. BMJ 2011; 343: doi 10.1136/bmj.d7434.
- Smith AJ, Dieppe P, Vernon K, et al. Failure rates of stemmed metal-on-metal hip replacements: analysis of data from the National Joint Registry of England and Wales. Lancet 2012: 379: 1199-204.
- Sedrakyan A. Metal-on-metal failures-in science, regulation, and policy. Lancet 2012: 379: 1174-6.
Posted 12/22/2010; updated 4/11/2012
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