The safety of Metal-on-Metal joint replacements with a particular focus on hip implants

There are two commonly used types of total hip arthroplasty (THA) (i) “stemmed” implants consisting of a ball head (either small-head diameter < 36 mm or large-head diameter ≥ 36 mm), which replaces the entire femoral head,  connected to a stem embedded in the femur as well as a cup embedded in the acetabulum. If both head and cup are metal, the implant is called “metal-on-metal” (MoM) total hip arthoplasty; (ii) “hip resurfacing arthroplasty” (HRA) in which the femoral head is resurfaced without a stem and a cup embedded in the acetabulum; both components in HRA consist of metal alloys and are always large diameter.
All types of MoM hip arthroplasties release metals in terms of particles, ions and/or metalloorganic compounds. The deposition of these substances in body fluids and tissue may lead to local and/or systemic adverse health effects. MoM implants with large diameters (large-head MoM THA and HRA) show the highest incidence of local reactions. For systemic health effects, no association with the type of implant or diameter has been established. Local as well as systemic adverse effects can also occur with other types of metallic implants (e.g. plates, screws).
Local tissue reactions: Broad clinical spectrum ranging from small asymptomatic tissue lesions to severe destruction of bone and soft tissues and include metallosis, aseptic lymphocytic vasculitis associated lesions, pseudotumours and adverse reactions to metal debris. These local responses can occur at any time after surgery (short, medium and long term).
Systemic adverse responses:  The toxicity of several metals present in implants  has  been evaluated in experimental and epidemiological studies. Extrapolation of these data to evaluation of metal particles, ions, and metallo-organic compounds released at the site of the implant and distributed into lymph nodes, bone marrow and internal organs is currently limited because the degradation products are not adequately characterised. There are some specific concerns related to the possible systemic exposure to cobalt and/or chromium after MoM-hip arthroplasty, such as systemic organ toxicity, carcinogenicity and teratogenicity.  There have been a few studies of carcinogenicity but with no consistent evidence of an overall increase in cancer associated with MoM HRA although there has been occasional reported elevation of hematopoetic malignancy, prostate cancer and melanoma rates.  Transplacental passage of metal ions has been demonstrated although without any teratogenic effect up to now.
The SCENIHR concludes that critical values for systemic effects are not yet established for patients after MoM implantation because of the lack of data and it is thus not possible to provide indications on limit values for the metals in any forms.
The SCENIHR decided to adopt the strategy as outlined in the European Consensus Statement, which includes recommendations on technical issues (e.g. metal ion determination for screening purposes should be performed in whole blood), determination of critical threshold ranges (e.g. for Co a range of 2 to 7 µg/L Co whole blood) and systematic follow-up for all patients and all implants due to the risks of MoM bearings.  Overall, the choice of the type of implant should be based on a detailed case-by-case evaluation taking into account risks and benefits relating to the characteristics of each patient such as age, gender, physical activity, occupation etc. The experience with MoM implants to date indicates that introduction of new or technically modified implants on the market should be made step-by-step. It is stressed that suitable preclinical and clinical studies are particularly important and that MoM implants requires close and comprehensive post-market surveillance.
Author(s) Source
European Comission, Scientific Committee on Emerging and Newly Identified Health Risks European Commission, SCENIHR 7th plenary of 24-25 September 2014
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