Metal-on-Metal Hip Implants May Cause Problems. The Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) has issued its final opinion on the safety of metal-on-metal (MoM) hip implants. In a report issued October 9th, SCENIHR concluded that all types of metal-on-metal hip implants release metal debris, such as particles and ions, can form metallo-organic compounds in the body.
There are two common types of total hip arthroplasty; one type is a “stemmed” implant where a ball attached to a stem is embedded in the femur to replace the femoral head. This type of hip replacement also utilizes a cup in the hip socket, or acetabulum. The device is considered a metal-on-metal hip implant if both the head and cup are made of metal. The second type of hip replacement is a “resurfacing” implant, where the head of the femur is resurfaced without using a stem. Resurfacing implants also have an acetabular cup, and both components are metal alloys. Stemmed implants can either have small-head diameters (<36 mm) or large-head diameter (> 36 mm) whereas resurfacing implants always have large diameters.
“we summarise that Metal-on-Metal (MoM) hip arthroplasty leads to a release of metal products (e.g. particles and ions) which can in addition form metallo-organic compounds in the body. These products can be deposited in draining lymph nodes and internal organs and may result in local and systemic adverse health effects.” SCENIHR said.
Local reactions occurred the most in large-head diameter implants, the report said. Local reactions refers to a spectrum of complications, “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.” The commission found that these reactions are short, medium and long-term, meaning that they can occur any time postoperatively.
Effects Of Systemic Exposure To Cobalt
The report notes that there are concerns about the effects of systemic exposure to cobalt and/or chromium with a metal-on-metal hip implant, including systemic organ toxicity, carcinogenicity and teratogenicity. The commission said that more data is needed to determine critical values for systemic effects.
Implants should be evaluated in an individual basis, the report said. For instance, the devices should not be used in women of child in patients who are allergic to the metals in the implant.
SCENIHR said that clinical and radiographic examination should be conducted for follow-up of all metal-on-metal hip patients. The commission also recommends that metal ion levels be obtained for large-head MoM total hip replacements in the first years after the surgery and also afterwards, depending on clinical outcomes. Ultrasound, CT-scan, and/r MARS-MRI are advised if Cobalt levels are above the range of 2 to 7 ul/L.