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Metal-on-Metal: Questions & Answers

My View:

call me a skeptic but talk about PR SPIN & Marketing: Download the Booklet here and judge for yourself…

Here is what S&N say in their Q&A Publication -read and be amazed…

What are the benefits of metal-on-metal hip replacement?

Cobalt Chrome alloy has been used since the 1930’s for a multitude of orthopaedic applications. Due to its relatively high hardness, high stiffness and resistance to damage, Cobalt Chromium alloy has widespread use in modern day orthopaedics as a bearing surface especially in joint arthroplasty. It is commonly used in such applications as total and uni knee femoral components, spinal implants, total hip arthroplasty femoral heads articulating against polyethylene, and metal-on-metal total hips including hip resurfacing.

Due to its material properties Cobalt Chrome can be used to produce very thin metallic devices without the risk of fracture. This makes it attractive for use in hip arthroplasty for active patients to produce bone conserving implants while maximising the size of bearing diameter to reduce the risk of dislocation. With the current technology, other materials used in total hip arthroplasty, such as ceramics and polyethylene, cannot practically be manufactured into anatomically relevant implant sizes without sacrificing acetabular bone stock or increasing their risk of fracture. Well functioning metal-on-metal hip replacements also have the potential to produce very low amounts of wear compared to metal on polyethylene implants.

Cobalt chrome implants are known to release metal ions within the body. Although metal-on-metal hip replacements have been used for over 40 years investigations are being carried out into any long term clinical implications.

What causes metal ions in the body?

Cobalt, chromium and molybdenum, which are the major constituents of the alloy used in artificial hip devices, are natural essential trace elements for humans and are found in the water supply and food. Cobalt is part of vitamin B12. Chromium facilitates insulin activity and is essential for the energy functions of the cell. Molybdenum is also essential for several cellular functions. Being essential elements there is an efficient continuous renal mechanism to get rid of much of the excess.

Measurable metal ion levels are present in the blood and urine of subjects with no artificial metal devices in the body. When artificial hip or knee devices are used, the circulating levels of these metal ions increase initially and then slightly reduce, but do not return to the levels that existed before the operation.

Is there a ‘normal’ level of metal ions?

There are measurable levels of metal ions in the urine and blood of all humans. In general they are of the order of 1 part per billion or less.

Are there set safety levels?

Although safe levels have been prescribed in Industrial Occupational Health settings, these are not directly applicable to patients with artificial hip devices. In one study of hip failures researchers found that serum chromium levels of 17 parts per billion (ppb) and serum cobalt levels of 19 ppb are unsafe and fail with excess wear and metal staining of tissues1 and more recently reported that a majority of patients with well-functioning hips had a serum cobalt level of less than 4 ppb and a chromium level of less than 5 ppb2.

What is the impact on the patient of metal ions in the body?

Metal-on-metal hip devices have been in use for over forty years. Since these devices are being increasingly used in young patients who are more active and are going to retain these devices over a longer life, there are concerns relating to potential unknown adverse effects. Although there are studies which show chromosome changes in association with increased metal ion levels, such changes have been seen in patients with different types of artificial hip and knee devices including metal-on-polyethylene and ceramic bearing devices. Scandinavian studies have shown no increase in the incidence of cancer in a group of patients with metal-on-metal devices followed up for over thirty years.

Recent studies show that some metal-on-metal devices are likely to release more metal ions and fail quicker because of suboptimal materials3 and design4. Surgeon experience and positioning of the implants also has an effect on the release of metal ions.