A metal hip replacement will be made of either: -
The metals used may be titanium, stainless steel or, most commonly nowadays, cobalt chrome alloy.
Metal-on-metal hip replacements tend to be used on younger, more active patients as the implant is likely to last longer than a metal-on-poly one will and so delay the need for any revision work.
They also allow the use of the largest sized femoral heads which reduce the risk of post-operative dislocation.
Wear rate is important because the process of wear (when the two surfaces rub against each other) leads to the production of debris. This debris sets up a reaction locally in the joint that leads to the aseptic loosening of the prosthesis. In turn that causes pain and the probability of revision surgery.
One of the most important factors in any hip replacement is the rate of wear. A metal-on-poly implant wears out about 10 times faster than a metal-on-metal one (0.1 and 0.01 mm per year respectively.
In either case this is higher than found in ceramic-on-ceramic implants.
The down side to metal-on-poly may be the higher wear rate but there is a a more serious down side to metal-on-metal hip replacements.
The problem here is related to the type of debris produced. Metal-on-metal implants release metal ions into the blood stream. These travel around the body and with time the concentration of these ions increases (metallosis).
And the end result of that is that thousands of people who had a metal hip replacement had to undergo revision surgery to have it removed.
Read more about the DePuy recall here, the warnings that were issued and ignored and the way attempts were made to shift responsibility onto the patients and their surgeons.
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