A metal hip replacement will be made of either: -
The metals used may be titanium, stainless steel or, most commonly nowadays, cobalt chrome alloy.
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. Wear rate is important because the process of wear 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.
The down side to metal-on-poly may be the wear rate but unfortunately there is a down side to metal-on-metal too. 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. With time the concentration of these ions increases. What is not known is the affect these ions will have on the body or on a foetus. It is for this reason that women of child-bearing age are advised to opt for a ceramic hip replacement rather a metal one.
To understand what can go wrong with a metal-on-metal implant have a look at this Medivisuals animated video.
There is currently a massive recall of this type of hip implant going on. DePuy (a subsidiary of Johnson & Johnson)is recalling 93,000 implants. It is anticipated that about 12,000 people will need to have their implant replaced. Read more about the DePuy recall here.
Metal-on-metal implants are more expensive than metal-on-plastic.
One advantage of metal-on-metal hip replacement is that they are self-polishing - that is they polish away any scratches on their own surfaces.
Metal-on-metal implants also allow the use of large sized femoral heads. The larger the ball the less risk there is of subsequent dislocation. Finally only metal can make the thinnest large sized cups.
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.
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