Over the past few decades, many advances in hip replacement have been made and several types of prostheses are available. They may include the traditional total hip replacement, which is a combination of metal and plastic, and the newer metal on metal total hip replacements.
What is the difference? A traditional total hip replacement uses a metal ball of 28 millimeters in diameter and a plastic socket for motion. While it has an excellent record for long life and durability, the plastic wears with each step and can lead to eventual loosening of the hip replacement. This process can take 15 to 25 years and is not a problem for most patients. However, younger patients may need their new hip to last substantially longer. These patients are candidates for metal on metal replacement.
The metal on metal total hip simply replaces the plastic material with metal, which is either a cobalt-chrome alloy, or a titanium alloy. These are “super metals” initially developed for the aerospace industry and now adapted for orthopedics. In laboratory simulations, the wear rate of metal-on-metal is as much as 1000 times lower than traditional metal on plastic. While metal-on-metal does not have the proven track record of metal on plastic, it is believed that it has the potential to last much longer than traditional hip replacements. This procedure has been performed successfully for the last ten years with excellent early results. The type of prosthesis used for your hip surgery is determined by a number of factors such as your height, weight, age, and bony structure.
The second major advantage is the very large ball component that is possible with metal on metal. Removing the thick plastic allows the use of heads that are very close to the size of the natural hip. This reduces the chance of a dislocation and allows much more natural motion. Heads up to 64mm are possible with metal on metal hips, with the average being around 46mm – almost twice the size of a metal on plastic hip
Because metal is much more durable than plastic, it should last much longer and should allow a higher activity level than hips containing plastic. At this point, I don’t know what the very long term outcome of metal on metal will be, but given the excellent results up until now, I remain optimistic that this procedure will help tens of thousands of younger patients lead much more active lives.