I believe metal on metal bearing is the best choice for hip replacement for the following two reasons:
- Extremely durable surface that is unlikely to ever wear out.
- Stability of the joint. Using a metal on metal bearing surface allows the manufacture of a large bearing hip joint that will not dislocate. Traditional hip replacements are not durable enough for many of today's younger more active patients. Traditional metal on plastic bearing devices have been shown to have a failure rate of (30 % ) by seven years in this patient group. Recently several new bearing couples have been developed as more durable options:
- Crosslinked polyethylene - more durable than standard plastic but still not well tested. Previous modifications in plastics have been very unsuccessful.
- Ceramic on Ceramic - very durable bearing surface. Unlikely to ever wear out. However , manufacture of this brittle material is very tricky , occasionally resulting in failure by fracture of the ceramic parts while in use.
- Metal-on-Metal - very durable bearing surface. Unlikely to ever wear out. No possibility of cracking like ceramic. Although metal-on-metal and ceramics are equally durable bearing surfaces , a metal on metal bearing has one distinct advantage over all other bearing surfaces.
Large diameter joint replacements can be manufactured which allow the surgeons to reconstruct the hip in a mechanically sound fashion similar to a patient's original hip and thus avoid dislocation after surgery. This is a major advantage that should not be underestimated. It is estimated that 5-7% of patients with total hip replacement will suffer dislocation over ten years. With large bearing joints the chance is less than 1 %. The improved stability makes it possible for us to offer patients a new hip without all of the traditional restrictions of total hip replacement. (Restrictions used to include no deep bending , crouching or crossing legs) , patients can now kayak , participate in dance , gymnastics , martial arts , etc. There is one potential problem with metal-on-metal bearings. Normal wear results in release of metal particles into the body. One problem with any artificial bearing surface is the wear regenerated by normal daily wear and tear. Large volumes of plastic generated by metal on plastic (traditional) replacements result in large amounts of bone destruction , (osteolysis) around implants and has been the major cause for failure in young patients. Both ceramic-on-ceramic and metal-on-metal devices generate about 99% less wear debris than traditional bearings and the debris generated seems to cause less irritation to the bone than plastic debris does. There has been some speculation of the potential for metal debris to cause cancer. However , with careful studies to date , no links have been demonstrated. My opinion is that the advantage of metal on metal bearings strongly outweighs the potential risks. Now we can return patients to normal function with almost no restrictions and expect their implants to last more than 10 years at very high activity levels. Most patients will never require another operation on their hip. If ceramic-on-ceramic or metal-on-crosslinked polyethylene bearings are used , wear is also not a problem , but significant restrictions remain. No running or jumping can be allowed due to the risk of fracture of the implant: no crossing of the legs or extreme bending can be allowed due to the risk of dislocation. I am surprised that any person would choose anything other than a large metal-on-metal bearing!