Alternative Bearing Surfaces
Highly Crosslinked Polyethylene
New, “highly crosslinked” formulations of polyethylene have shown vastly improved results in laboratory testing compared to standard ultra-high molecular weight polyethylene (UHMWPE). Although a few studies looking at this plastic retrieved from human beings have shown small cracks in the polyethylene after relatively short periods of use, it appears from most clinical studies that highly crosslinked polyethylene will probably wear at least twice as long as standard polyethylene. This advantage is magnified when used with a ceramic, rather than metal femoral head. Longer term clinical studies are now verifying the longevity of highly crosslinked polyethylene.
Standard polyethylene is a resilient substance that performs very well for the typical hip replacement patient, who is elderly and somewhat sedentary. It also gives good service for younger patients whose daily activity is restricted by multiple joint problems, such as rheumatoid arthritis, or by other chronic diseases that limit one’s activity level. However, for younger patients with no limitations other than an arthritic hip, standard polyethylene will not hold up like highly crosslinked polyethylene does.
As polyethylene wears, billions of microscopic particles are generated and released into the joint space. These foreign particles tend to provoke an inflammatory response that slowly destroys bone around the artificial joint replacement, resulting in holes in the bone that resemble Swiss cheese. This reaction is called “osteolysis” and is difficult to detect on x-ray until the bone damage is well advanced. The result is progressive bone loss and a painfully loose implant.
Ceramic on Ceramic
In contrast to the wear characteristics of UHMWPE, medical-grade alumina ceramic wears approximately 1,000 times less. The following chart shows the relative wear rates of different hip replacement bearing couples – the ball (head) and socket liner (cup).
Further, tissue studies directly comparing identically sized particles of UHMWPE and alumina ceramic have shown that wear particles from alumina ceramic are associated with very little inflammatory response from the body. The favorable wear characteristics of ceramics make it possible, at least in theory, that a ceramic total hip replacement could last a young, active person the rest of his/her life.
Although ceramics for use in total hip replacement have been available since 1970, the quality of the ceramic has improved dramatically. Currently, the most utilized ceramic material is alumina—termed Biolox Forte by its manufacturer, Ceramtec. As of 2009, all ceramic-on-ceramic total hips use Biolox Forte alumina for the ball and liner of the socket.
The mention of ceramic conjures, in some people’s minds, associations with fragile dinnerware. However, modern medical grade ceramic is literally “industrial strength” and is extremely strong. Biolox Forte alumina has been in clinical use in Europe since 1994 and has shown excellent long-term results in young, active patients. Currently, data from multiple studies suggests that the risk of a ceramic fracture in the hip is as low as 1 in 5,000 to 1 in 10,000. However, when a ceramic component fractures in the body, it requires immediate revision surgery and creates other potentially long-term problems.
Recently, a new type of ceramic has been released for use in total hip replacement. Biolox Delta ceramic is hard and smooth like Biolox Forte, but is much tougher and resistant to cracking and breakage. Delta ceramic heads were approved by the FDA in 2006, and delta hip socket liners are sure to be approved soon.
Metal on Metal
Metal on metal total hips have been gaining popularity recently, particularly with the FDA approval of resurfacing arthroplasty, which uses a metal ball against a metal socket. There is no doubt that metal against metal bearing surfaces wear much better than metal or ceramic against polyethylene (see graph above). In fact, metal on metal wears almost as well as ceramic on ceramic without the remote risk of fracture that ceramics carry.
So what are the downsides? Basically, 3 issues have lead to some hesitancy using metal-metal bearing couples across the board for young, active patients.
- Metal-metal wear particles create the highest levels of metal ions (i.e. atoms) in the bloodstream and other body tissues. Furthermore, these metal ions must be cleared from the body by the kidneys. So anyone who has kidney problems or who may develop kidney problems in the future may not be an ideal candidtae for a metal-metal hip. The problem is that we don’t know who will develop kidney problems in the future for reasons unrelated to their hip replacement.
- Although not entirely compelling, there is some literature that has suggested that these metal ions can potentially cause cancer. If this is an issue, it occures in an extremely small number of patient and may not justify the worry.
- Allergic (hypersensitivy) reactiosn to the metal particles have been documented in some patients. Allergic reactions can cause pain and osteolysis around the total hip replacement, not unlike that seen with polyethylene wear. Again, this is not a common problem and may be related to teh specific type of cobalt-chrome metal used.
Other concerns include the potential risks of high metal ion levels to an unborn fetus if used in a woman of childbearing age. Many of these risks are only theoretical and it will take years of follow-up studies to definitively determine whether any detrimental effect of metal ions outweigh the benefits of metal-on-metal total hip replacements.