Both metal on metal and ceramic on ceramic are collectively known as “alternate bearings”. To explain what is an alternate bearing, a brief discussion on what is a “normal” or “standard” bearing in terms of hip replacement.
The hip is a ball and socket joint. In a normal joint, both the end of the femur (thigh bone) and the portion of the pelvis called the acetabulum (the socket) are covered with cartilage. This is the glistening white stuff on the end of the chicken bone. Normal cartilage serves to absorb the shock of normal activities and is also very slippery. Normal cartilage on normal cartilage in a joint is five time more slippery than ice on ice.
In an arthritic hip, the cartilage surface is lost, and bone rubs on bone. In most hip replacements, the ball is made of metal and the surface of the socket is made of a special plastic. The plastic is the weak point link in the system. Eventually, the plastic wears out and needs replacement. Traditional plastics lasted about ten years in normal use. The plastics employed now are expected to last significantly longer.
Alternate bearings were designed to try to solve the problem of the plastic wearing out. Each has its own problems, some real, some only in theory. At this time, Dr. Harris usually recommends the “devil that we know” over the “devil that we don’t know”. As greater experience with these newer bearing materials becomes available, this page may change! The discussion in the office may have more up to date information, as Dr. Harris is also the webmaster.
While not truly an “alternate bearing”, modern plastics have advanced a great deal from the early days of arthroplasty. Today’s plastics are highly crosslinked, devoid of free-radicals, and are sterilized in oxygen free environments. Each company claims that it’s plastic is better than all of their competitors. It is not clear whose is best. Stryker claims to have the “newest and longest lasting plastic on the market.” This is the least likely to be accurate.
Metal on Metal is a complex issue. There are many design issues that are not yet optimized. For example, the radius of the ball cannot be exactly the same as the radius of the cup. If they were, the hip would seize like an automobile engine without oil. In general, the smaller the difference, the better. However, the manufacturing limits (how close the actual part is compared with the design) are similar to the best known differential. Also issues such as the best type of metal (alloy) is not known. Lastly, there is the question of metal ions. All patients with artificial hips have detectable levels of metal ions in their blood and in their tissues. Those with metal on metal have much much higher levels. Included in these alloys are cobalt and nickel. Both potential carcinogens. While carcinogenesis has not been proven, neither has it been disproved. More recently, there are several reports of a localized allergic reaction causing pain and loosening of the prosthesis. Further surgery is needed in these patients. It is not yet known if the reaction would carry over to the next prosthesis. It would make sense that the subsequent prostheses would come loose early also.
Dr. Joshua J. Jacobs is considered by many (Dr. Harris included) to be the “guru” of metal ion research. At this time, Dr. Jacobs still advocates considerable caution with regard to metal on metal replacements. At present, Dr. Harris believes that metal on metal should be reserved for a relatively small group of patients.
There are three issues with ceramic on ceramic bearings. Briefly, these are the brittle nature of the components, implantation tolerances, and the limited implant sizes. The first question really should be “what is a ceramic”? Most dinner plates are ceramic. While they are very hard, drop most dinner plates, and they shatter. All of that debris is very sharp. If the ceramic hip fractures, this debris gets into the surrounding muscles and other tissues. Getting it out is a bit like trying to get sand out of a sponge. You never get all of it out. That which is left will cause third body wear. Essentially, sand in the engine. This may make the success of subsequent replacements much lower.
Reported fracture rates, information from the companies that sell these products,varies widely. The actual rate may vary with many factors not completely understood. Until recently, ceramic heads were limited in size. Fracture rates may be substantially lower for larger heads. The larger heads may represent an acceptable risk.
Ceramic hips are also more susceptible to edge impingement than other designs. Edge impingement is when the side of the “neck of the femur” contacts the side of the cup. The neck of the femur is always made of metal. Metal striking the ceramic is a risk factor for fracture of the ceramic cup. Some modern designs bury the cup under a metal rim. Then, the potential range of motion is diminished. At extremes, the metal neck strikes the metal cup sometimes causing notching of the neck. This can lead to fracture of the stem, forcing revision. Others employ larger ceramic heads, improving motion and diminishing the potential for impingement.
Some ceramic on ceramic hips squeak. People, with these hips can be heard, often from some distance, as they walk. In order to generate a squeak, there must be friction. This indicates a failure of one of the key reasons to use alternate bearing, namely to decrease friction. The source of this squeaking is now well understood. The joint bearing surface becomes a high friction energy source. The energy generated may resonate in the audible range depending on the design of other parts. Further, it occurs more often in certain designs or in hips made by certain manufacturers. Hips made by Stryker seem to squeak more often than do hips from DePuy or Wright Medical. Whether the squeaking occurs in the audible range, the energy transfer from the high friction in the joint to the adjacent is a cause for much concern.
Ceramic on polyethylene or Oxinium (a metal/ceramic hybrid) may offer lower wear and longer life of a total hip arthroplasty compared to metal on polyethylene. When used in conjunction with modern plastics, though, the differences may be small. Lastly, ceramic on metal is a promising combination, but is not yet approved by the FDA.
A diamond on diamond prosthesis is also under development. At present it is cost prohibitive. It is not expected to be on the market any time soon.
Again, the bottom line on alternate bearings is that the jury is out. For some, the risks associated with metal on metal bearings may be worthwhile. At present, Dr. Harris is very cautious about ceramic on ceramic. Some ceramic materials that may be available soon might make the risks associated with ceramics worth the potential benefit. For most patients, at present, Dr. Harris recommends the “devil that we know” over the “devil that we don’t know”, and he implants metal or large head ceramic on the current generation of plastics.
