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| Alternative Bearing Surface Overview The bearing surfaces—the artificial femoral head (ball) and cup liner (socket)—are an important determinant of the longevity of your new prosthesis. In young, active patients, standard polyethylene cup liners used with metal femoral heads tend to wear out quickly, often within 7-10 years. Several alternative bearing surfaces are available for the young, high-activity patient. 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 about twice as well as standard polyethylene. This advantage is magnified when used with a ceramic, rather than metal, femoral head. Of note, it is still wise to exercise some caution (as with any new technology) since the longest follow-up studies currently available are 5 years. By contrast, the characteristics of standard UHMWPE are well researched. It 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 in 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 has serious drawbacks for use in total hip replacement. As standard UHMWPE wears, billions of microscopic polyethylene 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 bearing couples (the ball-head and socket liner) for total hip replacement:
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. 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. The current “third generation” of alumina ceramic 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 approximately 1 in 5,000. However, when a ceramic component fractures in the body, it requires immediate revision surgery and creates other potentially long-term problems. New ceramic options are being developed, including a new “delta” ceramic and ceramic against metal. Time will tell whether these are found to be beneficial and safe Metal on Metal Metal on metal total hips are 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.
Disclaimer:The information provided here is intended to educate the reader about certain medical conditions and certain possible treatment. It is not a substitute for examination, diagnosis, and medical care provided by a licensed and qualified health professional. If you believe you, your child, or someone you know, suffer from the conditions described herein, please see your health care provider immediately. Do not attempt to treat yourself, your child or anyone else without proper medical supervision. ©2006 Todd Swanson, M.D. The minimally invasive total hip replacement was developed by Dr. Swanson after being introduced to the concept in early 1997. Dr. Swanson began using the technique in May of 1997 and has continued to make improvements that now allow reliable surgery and reproducibly good results in most patients. Standardization of the technique has allowed teaching to other surgeons. Numerous physicians and orthopedic companies have shown interest in promoting “minimally invasive surgery” for total hip replacements. The minimally invasive technique utilizes a 3-4 inch incision, much smaller than the 8-10 inch approach historically used for hip replacement. Patients require less anesthesia and pain medication, and mobilize more quickly. This results in both quicker recovery for the patient and a lower incidence of complications. Patients typically recover from minimally invasive surgery in about half the time it takes to recover from standard incision surgery (6 weeks vs. 12 weeks). |
  
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