The most commonly used FDA-approved joint prostheses for knees and hips are made of metal and plastic. The metal is usually titanium or a mixture of cobalt and chromium. The plastic is a high-density polyethylene.
Although the metal in a prosthesis is highly polished and the polyethylene is intended to be wear-resistant, the daily rubbing of these surfaces against each other during normal movements creates tiny particles of debris. After many years, these wear particles may damage the surrounding bone, loosen the prosthesis, and require another knee or hip joint replacement.
"The 'Achilles tendon' of any artificial joint over the long term is wear," says Anderson Orthopaedic's Engh. "Any time you have parts moving against each other, there has to be wear."
In an effort to solve the wear problem of metal-on-polyethylene in the hip joint, manufacturers have produced hip prostheses with three other kinds of surfaces: metal-on-metal, ceramic-on-polyethylene, and ceramic-on-ceramic. Unlike the clay ceramic used in pottery, the ceramic used in hip joint replacements is made from aluminum or zirconium chemically combined with oxygen for strength and durability.
Metal-on-metal and ceramic hip prostheses are decades old, but modern materials, designs, and manufacturing methods have improved upon earlier versions, says Engh. He cautions that, although modern investigational products have shown good wear in mechanical simulations in the laboratory, it's how well they work in people over the long term that is the real test. "Very often it's best to select an implant that's been on the market for a while rather than something that's brand new," says Engh.
A few metal-on-metal and ceramic-on-ceramic hip prostheses are FDA-approved for use in the general population; others are approved only for use in carefully controlled studies. However, a large number of ceramic-on-polyethylene prostheses are available for use in the general population.
When choosing a prosthesis, the surgeon will consider many factors, including the patient's age, weight, gender, anatomy, activity level, medical history and general health, says A. Seth Greenwald, D.Phil., director of orthopaedic research and education at the Lutheran Hospital in Cleveland, part of the Cleveland Clinic Health System. The device's performance record and the surgeon's own experience with the device also will be considered.
"All You Need to Know About Joint Surgery," © 2002, Arthritis Foundation
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