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Where the FDA Fits In

Artificial joints are medical devices, which must be cleared or approved by the FDA before they can be marketed in the United States. In addition, FDA permission is required before a company can test a new or redesigned prosthesis in human studies. The data gathered in these studies, which take place in specific hospitals, may then be used to support a company's application for marketing its prosthesis to surgeons and hospitals throughout the United States.

What does the agency look for before clearing a prosthesis for marketing? "It has to be proven safe and effective," says Barbara Zimmerman, chief of the FDA's orthopedic devices branch. "FDA assures safety and effectiveness using different means depending on the risks of a particular device and the technology that it presents."

For devices with a history of safe and effective use, frequently those using established technology, the FDA relies on a set of general controls to determine which devices can be marketed, says Zimmerman. "These general controls are augmented with special controls such as standards or standard test methods.

"For devices involving new uses or advanced technology, FDA often requires that a particular device be demonstrated to be safe and effective through clinical trials," she says.

The Risks of Replacement

Like any surgery, hip and knee joint replacement carries certain life-threatening risks, such as infection, blood clots and complications from anesthesia. Other complications include nerve damage, dislocation or breakage after surgery, and wearing out or loosening of the joint over time. After hip replacement surgery, one leg may be shorter than the other.

Infection is an ongoing risk for people with joint replacements. Not only can it occur in the hospital, but it can happen years later if bacteria travel through the bloodstream to the replacement area.

In the rare case that an infection spreads to the new joint and does not clear up with antibiotic treatment, the joint must be replaced. This usually requires two surgeries — one to remove the infected joint and another surgery later to insert the new joint. Between surgeries, the infection is treated with antibiotics.

In 2001, the FDA approved a temporary artificial hip for people with hip joint infection. The temporary hip, called Prostalac, can be inserted and left in place for about three months after the infected hip is removed. It consists of a metal stem and ball that fits into the thighbone, a plastic cup that attaches to the hipbone, and a bone cement that contains antibiotics. The antibiotics in the cement, along with oral antibiotics taken by the patient, help to treat the infection. The temporary hip allows a person some movement while healing.

The Wear Problem

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.

Tags: Health

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.