Although the procedures are still mostly performed on people over 65, there is a growing population of individuals who range from 38 to 56 who are getting hip and knee implants or prostheses.

According to the American Academy of Orthopaedic Surgeons, individuals aged mid-thirties to late fifties had 35,000 hip replacements or 21 percent of the total procedures in 2003, the last year for which figures are available. That's up from 26 percent of all procedures in 1997.

Additionally individuals in this age group had 48,000 knee replacements or 15 percent of the total 2003, up from 22 percent in 1997.

Many doctors estimate that over 90 percent of joint replacements are done because of osteoarthritis, which affects nearly 21 million people and is the most common form of arthritis. Osteoarthritis is a degenerative disease characterized by the breakdown of a joint's cartilage, and is caused by a variety of factors including injuries, obesity and genetics. The breakdown causes bones to rub against each other, resulting in pain and loss of movement. The incidence of arthritis increases as people age.

In 2002, the latest year that statistics are available, the U.S. Centers for Disease Control and Prevention (CDC) estimated that 43 million adults reported being told by a doctor that they have some form of arthritis, rheumatoid arthritis, gout, lupus, or fibromyalgia.

Also 23 million adults reported chronic joint symptoms (possible arthritis) but had not been diagnosed with arthritis. One in five, or 21%, of adults report having doctor-diagnosed arthritis. In total 51% of adults 75 years and over reported an arthritis diagnosis.

The CDC estimates that by 2030, approximately 64.9 million of Americans aged 18 years or older will have doctor-diagnosed arthritis.

A decade or so ago, doctors and patients chose to put off implants as long as possible because prostheses would only last about 10 years and replacement surgery becomes less effective and more dangerous each time it is done. Now some prostheses are expected to last 25 years, but there are still risks associated with implants and with the surgery in general.

Artificial joints or prostheses are medical devices; they must be cleared or approved by the Food and Drug Administration (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. The product or device must be proven safe and effective.

The FDA assures safety and effectiveness through 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 deployed. The general controls are augmented with special controls such as standards or standard test methods.

However, for devices that involve new uses or advanced technology the FDA often requires that a particular device be demonstrated to be safe and effective through clinical trials. As has been proven with some devices in the past these clinical trials do not always identify all of the potential issues that may be associated with a new type of hip or knee prostheses.

In addition just 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 also 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.

An individual's choice of device and surgeon to repair hip or knee injuries is critical to avoiding serious future complications. If problems relating to prosthesis exist the person should in many cases seek legal advice.