Chicago ? As Americans live longer, exercise more and become increasingly obese, the number of artificial joints that will be needed to replace damaged knees will jump 673 percent by 2030 to 3.48 million a year, according to a new study.

The need for artificial hips will climb 174 percent over the same period to 572,000 a year, said Steven M. Kurtz, of Exponent Inc., a Philadelphia-based engineering and scientific consulting firm. The study is to be presented today at the American Academy of Orthopedic Surgeons annual meeting in Chicago.

The huge increase in artificial joints is the result of two converging trends – more people suffering wear and tear on their joints in a condition called osteoarthritis, and improvements in artificial joint technology that dramatically relieve pain and disability, experts say.

“A generation ago, when artificial joints were not available, individuals that had joint problems were crippled,” said Dr. Joshua J. Jacobs, associate chairman of orthopedic surgery at Rush University Medical Center. “They couldn’t participate in the work force and there was an enormous economic impact.

“Now that these artificial joints are available people can be returned to the work force and returned to a higher quality of life,” said Jacobs, who was not involved in the study.

Nine out of 10 people receiving an artificial knee replacement report fast pain relief, improved mobility and better quality of life, according to a 2003 National Institutes of Health report.

“Five to 10 years ago people who got into their 70s would think they were maybe too old for a joint replacement. Now you never hear that,” said Dr. David Stulberg, an orthopedic surgeon at Northwestern University Feinberg School of Medicine. “People have become more comfortable that these things are going to last a long time, even when used for aggressive activities.”

Joints normally sustain wear over time. The cartilage cushion between bones gradually erodes, causing pain when the bones rub together. This process is exacerbated by vigorous exercise or sports activities, which may injure joints, and putting on excessive pounds, which places greater pressure on joints. Artificial replacements are made of metal and plastic.