Updated

Joint-replacement patients these days are younger and more active than ever before, The Wall Street Journal reported Tuesday, noting that half of all hip-replacement surgeries performed this year are expected to be on people under 65, with the same percentage projected for knee replacements by 2016.

The fastest-growing group is patients 46 to 64, according to the American Academy of Orthopaedic Surgery.

Many active middle-agers are wearing out their joints with marathons, triathlons, basketball and tennis and suffering osteoarthritis years earlier than previous generations. They're also determined to stay active for many more years and not let pain or disability make them sedentary.

To accommodate them, implant makers are working to build joints with longer-wearing materials, and surgeons are offering more options like partial knee replacements, hip resurfacing and minimally invasive procedures.

More younger people also need joint-replacement surgery due to obesity, and some orthopedists refer them for weight-loss surgery first to reduce complications later.

Even the most fit patients face a long period of rehabilitation after surgery and may not be able to resume high-impact activities.

"There is, to be honest, some irrational exuberance out there," says Daniel Berry, chief of orthopedic surgery at the Mayo Clinic in Rochester, Minn., and president of the American Academy of Orthopedic Surgeons. "People may be overly optimistic about what joint replacement can do for them."

One big unknown: How long will the replacement joints last? In the past, many doctors assumed implants would wear out in about 10 or 15 years, and they urged young patients to put off surgery as long as possible to minimize the risk of needing a costly and difficult revision surgery -- or even two.

Several studies of hips and knees implanted in patients long ago have found that 85 percent to 90 percent are still functioning well after 20 years. Advances in wear-resistant materials may make implants being used today last even longer.

For example, many hips and knees use a combination of metal and polyethylene parts, and continuous motion can wear away small polyethylene particles. Scientists have found that irradiating the polyethylene removes free oxygen radicals that contribute to degradation, so the polyethylene components are expected to wear much longer.

Click here to read more from the Wall Street Journal.