As the U.S. population ages and continues packing on the pounds, knee replacement surgeries are becoming increasingly common. More than 650,000 total knee replacements were performed in 2008 (according to the latest data available).

And as materials and surgical technologies improve, the promise of newer and better implants is making the procedure even more appealing to the millions of people who suffer from arthritis of the knee. Many of these implants have yet to prove their mettle when it comes to long-term efficacy and safety, however, according to a new report published online Monday in The Lancet.

"In the past 40 years, the number of implants available on the market has substantially proliferated, often with little or no evidence of effectiveness or cost-effectiveness," noted the authors of the report, led by Andrew Carr, of the University of Oxford's Department of Orthopedics, Rheumatology and Musculoskeletal Sciences. He and his co-authors reported that there simply isn't enough data to be sure implants—especially new designs—provide the most pain relief and hold up well to wear and tear.

Unlike new medications, knee replacements and other implants often get far less scrutiny from regulatory agencies, such as the U.S. Food and Drug Administration. A 2011 report from the Institute of Medicine found that medical devices have long lacked adequate safety and efficacy testing. Information on the safety or success of knee replacement implants, in particular, comes primarily from small studies done by individual surgeons who might be the inventor or co-owner of the technology. That connection can lead to bias and conflict of interest in reporting results. "Without high-quality, unbiased and reliable information, surgeons can not make informed decisions," Carr said in a prepared statement.

If artificial knee implants aren't working well, doctors and patients should know, Carr noted. So should the public: In 2008 alone, full-knee replacement procedures cost the U.S. health system more than $10.4 billion.

With so much money—and so many knees—at stake, "widespread surveillance of existing implants is urgently needed alongside the carefully monitored introduction of new implant designs," the researchers wrote in the Lancet paper.

Close scrutiny is all the more important because the demand for knee replacements is expected to increase. Most total knee replacements are recommended for people with osteoarthritis who have severe pain that is hampering their daily lives, although specific criteria for candidates for the procedure are lacking. The main two causes for such pain are age and obesity—both of which are on the rise in the U.S. and across the globe.

Successful surgery can vastly improve a person's mobility and quality of life. "The outcome of modern knee replacement is very good and continues to improve," the researchers wrote. And by many counts, some 80 percent of total knee replacements leave patients relatively pain free and without serious problems.

But complications such as infection, implant wear or malfunction are not uncommon. Carr and his co-authors argue that doctors and patients need to be more familiar with the risks and benefits of the procedure—and the vast array of implant options. They noted that implants should be tested, like drugs, in large randomized controlled studies and that countries should continue to create long-term registries for the implants so that their track records can be tracked.

In the meantime, the researchers caution that even though doctors and patients often expect that the latest technologies will automatically be better, that is not always the case. Sometimes the tried-and-true device might indeed be the best choice. And because any surgical procedure carries risks, the goal for individuals as well as societies should be to develop ways to avoid the need for knee replacements in the first place.