What's more, most study participants showed no further cartilage loss a year after the PRP injection. Previous studies have found that people with knee OA tend to lose around 5 percent of cartilage per year.  

Dr. Halpern stresses that the positive results occurred in people with early OA. "You're not going to be able to do a lot in the regenerative sense for people with bone-on-bone arthritis. By then, the horse is already out of the barn," he says. "But in the earlier stages, you can improve symptoms and the environmental milieu enough to delay or maybe even prevent knee replacement. But that's a long way off and we need to look at a lot more numbers [before we can say that with certainty]."

Still, Dr. Halpern says the study results have prompted HSS to create a registry that will track future osteoarthritis patients over time. Researchers will be able to compare outcomes for various OA treatments, such as exercise, weight control, bracing and hyaluronic acid injections as well as PRP. And for the first time, imaging tests will be used to detect changes in joint cartilage.

It's hoped that the HSS data will help answer some of the many questions that arise as PRP treatments become more common.

"Many more folks are doing it now, especially sports medicine orthopaedists, and they are learning more about why it can and can't work," says Dr. Halpern, noting that he has injected more than 400 patients with PRP over the last five years and all had outcomes consistent with the study results.

"One example is that it appears the platelet concentration has an effect on efficacy, but we don't know what the optimum [level] is right now. That's another thing that has to be explored,” he says. “By definition, PRP formulations have to be two to five times greater than the platelet concentration in the blood. But it seems that if platelet concentration gets as high as eight times greater, it can actually have a deleterious effect."

Dr. Halpern stresses that PRP is not a panacea. "It certainly won't help everybody with everything," he notes. "This is an evolving field, and we need to learn much more."

Eric L. Matteson, MD, chair of rheumatology at the Mayo Clinic in Rochester, Minnesota, agrees that PRP needs more study.

"The real issue from a biological standpoint is whether PRP contains factors that can somehow stimulate cartilage and in so doing improve arthritis. There has to be some demonstration of actual effect. If there is improvement in cartilage, we should be able to see it, but this study didn't show that,” says Dr. Matteson. “And if MRI didn't show an improvement, then what accounts for the decrease in pain? Perhaps the placebo effect. We really have no idea. We need studies that show the biological plausibility of this treatment."

Orthopaedic surgeon Jason Scopp, MD, director of the Joint Preservation Center at Peninsula Orthopaedic Associates in Salisbury, Md., says that in addition to failing to show biological plausibility, the HSS study has other shortcomings.

"This is a very small sample size, which means the study is underpowered, and there is no control group for comparison. Other studies of PRP have enrolled more patients, are better powered and compare PRP to viscosupplementation [hyaluronic acid injections]," he notes.

Still, despite what he sees as the limitations of this study, Dr. Scopp is a fan of PRP. "PRP is a great product, and several studies have shown it to be superior to cortisone and even viscosupplementation, but there is still no consensus on the best preparation, best volume of injections and number of injections," he says.

Dr. Matteson agrees that PRP procedures need to be standardized. "There is a whole conglomeration of stuff out there," Dr. Matteson says, "and we need to sort out what may or may not have a biological effect."