The companys website, loaded with impressive videos and testimonials from patients, is a major draw for aging gym rats searching the internet for relief from sore knees, shoulders, hips and backs.

Through the website, Dr. Centeno regularly criticizes other stem cell businesses, and has acted as an expert witness for injured patients suing his competitors.

The company did run afoul of the F.D.A. in 2008 over its use of cells cultured and multiplied in a lab to increase the stem cell count. After a protracted legal battle, Regenexx quit using that technique in the United States, but began offering it at a clinic on Grand Cayman.

This has always been about creating a less invasive orthopedic solution, what I call interventional orthopedics, Dr. Centeno said. He predicts a sea change in orthopedics similar to the revolution in cardiology, where much open-heart surgery was replaced by less invasive procedures.

While regulators may not consider them high risk, stem cell treatments involving bone marrow are not trivial. Collecting bone marrow involves forcefully puncturing the back of the hip bones in several spots, a painful process that requires local anesthesia. Then, pressure is applied to prevent bleeding, and the sites are bandaged to prevent infection.

Injecting the bone marrow or platelet extracts into the knee takes skill, even with X-rays to guide the needle. The injections can cause pain and irritation, and patients are usually sent home with leg braces that they will wear for a few weeks.

Sterile techniques are essential.

Whenever injections are administered to the joint, there is always a risk of introducing infection, said Dr. Kiran M. Perkins, who has investigated such illnesses at the Centers for Disease Control and Prevention. With stem cell treatments, she added, there are a lot of steps along the way where something could go wrong and you could have the introduction of microorganisms.

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