Image: Peter Ryan

In 2005, at the age of 32, then Los Angeles Angel Bartolo Coln won the American League Cy Young Award for best pitcher, one of professional baseball's top honors. He stumbled through subsequent seasons, however, after a series of rips and strains in the tendons and ligaments of his throwing arm, shoulder and back. In 2009 he all but quit baseball. Desperate to reclaim his career, Coln flew home to the Dominican Republic in 2010 for an experimental procedure not vetted or approved by the U.S. Food and Drug Administration. Doctors centrifuged samples of Coln's bone marrow and fat, skimmed off a slurry containing a particular kind of stem cellimmature, self-renewing cells that can turn into a variety of tissuesand injected it into his injured shoulder and elbow. Within months of the procedure the then 37-year-old Coln was once again pitching near the top of his game for the New York Yankeescommanding a 93-mile-per-hour fastball.

Whether the injected stem cells rejuvenated his arm is an open question. The fda and the International Society for Stem Cell Research warn that no rigorous studies have demonstrated that such treatments safely and effectively repair damaged connective tissue in people. The results of related animal studies, though promising, have raised more questions than answers. The term stem cell makes it sound cutting edge and exciting, says Paul Knoepfler, a cell biologist at the University of California, Davis, who also writes frequently on policy surrounding stem cells. But the role of these cells in sports medicine is essentially all hype.

No matter, apparently, to the aging, injured athletes who have followed Coln's lead. Lefty pitcher C. J. Nitkowski, who underwent the same procedure in 2011, told readers of his personal blog that he did not mind the lack of carefully controlled research. My attitude is I don't have the time to wait for the five- or 10-year study to come out, the then 38-year-old relief pitcher wrote, so I'm taking a chance now. Besides, Nitkowski figured, even if the treatment did not work, any health risks ought to be slight because the cells involved were his own.

That might not be such a safe bet. Numerous studies suggest that Coln, Nitkowski and others trying untested stem cell treatments may be risking more than they think. Even a syringe of one's own stem cells taken from one part of the body and squirted into another may multiply, form tumors, or may leave the site you put them in and migrate somewhere else the fda warns on its Web site. More clinical research is needed to define safety procedures, as well as how many cells of which types and what other tissue factors produce the desired results. In some animal studies, for example, the regenerated tissue is not as strong or flexible as the original. In other cases, an overgrowth of scar tissue makes the injected tendon or ligament adhere to the overlying skin. By preventing different tissues from gracefully sliding past one another, these adhesions sometimes pull an even bigger tear in an already serious wound.

In addition, Knoepfler worries that high-profile sports testimonials by Coln, Nitkowski and others will encourage joggers with blown-out knees and the parents of sore-armed Little Leaguers to demand the procedure before it has been thoroughly tested. When celebrities take to a new treatment, many other people follow suit, he says. Such premature enthusiasmor an unforeseen tragedy that results from proceeding too fast too sooncould also prevent serious researchers from getting funding to do the kinds of careful experiments that might eventually lead to safe and reliable treatments.

Seeds of Repair

The need for better ways to reknit damaged tendons and ligaments is painfully apparent to the roughly two million Americans in a given year who seek medical help for tears in their shoulder's rotator cuff, for example, or the 100,000 patients in the same year who undergo surgery in the U.S. to repair a ripped or ruptured anterior cruciate ligament (ACL) of the knee. Tendons and ligaments are tough, fibrous bands, made mostly of collagen, that anchor networks of muscles to a bone or link bones and cartilage across crucial joints. They lend strength, flexibility and stability to your daily twists and turns, whether you are rocketing a baseball across home plate or hefting a suitcase into an overhead bin. Once frayed or snapped, they can take many months or longer to mendeven with surgery.

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A Dangerous Game: Some Athletes Risk Untested Stem Cell Treatments