Doctors injected stem cells into pitcher Bartolo Colon’s injured arm.

Doctors extracted fat and bone marrow stem cells from baseball pitcher Bartolo Colon and implanted them in his injured shoulder and elbow in April 2010 to help rebuild tissue. The 37-year-old Yankee is now pitching like he did when he was a younger man, leading some to question whether banned substances, such as human growth hormone, were involved. Can injecting some stem cells into a damaged joint really help that much?

Maybe. In recent years, orthopedists have been tinkering with the use of stem cells to help heal fractures, replace worn cartilage, and repair torn ligaments. As David Epstein explained in Sports Illustrated, the most popular technique is called micro-fracture surgery, in which the surgeon drills holes in the bone and allows the marrow to seep out into the area of the injury. The hope is that the stem cells in the bone marrow will release a cocktail of proteins and other chemicals to stimulate tissue repair. Researchers recently postulated that, if micro-fracture accelerates recovery, an even bigger shot of stem cells might speed it further. So they began experimenting with extracting bone marrow, centrifuging it to concentrate the stem cells, and injecting them into the injured joint. Implanting stem cells, either as the only treatment or in conjunction with micro-fracture surgery, has shown promise in a variety of animals, including horses and goats. But there have been very few trials on human patients, and it’s considered experimental at this point.

The few human trials that have been performed have yielded underwhelming results. The Maryland-based biotech company Osiris Therapeutics collected 55 patients with injured menisci, the disk of cartilage in the knee. After removing the damaged cartilage, they injected Chondrogen, the company’s special brew of stem cells, into some of the patients’ knees. After a recovery period, the patients who got the stem cells reported less pain than the control group. That’s nice, but the more important goal was regenerating cartilage, and there was no evidence of that.

Shouldn’t the animal trials be enough to prove the technique’s efficacy? Not really. Horses, for example, have thicker cartilage than humans, which may alter the healing process. They also get arthritis, one of the main disorders in the trials, in different places than humans do.

In fairness to Colon’s doctors, convincing evidence can be hard to come by for any surgical procedure. Unlike in animal studies, in which doctors can surgically impose the same injury on all the patients, every human injury is a little different. The human patients’ behavior after surgery, including their commitment to rehab, can vary widely. Surgeons are famously individualistic, each putting their own personal twist on the procedure. Some are also just better than others. All these factors combine to make it difficult to coordinate large-scale, well-controlled trials.

As for the murmurs about Colon’s surgeon including HGH along with the stem cells, there’s no evidence at this time. But the principle behind the two therapies is the same. HGH is a single- ingredient growth-promoter. Stem cells release a wide variety of growth-promoting chemicals. That diversity might, in theory, offer more benefit. Stem cells are also smarter than HGH, because they can respond to their environment. For example, if there is significant inflammation in an injured joint, the stem cells seem to ramp up their production of anti- inflammatory chemicals.

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