The resurgence of one previously out-of-work major league pitcher has increased demand for a controversial new procedure that could usher in new forms of athletic-injury treatment.

Bartolo Colon was an elite hurler for the Cleveland Indians and Anaheim Angels, before injuries and workload took their gradual toll on his right shoulder. His baseball career appeared over when he was released by the Chicago White Sox during the summer of 2009, thanks to recurring bone spurs in his elbow. His fastball speed had dropped several ticks, and for a power pitcher like Colon, that often signals the beginning of the end.

And yet Colon has emerged this season with a fastball on par with his 2007 numbers, and his strikeout rate (if it keeps up) would be the best of his career since 2000. Testing for performance-enhancing drugs has never been more rigorous in baseball, so how did this happen to a 38-year-old has-been?

What no one knew until recently — not even the New York Yankees, who signed Colon to a minimum contract last offseason — was that Colon had a semi-experimental procedure done in the spring of 2010 in the Dominican Republic.

Fat and bone-marrow stem cells were extracted from Colon and then re-injected into his elbow and shoulder, a move designed to help regenerate and repair tissue — including his rotator cuff, which had been torn.

In some circles, the use of stem cells has become commonplace in treating athletes with stress injuries and ligament damage. By “some circles,” though, I mean “those who train thoroughbred racehorses.” For several years now, thousands of racehorses around the world have been treated with stem-cell-rich injections, with some analyses suggesting a 50 percent reduction in repeat leg injuries.

(Of course, even innocuous-looking injuries can be far more fatal in horses than people, so the stakes are higher to find faster, less invasive treatment options.)

Even today, only a handful of companies engage in stem cell treatments for horses, but they’re present in some of the most equine-centric parts of the world. MediVet America, which opened in Kentucky earlier this year, is already treating horses at up to $2,400 a pop.

The practice, especially as it pertains to injured tendons, wasn’t introduced for human use until 2009, when MedCell Bioscience announced it would begin treating patients in such a way. And because the tendon and muscular systems of horses are very similar to those of humans, advances in one regimen often mean progress for the other.

Colon’s surgery and injection was overseen by doctors from Regenocyte, which is based in Florida but does most of its work in the Dominican Republic. It was there that Colon was treated under the supervision of Leonel Liriano, the company’s medical director in the Dominican Republic.

Joe Purita, an orthopedic surgeon based in Boca Raton, Florida, was part of that medical team.

“This is not hocus-pocus,” he told The New York Times. “This is the future of sports medicine, in particular. Here it is that I got a guy back playing baseball and throwing pitches at 95 miles an hour.”

Except that there still is much we don’t know. SI.com’s Will Carroll, who specializes in athletes’ injuries, says we still have more questions than answers when it comes to Colon’s surgery. He thinks we should be careful not to conflate correlation and causation:

Colon’s procedure was completely legal and doesn’t appear to even walk any ethical lines. The use of stem cells, the avoidance of HGH (which would have necessitated a waiver if used medically), and rest seem little different than platelet rich plasma, which is being used more and more in sport. The question now is whether pitchers seeking that one last chance will flock to this doctor if Colon holds up this season.

But that raises another question that hasn’t been answered yet: What was the surgery?

Injections of stem cells and other similar (and I’ll say again, legal and not banned) substances don’t require surgery. Did Colon’s doctor, an orthopedic surgeon, also perform more standard repairs on his damaged elbow and shoulder? Moreover, this surgery took place in April of 2010. Given that, his recovery time wasn’t stellar, depending on what was done. Until we know the complete story, we have no way of knowing whether the additional injections had any effect whatsoever. Then again, these are questions that pitchers with sore arms and one more shot at the major leagues probably won’t ask.

Still, even with Major League Baseball actively investigating the matter on its own, more pitchers are approaching the company that did Colon’s procedure, hoping perhaps to attempt a trip back in time themselves, courtesy of their own stem cells.

Before too many plunk down thousands of dollars for this treatment, perhaps we could get a medically viable confirmation that Colon’s stem cell treatment was the true reason behind his sudden and startling return to form.