This year, Kansas City Chiefs star running back Jamaal Charles went down with a torn ACL in his right knee, ending his season. To recover, he opted to use stem-cell treatment because he believed it'll help him return to the field quicker.

Charles is one of dozens of NFL players in recent years to use stem cell treatment to heal injuries. Chris Johnson, a former running back for the Tennessee Titans, tore his meniscus during the first month of the 2013 NFL season. Unlike Charles, he continued playing but had renowned orthopedic surgeon James Andrews fix the meniscus when the season ended. Andrews's advice to Johnson following the surgery was to use adult stem cells.

Charles is one of dozens of NFL players to use stem cell treatment.

"When I tore my meniscus and played the season out, through the wear and tear, I lost a lot of cartilage," said Johnson. "When you put the stem cells in, it might be able to help rebuild that cartilage in your knee. Hopefully, it makes your knee better for even more years."

To complete the surgery, Andrews removed about 60 milliliters of bone marrow—about the volume of a shot glass—out of the iliac crest of his pelvis. He used a long needle pushed through a tiny incision in Johnson's skin. The procedure took about an hour.

For Charles, the procedure was similar. Doctor James Andrews took a small amount of his bone marrow, extracted the stem cells and then injected the cells into his injured knee. He had the procedure done again six weeks later.