Study reveals the body's systemic stem cell response to ACL injury

You might think stem cells only exist inside a fetus, but your adult body has a stockpile of stem cells, armed and ready to respond. These remarkable cells can develop into any other type of cell, like muscle or bone or nerve cells.

Researchers know heart attacks and strokes summon these cells. They flock to your heart or brain from all over your body to help you stay alive.

But, scientists did not realize other injuries, like a torn ACL of the knee, could command the army of stem cells to deploy.

Kevin Baker, Ph.D., Beaumont director of Orthopedic Research, conducted a study with Beaumont orthopedic surgeon Kyle Anderson, M.D., and others that revealed ACL tears send a signal to stem cells throughout our body.

After an ACL tear, Dr. Baker and his colleagues found a six-fold increase in stem cells circulating around the knee, similar to the body's response to a major, life-threatening event like a stroke or heart attack.

However, when the stem cells arrive to help regenerate and repair the injured ligament, they get stuck. They can't get through the thick membrane that surrounds the knee joint.

"We think this discovery will help us to understand how the body responds to an ACL injury, and also how post-traumatic osteoarthritis develops after a joint injury," Dr. Anderson said.

Post-traumatic osteoarthritis is a form of arthritis that develops after a knee injury. It's a common injury that affects veterans, athletes and anyone who puts stress and strain on their knees. But, until now, little was known about how the body attempts to heal these injuries.

As we age, the number of stem cells in our body declines. "This could explain why your knee joint doesn't heal as well after a trauma when you are older," Dr. Baker said.

Osteoarthritis affects more than 30 million adults in the United States, according to the Centers for Disease Control and Prevention, and many of these cases occur after trauma to a joint. It's also a leading cause of disability.

"The next step of our research will be finding methods to get the stem cells inside the joint. If the stem cells can get through the membrane around the knee, they could help speed up the healing process and perhaps delay or prevent arthritis," Dr. Baker added.

The study, funded in part by the American Orthopedic Society of Sports Medicine, is entitled, "Acute mobilization and migration of bone marrow-derived stem cells following anterior cruciate ligament rupture." The authors believe it is the first study of its kind to reveal the body's systemic stem cell response to an ACL injury.