Your high school soccer career ended with a torn anterior cruciate ligament (ACL) and knee surgery. Now, 20 years later, you’re experiencing increasing pain in that same knee and the doctor diagnoses the problem as post-traumatic osteoarthritis (PTOA).

“How can I have arthritis?” you groan. “I’m only 35 years old!”

This kind of case is not unusual. Nearly 4 million Americans are estimated to have post-traumatic osteoarthritis (PTOA). Many suffered ACL tears or other joint injuries early in life, and despite reconstructive surgery, later developed PTOA.

PTOA is a form of osteoarthritis (OA). OA affects about 33 million American adults and is the most common form of arthritis.

“OA is a huge source of disability and a huge part of our national expenditure for health-related issues,” said orthopedic surgeon Andrew Spitzer, MD.

The cause is a degeneration of the articular cartilage that sits between bones to help them move smoothly against one another.

“Healthy cartilage has a coefficient of friction lower than anything else known to man,” said Spitzer. “It's very smooth and pristine, white and shiny.”

It can become damaged over time, making age a significant risk factor. Almost half of all OA sufferers are over the age of 65.

But an early injury to a joint—such as an ACL tear, meniscal tear or fracture—can lead to the much earlier onset of OA known as PTOA. About 12% of all osteoarthritis is estimated to be post-traumatic. In fact, more than 50% of patients who suffer an early ACL tear are found to have PTOA 20 years after reconstruction.

“Anything that disrupts the pristine interior of the joint—whether it’s a meniscal injury or a ligamentous injury or something more serious, like a broken bone—can change the way the bones glide upon one another,” explained Spitzer. “And that can change the wear patterns on the cartilage and accelerate the degeneration of the cartilage, leading ultimately to arthritis.”

Read: Breakthrough Technology Can Help Heal ACL Injuries

There is currently no treatment that can repair OA. There are medications and injections to help relieve pain and inflammation, and physical therapy can help stabilize and support the joint. But once the cartilage is damaged, it has a hard time healing itself.

“Unlike muscles and bones, cartilage has no blood supply,” explained Spitzer. “So, its healing capacity is very limited. When it does heal, it heals to fibrocartilage or fibrous tissue rather than the smooth natural cartilage.”

About 50% of patients with knee OA will resort to knee replacement surgery at some point in their lifetime.

Read: Slowing the Progression of Osteoarthritis – What Works, What Doesn’t and What’s Next?

There may be some light on the horizon. Dmitriy Sheyn, PhD, research scientist at the Cedars-Sinai Board of Governors Regenerative Medicine Institute, is working on a treatment to keep patients from developing PTOA, or at least delay its onset.

Sheyn says inflammation, the body’s immune response to injury and surgery, is a significant contributor to the development of PTOA—so is the introduction of blood and other cells into the knee joint. To combat this problem, Sheyn is growing cells called macrophages in his lab.

“We know that macrophages have a role in both inducing and reducing inflammation,” he said. “And to clean up the mess after injury.”

Sheyn is using induced pluripotent stem cells (iPSCs)—cells that can be reprogrammed to become any cell in the body—to create exactly the right kind of macrophages for the job.

“We have to control them to be anti-inflammatory and to physically remove the factors and triggers for inflammation.”

Sheyn has been testing his macrophages in animal trials and in vitro with human cells and is optimistic about the results so far.

“We can see the effect of the macrophages slowing the development of PTOA,” he said.

Read: Regenerative Medicine for Orthopedics

There may be further hope for millions of PTOA sufferers in the future. Work is being done in regenerative medicine labs to build brand new replacement cartilage using iPSCs.

“Attempts at that for either cartilage or meniscal injuries have been fraught with all sorts of challenges and have not yet been successful,” said Spitzer. “But certainly a regenerative medicine intervention that would restore a more normal anatomy to the damaged joint would really be the holy grail.”