By Robert Ashley, M.D.

A: "Worn out" is a good way to term what happens to the knee joint with prolonged use. Let's look at how this happens, starting with cartilage.

The lower portion of the knee joint (at the tibia) contains shock absorbers called menisci made of cartilage. You have one on the inner portion and another on the outer portion of each knee. The upper portion of the knee joint (at the femur) is lined with cartilage as well. All of this cartilage helps protect the bones at the joint but it doesn't heal or regenerate well due to limited blood supply. When severe, worn cartilage leads to arthritis of the knee. In knee X-rays of people over the age of 60, 37 percent have shown evidence of arthritis of the knees.

The intriguing thing about stem cells is that they have the ability to become any type of cell that the body needs. The cells used for stem cell injections in the knees are called mesenchymal stem cells, and they can differentiate into bone, fat or cartilage cells. These stem cells can come from the fat cells of your body, from your bone marrow or from the inner lining of your knee joint; they're then replicated in the laboratory and injected into the knee joint.

Here's what the research shows so far ...

In a 2013 study, 32 patients with meniscal tears of the knee were injected with a combination of stem cells, platelet-rich plasma and hyaluronic acid. The study reported improved symptoms and even MRI evidence of meniscal cartilage regeneration.

In a 2014 study, 55 patients who had surgery for meniscal tears of the knees were separated into three groups, with two of the groups receiving stem cell injections. Researchers found that, after six weeks, pain had decreased substantially in the two groups that received stem cell injections and that the decrease was even greater at one and two years after the injection.

In a 2017 study in the British Journal of Sports Medicine, researchers analyzed six studies that used stem cells for osteoarthritis of the knees. In five of the studies, stem cells were given after surgery to the knee; in the other study, stem cells from a donor were administered without surgery. All the studies showed reduced pain and improved knee function. Further, in three of the four trials, MRIs corroborated the cartilage improvements. However, the authors noted, five of the six studies were of such poor methodology that an overall conclusion about the stem cells' effectiveness could not be made.

In all these studies, the most common side effect was knee swelling and stiffness, which improved over time.

There may be benefit to stem cell injections for cartilage loss of the knees, but more data are needed, especially in those who aren't having surgery of the knee. I'd also like to see more data on this type of therapy as a preventive measure for younger patients before their knees are worn out.

Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles. Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.

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