Washington ? Doctors are testing new ways to spur cartilage to regrow in damaged knees, from implanted “cartilage plugs” to injections of bone-marrow stem cells.

The need is huge. Knees are the joint most likely to go bad, and the cartilage that cushions them has only a limited natural ability to repair itself.

The question is how to unlock that ability and give it a boost.

The potential new options are being tried first in people who injured their knees and thus need small amounts of cartilage to regrow. But if they truly work, the techniques one day might offer hope for arthritis sufferers, too, whose cartilage over time completely erodes.

Most eagerly anticipated: the first clinical trial using stem cells from donated bone marrow to try to regenerate the knee’s shock absorber, a cartilage pad called the meniscus.

Meniscus injuries are common and not just among young athletes. Because the pad weakens with age, a simple wrong step can leave an older person with the painful tear. About 800,000 Americans per year have part of the meniscus surgically removed, to relieve pain when jagged edges won’t heal.

Stem cells are building blocks for tissue. Mesenchymal stem cells are a type of adult stem cell that live in the bone marrow and can transform into cartilage-forming cells called chondrocytes. Mixing these stem cells with a knee-lubricating fluid helped the meniscus regrow in goats. Will it work in people?

Researchers enrolled 55 patients who had about half their meniscus removed. About a week after that surgery, each participant received a single injection – either a dummy shot or one containing millions of mesenchymal stem cells.

With a high-powered MRI machine, scientists now are measuring each patient’s remaining meniscus, for signs of regrowth.

“No one’s ever looked at the meniscus in terms of volume,” said Dr. C. Thomas Vangsness Jr. of the University of Southern California, lead researcher for the study funded by stem cell producer Osiris Therapeutics. “It’s very interesting what I’m seeing.”

While he reports no safety problems, he can’t say yet if the stem cells are working because the study still is blinded, meaning researchers don’t yet know which patients got the real injections. Initial results are due in October.

“It’s very, very exciting research,” said Dr. David C. Johnson, an orthopedic surgeon and sports medicine specialist at Washington Hospital Center in the nation’s capital, who is closely watching the work.