The fallout from nuclear bomb testing decades ago is now helping researchers better understand knee joints. By tracking radioactive carbon absorbed in knees, a team of Danish researchers has found that the structure of cartilage is determined by early adolescence and doesn’t change later in life. They published their findings Wednesday in Science Translational Medicine.

Why it matters:

Cartilage is often thought of as a human shock absorber. When it’s damaged or depleted, it can result in painful friction between bones of the joint. In 2005, the Centers for Disease Control and Prevention estimated that nearly 27 million adults in the United States were affected by osteoarthritis, a condition characterized by the progressive loss of joint cartilage.

Researchers hope understanding how cartilage forms normally can help them target the best way to treat cartilage damage. The ongoing debate over whether collagen — a fibrous component of cartilage that determines its overall structure — is renewed in adulthood will influence the type of drugs and bioengineered tissue that researchers create to address osteoarthritis and similar conditions.

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The nitty gritty:

Nuclear bomb explosions release a radioactive form of carbon, called carbon-14. A series of nuclear bomb experiments in the 1950s and 1960s caused levels of carbon-14 in the atmosphere to spike — peaking around 1964. That carbon got incorporated into developing organic matter — including our own.

Researchers measured levels of carbon-14 in cartilage taken from knee joints of people born before and after this peak. Some donors had osteoarthritis while others had healthy joints. Carbon-14 levels in the knee tissue of both groups matched the atmospheric carbon levels prevalent when the donors were preteens, suggesting permanent collagen structures are established in childhood.

You should know:

If cartilage were capable of naturally renewing itself in adulthood, treatments to amplify that process might help people with cartilage loss. But these findings suggest that such an approach will not be effective, said Peter Schjerling, a molecular biologist at the Bispebjerg Hospital in Copenhagen and one of the study’s senior authors.

“There is nothing to stimulate,” he said. It might make sense to go the other way and try to save what is left and make the remaining part stronger.”

Keep in mind:

The carbon dating method may not detect transient changes that occur during tissue repair, said Rocky Tuan, a bioengineer and cellular and molecular biologist at the University of Pittsburgh, who praised the study.

“You’d better do well why you’re young. That’s what that study is essentially telling you,” he said.

Tuan’s own research in animals and human cells suggests that collagen can be regrown at the site of injury or disease, though — in light of the new research — perhaps not enough to solve cartilage problems.

The bottom line:

Our overall cartilage structure is generally set by adolescence, with little chance to restore or grow replacements later in life.

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