If you fall on the ski slopes or slip on a patch of ice, you’ll probably be better off if you break your leg than if you rip the cartilage in your knee. Unlike bones, your cartilage is never going to regrow or heal, according to a new study based in part on fallout from past nuclear explosions.

“The surgeons who do joint replacements should not be afraid,” says study co-author and rheumatologist Michael Kjær of the University of Copenhagen. “They are going to be in business for some time.”

As plenty of athletes and weekend warriors can attest, damaged knee cartilage is reluctant to mend. But because measuring cartilage turnover is difficult, researchers have never been sure whether adults replace any of the material.

To find out, Kjær and colleagues used a technique that determines the age of molecules based on levels of the carbon-14 isotope, a hefty version of carbon. The amount of carbon-14 in the atmosphere surged in the 1950s because of above-ground nuclear weapons testing, but it declined rapidly after a 1963 treaty banned these explosions. Measuring the abundance of the isotope can reveal how old a molecule is. If the molecule is continually being replaced, it should appear young—the quantity of carbon-14 should be close to current levels in the atmosphere. But if the molecule remains stable for a long time and isn’t swapped out, its carbon-14 content should match the atmospheric levels from when it was made.

Kjær’s team measured carbon-14 levels in knee cartilage from one donated body and from 22 patients born before the year 2000 who had undergone knee replacement surgery. Some of these people were getting new knees because they suffered from osteoarthritis. Others had healthy joints but needed replacements because of bone tumors. The researchers analyzed cartilage from the middle of the knee joint, which endures the most strain, and from the edge of the joint, which carries a lighter load.

Carbon-14 levels in the subjects’ knee collagen—the protein that provides cartilage’s tensile strength—corresponded to atmospheric levels from when they were 8 to 13 years old, suggesting that they didn’t produce new collagen after they became adults, the team reports online today in Science Translational Medicine . One of the patients, for example, was born in 1935 and had little carbon-14. Collagen from patients born in the 1950s, by contrast, showed the largest amounts of the isotope, reflecting the rapid rise in atmospheric carbon-14 after nuclear tests began.

In some previous studies, scientists saw an increase in collagen synthesis in patients with osteoarthritis, which could represent the joint’s attempt to repair itself. But Kjær’s team didn’t detect this effect. An explanation for this difference, the scientists suggest, is that previous studies used indirect measures of collagen turnover in the joints. Even in the areas of the joint that are under the most stress, adults didn’t make new collagen, the team found.

Although researchers have tried several approaches to induce regrowth of knee cartilage, such as inserting stem cells or slivers of healthy cartilage into the joint, they haven’t worked. Developing methods to prevent cartilage from deteriorating might be more successful, Kjær says.

The study highlights the importance of protecting our cartilage, agrees cartilage biologist Richard Loeser of the University of North Carolina School of Medicine in Chapel Hill, who wasn’t connected to the research.

“You need to take care of your joints while you are young,” he says. “Once you have damage to the cartilage, it’s not going to repair itself.”