Knees hurting? You wouldn’t be alone. The prevalence of knee arthritis among people in the United States has doubled since the start of World War II, according to an unusual study of more than 2500 skeletons, including some dating as far back as 6000 years. Even after researchers corrected for the recent growth in U.S. waistlines and life spans, the increase still held, meaning—say scientists—that other, still unknown, factors are causing more people to suffer from aching knees.
“This is really important work,” says biomechanist Louis DeFrate of the Duke University School of Medicine in Durham, North Carolina, who wasn’t connected to the study. Knee arthritis is one of the leading causes of disability, he says, and the findings may help researchers better understand why more and more people are developing it.
Today, almost 20% of people in the United States over 45 years old suffer from knee osteoarthritis, in which joint cartilage breaks down; the odds of developing it climb as we get older. Scientists have long suspected that number has risen in recent generations. Because most Americans are living significantly longer than their grandparents, researchers have speculated that the graying population could be one culprit. Another joint-straining suspect is obesity, which now affects more than one-third of adults in the United States, up from 13% in the early 1960s.
To tease out the influences of weight and age, paleoanthropologists Ian Wallace and Daniel Lieberman of Harvard University and their colleagues examined nearly 2600 skeletons that had been saved for research or teaching. The team sorted the skeletons, which came from middle-aged and elderly people, into three groups. Nearly 1600 belonged to people who died between 1905 and 1940. Another 819 were from individuals who died between 1976 and 2015. And 176 were from Native Americans who lived between 300 and 6000 years ago.
To diagnose arthritis, the scientists checked for the smooth patches that form when cartilage in the knee joint erodes and allows the upper and lower leg bones to grind against one another. “When you see this polish on a bone, you know there was no cartilage there,” Wallace says. Using that criterion, the researchers compared the two modern groups and determined that the rate of knee osteoarthritis has more than doubled since 1940, from 6% to 16%, they report today in the Proceedings of the National Academy of Sciences. Even among the Native Americans—for whom the scientists didn’t have weight data—the arthritis rate was about 8%. “There was never a time when knee osteoarthritis didn’t exist,” Wallace says.
When the team factored out the effects of weight and age in the two modern groups, knee arthritis was still more than twice as common in the group of people who died after 1976, suggesting other factors are involved. What changed to touch off the explosion in knee arthritis cases is still a mystery, Lieberman says.
One modern bad habit that may be partly to blame is inactivity. Because we spend more time sitting and looking at screens, from smartphones to TVs, our leg muscles and cartilage might be weaker, causing our joints to break down faster. Wallace says that the team is testing this hypothesis by studying guinea pigs, which are the only lab animals that naturally develop knee arthritis, and the famous long-distance Tarahumara runners of Mexico.
The study is important because it emphasizes that the causes of knee arthritis are complex, says bioarchaeologist Elizabeth Weiss of San Jose State University in California. Paleopathologist Anne Grauer of Loyola University Chicago in Illinois agrees and says that our remedies will also have to be. “People losing weight isn’t going to solve the problem.”