Knee osteoarthritis, a painful condition that often strikes older people, is twice as common today in the United States as it was prior to World War II, researchers said Monday.
But the reasons for the rise are not simply because obesity is increasingly common, or that people are living longer, said the researchers from Harvard University.
The study in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal, did not identify the real reasons, but suggested they may include physical inactivity, "which has become epidemic during the postindustrial era."
Other factors could be the modern diet, which is high in refined sugars and carbohydrates and can lead to chronic low grade inflammation, which wears down joints.
Footwear, sidewalks, and increasing body weight could also be changing the way we load our joints, researchers said.
More study is needed to narrow down the causes of osteoarthritis, and to find ways to prevent it, they said.
"We were able to show, for the first time, that this pervasive cause of pain is actually twice as common today than even in the recent past," said Ian Wallace, the study's first author and a post-doctoral fellow at Harvard University.
"But the even bigger surprise is that it's not just because people are living longer or getting fatter, but for other reasons likely related to our modern environments."
The study was based on a review of more than 2,000 skeletons from across the United States.
"This the largest sample ever studied of older-aged individuals from three broad time periods -- prehistoric times, early industrial times (mainly the 1800s), and the modern post-industrial era," said the report.
-- 'Partly preventable disease' --
Osteoarthritis is easy to diagnose in skeletal remains. When cartilage erodes, and the bones of a joint rub together, the friction causes a glass-like polish to develop, called eburnation, which is obvious to the naked eye.
Osteoarthritis has been around for ages, with evidence of the condition even turning up in Neanderthal fossils.
Experts suspected it has become more common in recent times, but researchers had not studied the trends in a systematic way until now.
"The most important comparison is between the early industrial and modern samples," Harvard researcher and co-author Daniel Lieberman said.
"Because we had data on each individual's age, sex, body weight, ethnicity, and in many cases, their occupation and cause of death, we were able to correct for a number of factors that we considered important covariates," he added.
"So using careful statistical methods, we are able to say that if you were born after World War II you have approximately twice the likelihood of getting knee osteoarthritis at a given age or body mass index (BMI) than if you were born earlier."
Nearly one in three adults over 60 suffer from osteoarthritis, which cannot be cured without a total joint replacement.
"Knee osteoarthritis is not a necessary consequence of old age. We should think of this as a partly preventable disease," Lieberman said.