Knee osteoarthritis (OA) has more than doubled among Americans since 1940, say researchers, and the increase can't be explained by longer lifespans or a higher prevalence of obesity and overweight in recent decades. Instead, the real culprit could be physical inactivity, which authors describe as "epidemic in the postindustrial era."

The study, appearing in the Proceedings of the National Academy of Sciences, compared knee joints of 2,756 skeletons from 3 groups of individuals: those who lived in the 1800s and early 1900s ("early industrial," N=1,581), those who lived during the late 1900s through the early 2000s ("postindustrial," N=819), and prehistoric hunter-gatherers who lived between 6,000 and 300 BCE (“prehistoric,” N=176). Researchers were looking for knee joint eburnation—the ivory-like result of bone-on-bone contact that occurs after cartilage erodes—as the indicator for moderate to severe OA.

Here's what they found:

• The prevalence of knee OA in the postindustrial skeletons was about 16%, a rate 2.6 times higher than the early industrial group, which had a 6% incidence rate. Knee OA prevalence among the prehistoric sample was 8%.
• After controlling for body mass index (BMI) and age when that information was available (1,859 of the 2,756 skeletons), researchers were unable to establish a correlation between these factors and prevalence of knee OA—instead, rates remained 2 times higher for the postindustrial group even when compared with early industrial skeletons with similar ages and BMIs. BMI for the prehistoric sample could not be estimated.
• In the postindustrial individuals with knee OA, 42% had the disease in both knees. Bilateral occurrence was 30% among the early industrial samples with knee OA, and 17% among the prehistoric group.

"Although knee OA prevalence has increased over time, today's high level of the disease is not, as commonly assumed, simply an inevitable consequence of people living longer and more often having a high BMI," authors write. "Instead, our analyses indicate the presence of additional independent risk factors that seem to be either unique to or amplified in the postindustrial era."

The researchers believe that risk factor could have to do with "environmental changes"—namely, the reduced levels of physical activity associated with the postindustrial era, despite the human body's need for regular exercise. It's a phenomenon known as a "mismatch disease," when the human body can't easily or rapidly adapt to changes in the lived environment.

"Although altered loads generated by walking more frequently on hard pavement … or with certain forms of footwear … might be factors, another possibility that merits more study is physical inactivity, which has become epidemic during the postindustrial era," authors write. "Less physically active individuals who load their joints less develop thinner cartilage with lower proteoglycan content … as well as weaker muscles responsible for protecting joints by stabilizing them and limiting joint reaction forces."

The good news, according to the researchers, is that their findings point to the possibility that knee OA is a largely preventable condition—providing there's a widespread "reappraisal of potential risk factors that have emerged or intensified only very recently."

"As with other mismatch diseases, it is likely that any effective prevention strategy will involve adjusting physical activity patterns and diets to approximate more closely the lifestyle conditions under which our species evolved," authors write.

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