Since World War II, arthritis rates have doubled.

However, a longer life expectancy along with increasing obesity rates aren’t the only causes for the surge, according to a released today.

Researchers from Harvard University examined more than 2,000 skeletons. They found a vastly increased number of arthritic knees in people who died during the post-industrial era compared to those who were born in the late 1800s.

They found the rate of arthritic knees has doubled since the middle of the last century, according to the study published in Proceedings of the National Academy of Sciences.

Ian Wallace, PhD, a research fellow in the Skeletal Biology and Biomechanics Lab at Harvard University’s Department of Human Evolutionary Biology, said researchers wanted to look at skeletons because there was no set way to diagnose arthritis before the 1950s.

“When your cartilage erodes away and two bones that comprise a joint come into direct contact, they rub against each other causing a glass-like polish to develop,” Wallace said in a statement. “That polish, called eburnation, is so clear and obvious that we can use it to very accurately diagnose osteoarthritis in skeletal remains.”

The team looked for signs of knee osteoarthritis in 1,581 skeletons of individuals who died between 1905 and 1940.

They also looked at 819 skeletons of people who died between 1976 and 2015.

All of the remains were from people over the age of 50.

They found that knee arthritis was 2.6 times more common in remains of people born in the post-industrial age compared to those people who had been born in the late 1800s.

The researchers found that 42 percent of people with arthritis in the post-industrial age had the condition in both knees. This was 1.4 times higher than in the remains from the early-industrial period.

“We were pretty stunned by how much prevalence has spiked,” Wallace told Healthline.

Reasons for rising arthritis rates

In recent decades, life expectancy has increased and obesity rates have skyrocketed.

Doctors had believed these factors likely led to the spike in arthritis.

But this new study finds there may be other factors. This is because the increase was seen even after the researchers controlled the findings for age and body mass index (BMI).

“Although knee OA [osteoarthritis] prevalence has increased over time, today’s high levels of the disease are not, as commonly assumed, simply an inevitable consequence of people living longer and more often having a high BMI,” Wallace and his co-authors wrote. “Instead, our analyses indicate the presence of additional independent risk factors that seem to be either unique to or amplified in the postindustrial era.”

Wallace said one possible factor may be a decline in activity.

As people have become more sedentary in recent decades, they may be more at risk for arthritis. Being sedentary can lead to weakened joints and surrounding muscles.

But Wallace explained that more study needs to be done to understand the factors that put people at risk for arthritis.

Additionally, he said, if they identify new risk factors — like leading a sedentary lifestyle — this research may help physicians identify ways to prevent the condition from developing in the first place.

“I think that one of the most important take-home messages from the study is that knee osteoarthritis and probably arthritis in general is probably more preventable than we assumed,” Wallace said.

Challenging established theories

Dr. Matthew Hepinstall, the associate director at Lenox Hill Center for Joint Preservation and Reconstruction, said more research needed to be done to confirm these findings.

“I would say this is an interesting study because it challenges some commonly held beliefs about osteoarthritis,” Hepinstall told Healthline.

Hepinstall also emphasized that age and obesity are still risk factors in arthritis, even if they aren’t the only reasons that there has a been a jump in the disease since the middle of the last century.

He said right now family history, obesity, injury, limb deformity, and age are the known risk factors for arthritis.

“Obesity has been shown to have higher incidence of arthritis and [worse pain],” he said.

He also pointed out there aren’t many 20-year-olds — even those who are obese — with osteoarthritis compared to people in their 80s.

He said the study could help the medical community focus on possible ways to slow down the progression of the disease.

“Certainly, at the current time we don’t know of any specific treatments that do delay” the onset of osteoarthritis, he said.