Knees weren’t always as knocked around as they are today. Something about modern life has more than doubled the rate of knee osteoarthritis since the mid-20th century.
And increased wear and tear from obesity and longevity doesn’t fully explain the upswing, a study of thousands of human skeletons has suggested.
Dr Ian Wallace, an evolutionary biologist at Harvard University in the US, examined more than 2,500 knee bones from human remains dating back millennia.
Knee osteoarthritis can be detected, even in ancient human remains, through the presence of eburnation, a sclerotic, ivory-like reaction that occurs from bone-on-bone contact at sites exposed by advanced cartilage erosion.
In a paper published in the Proceedings of the National Academy of Sciences this month, Dr Wallace and his colleagues reported a doubling in knee osteoarthritis since the early 1900s.
The study found prehistoric hunter-gatherers and farmers had a similar rate of the condition as people who lived during the early industrial era.
But the rate increased from around 7% in these samples to 16% in post-industrial skeletons.
What could explain this rise? The commonly cited causes are increased BMI and longer life spans, which are assumed to place extra strain on the knee joints.
Dr Wallace’s team re-ran the analysis taking into account the average 41% increase in BMI and average increase in longevity since the early 1900s.
After controlling for these two factors, knee osteoarthritis was still at least twice as high in the post-industrial sample.
“Our analyses contradict the view that the recent surge in knee osteoarthritis occurred simply because people live longer and are more commonly obese,” said Dr Wallace.
“Instead, our results highlight the need to study additional, likely preventable risk factors that have become ubiquitous within the last half-century.”
One major factor not included in the study was differing rates of exercise, Professor David Hunter, a rheumatology clinician researcher at The University of Sydney, said.
“Joints want to be active,” he said. “They are what we call trophic organs and they benefit from the physical- activity stimulus on a regular basis.”
Maintaining good knee health involved scattering exercise throughout the course of the day – as it was in the 1800 and early 1900s – instead of having long periods where workers were completely sedentary punctuated by bursts of exercise, Professor Hunter said.
“If anything, the ‘weekend-warrior style’ where the person works solidly all week and then goes out on the weekend and does two to three hours of activity increases injury risk and injury rates,” he said. Knee injuries caused around 25% of osteoarthritis cases.
Hard pavements, roads, more rigid footwear, and even higher heels, could also be increasing the rate, Professor Hunter said.
PNAS 2017, August 14