Aging and Weight Cannot Explain Drastic Rise in Knee Arthritis Since 1950, Study Shows

Researchers studied the bones of ancient people to arrive at their findings that knee osteoarthritis has doubled in prevalence recently; you can see a light colored, polish-like patch toward the top of the image where the bones rubbed together. Heli Maijanen

New research shows that knee osteoarthritis, a painful and disabling condition in which cartilage wears down and bones rub against each other, has doubled in the last 50 years. This surprising spike raises serious questions about what causes the disease, and suggests it may be more preventable than previously thought.

The primary risk factors for the condition, which affects nearly one in five Americans over the age of 45, were thought to be the aging of the population (that is, the longer lifespans in modern times) and increased obesity rates. But this new study, published August 14 in Proceedings of the National Academy of Sciences, suggests that the doubling in prevalence cannot be explained by these factors.

Ian Wallace, paleoanthropologist at Harvard University who studies how human health and disease have changed over time, and colleagues studied several thousands of skeletons from around the United States, from pre-industrial, early industrial, and post-industrial periods. For each skeleton, the authors analyzed the wear and tear on the knee joint. Those with knee osteoarthritis were relatively easy to spot, as the condition causes polish-like marks on skeleton where the bones rub together. They then noted the ages and body mass index of the cadavers when available, and used a mathematical process called regression analysis to estimate prevalence and control for the influences of aging and obesity.

The researchers found that 18 percent of the skeletons they examined from the post-industrial period had signs of advanced knee arthritis, compared to six and eight percent of the early industrial and prehistoric bones, respectively. The condition was 2.1-fold higher in the post-industrial group, and the regression analysis showed that neither aging nor obesity was to blame for the increase.

The findings runs counter to the idea that with age (and excess weight), arthritis is inevitable, says Wallace, study first author. And that conclusion provides unexpected hope that further advances in better understanding the condition will help to ward it off.

"It points to this mysterious conclusion: A lot of cases of osteoarthritis, which we thought might be inevitable, may be preventable... and are due to unknown factors," Wallace says.

The authors stress that obesity and aging do still clearly play a big role in the disease. Excessive weight subjects the joints to forces that they cannot accommodate, damaging the cartilage, says David Felson, study co-author, a renowned arthritis expert and physician at Boston University. And some breakdown of connective tissue is natural with aging. Left unanswered is the obvious question: What are the other factors behind the increased prevalence?

The researchers don't know for sure, but have some leads. Felson says that physical inactivity, which has become much more common since the mid-twentieth century, likely plays a part, as it allows for muscles surrounding the joint and the joint itself to weaken and become more vulnerable. "Our joints don't do well when they aren't active much of the time," Felson says.

Inflammation may also share some blame. Although inflammation is a natural reaction to cellular damage and infection, it may make joints more prone to injury and prevent proper healing, says Francis Berenbaum, a researcher and physician at Pierre and Marie Curie University and AP-HP hospital in Paris, France, who wasn't involved in the study. He suspects that conditions like hypertension and diabetes, which can cause low but persistent levels of inflammation in the body, may help increase the prevalence of knee osteoarthritis. There is also some evidence that a diet high in processed sugars and grains may increase levels of inflammation, and Wallace suspects this could help increase the incidence of arthritis.

Injuries to the knee also play a clear role, as they greatly increase the risk for arthritis. (And it could be that higher levels of inactivity and resulting weakness, combined with inflammation, make injuries damaging.) Berenbaum advocates that adults, especially older adults, seek out coaching or physical therapy when attempting new or strenuous exercise.

To avoid knee arthritis, Felson recommends staying active and keeping weight down. As for the other as-yet-mysterious risk factors, he can't say. "I study this, and I don't know... what [more] can be done to prevent it," he says.