The prevalence of knee arthritis in the United States has doubled in adults since World War II. But while many will have already jumped to conclusions as to why this might be the case, it is not for the reasons you probably think, as obesity and longer life expectancy were not behind the dramatic rise observed.

Currently in the United States, close to 20 percent of people over the age of 45 are suffering from knee osteoarthritis. It has long been assumed that this figure has been changing over time, but to date there has been no study confirming this. Now, researchers from Harvard have finally managed to quantify that the rate is indeed increasing, publishing the results in the Proceedings of the National Academy of Science. But that was not the most surprising aspect of their work.

“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 BMI than if you were born earlier,” explained study co-author Daniel Lieberman in a statement.

To test how the prevalence of the condition has changed over time, the researchers studied over 2,500 skeletons spanning over 6,000 years of history. Because of how knee osteoarthritis manifests itself in the human body, with the bones in the knee directly rubbing on each other to form a polished surface, the researchers were able to assess its prevalence in three main time periods, prehistoric, early industrial, and modern post-industrial.

The most important comparison was between the last two of these three periods, as the researchers had access to vast amounts of data on each person’s age, sex, body weight, and even occupation and age of death. This allowed them to correct for many factors that we might expect to influence the development of knee arthritis, including weight and age.

It is often assumed that the increase in arthritis in the modern age is due to an increase in rates of obesity, as well as the simple fact that people are living longer. But this latest research found that neither of these factors had an impact on the rate of knee arthritis, and that even when both were corrected for, the prevalence still increased after the Second World War.

“Knee osteoarthritis is not a necessary consequence of old age. We should think of this as a partly preventable disease,” said Lieberman. The researchers are now working on trying to identify exactly what other factors may be behind this increase in the condition, focusing on changes that have occurred post-war. In the long run, this might help others find a way to prevent it in the first place.

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