The fabella (small dot on the right of each scan) is a little bone embedded in a tendon of the knee in some people.

MICHAEL BERTHAUME, IMPERIAL COLLEGE LONDON

Textbooks will tell you that the human body contains 206 bones. But sometimes, there are 208. The fabella, a small bone in a tendon behind the knee, was lost over the course of early human evolution, but these days it’s becoming more common, according to a study published this week (April 17) in the Journal of Anatomy.

The bone has been linked to knee problems, and the authors argue that the fabella should be taken into account when treating people with knee pain.

Michael Berthaume of Imperial College London and his colleagues gathered data from more than 21,000 studies of the knee spanning the past 150 years, and found that between 1918 and 2018, the fabella has become more than three times more common. In 1918, the bone was found in just 11 percent of the world population, according to their data. Last year, it was present in 39 percent of people. The researchers’ analyses controlled for the method of data collection, which included X-rays, dissection, and MRI scans, as well as country of origin.

“The average human, today, is better nourished, meaning we are taller and heavier,” Berthaume says in a press release. “This came with longer shinbones and larger calf muscles—changes which both put the knee under increasing pressure. This could explain why fabellae are more common now than they once were.” The researchers suggest that genetics may influence whether people have the ability to develop fabellae, but if they do, environmental factors such as the mechanical forces that the knee experiences likely drive the bones’ formation.

The fabella’s function is unknown. In old world monkeys, it appears to play a role in knee muscle mechanics. But as the ancestors of great apes diverged from the monkey lineage, the bone apparently disappeared. Now that it’s reappearing in humans, its function is anybody’s guess. It might help “reduce friction within tendons, redirecting muscle forces, or, as in the case of the kneecap, increasing the mechanical force of that muscle,” Berthaume says in the release. “Or it could be doing nothing at all. . . . Perhaps the fabella will soon be known as the appendix of the skeleton.”

Regardless of whether it provides a functional advantage, the bone has been linked to various ailments. Its presence can cause knee pain, for example, and people who suffer from osteoarthritis in their knees are about twice as likely to have it than those without osteoarthritis. The fabella can also create additional challenges for knee replacement surgery.

“As we evolved into great apes and humans, we appear to have lost the need for the fabella,” Berthaume says in the release. “Now, it just causes us problems—but the interesting question is why it’s making such a comeback.”

Correction (April 22): This story has been updated from to correct the number of bones people with fabellae have—208, not 207. The Scientist regrets the error.