A Tiny Bone Is Sneaking Back Into Human Knees

Somewhere behind your knee, tucked inside a tendon, there might be a bean-sized bone you never knew about. Plenty of people have one. Plenty of others don't.

It's called the fabella, Latin for "little bean." In many Old World monkeys it can act like an extra kneecap, but it seemed to fade away when the ancestors of great apes and humans evolved.

In 2019, a team led by Dr Michael Berthaume at Imperial College London reviewed 21,676 knees across 27 countries. The results appeared in the Journal of Anatomy, and they were not what anyone expected.

In 1918, about 11.2 percent of people had a fabella. By 2018, that figure hit 39 percent. That's a 3.5-fold jump in one century, which is a blink in evolutionary time.

The researchers' leading idea is nutrition. Better diets have made people taller and heavier, with longer shinbones and bigger calf muscles that squeeze the knee harder. That's a hypothesis, not a proven cause.

Where you live matters too. A 2021 meta-analysis of 34,733 knees found rates of about 41 percent in Asian populations and 15 percent in Europeans, using the study's own labels.

Then comes the odd part. Berthaume admitted, "We don't know what the fabella's function is." Nobody has ever really studied it, though it's linked to knee pain and can complicate knee replacement surgery.