The fabella (named after the faba/broad bean) is a small sesamoid bone located in the tendon of a muscle at the back of the knee. It is common in four-legged mammals, but curiously it also occurs in
some humans.
Although the fabella seems to have disappeared with the evolution of great apes, it mysteriously reappeared in humans sometime after they diverged from chimpanzees.
In 1918, an average of 11% of human knees, worldwide, had fabellae, with bilateral cases (one/knee) being more common than unilateral ones (one/person). But today, the average prevalence is 39%. With better nourishment people are getting taller and it is postulated that the fabella provides extra stability to the cope with the increased forces in the knee.
In humans, it is more common in men than women, older individuals compared to younger, and there is high regional variation, with fabellae being most common in people living in Asia and Oceania and least common in people living in North America and Africa.
NB. Sesamoid bones (named after sesame seeds) are bones embedded in tendons/muscles.