New knee bone

Three knee joints with different sizes of fabella, the small bone to the right of each image.

Textbooks tell you that the human skeleton has 206 bones. A new study from Imperial College London says there could soon be 208. A tiny knee bone called fabella, with no apparent function, believed to be lost in evolution, seems to have re-emerged. A mystery knee-bone arthritis which has increased threefold over the last 100 years may be linked to the fabellae and should be taken into account in treating patients with knee problems and pain, the authors say.

Known to be rare in humans in recent history, The fabella is buried in a tendon behind the knee. Based on 21,676 individual knee studies over 150 years in 27 countries, the research has shown that between 1918 and 2018, the rate of fabellae occurrence in humans increased more than threefold.

In the earliest records from 1875 that the researchers examined, fabellae were found in 17.9 per cent of the population. From this, the researchers created a statistical model that predicted the prevalence rate while controlling for country of study and method of data collection, such as X-rays, anatomical dissection and MRI scanning. Their analysis showed that in 1918, fabellae were present in 11.2 per cent of the world population, and by 2018 they were present in 39 per cent, a 3.5-fold increase.

According to the Imperial College London press release, the fabella is a sesamoid bone, which grows in the tendon of a muscle. The kneecap is the largest sesamoid bone.

“The fabella may behave like other sesamoid bones to help reduce friction within tendons, redirecting muscle forces or, as in the case of the kneecap, increasing the mechanical force of that muscle. Or it could be doing nothing at all,” said Michael Berthaume of the college’s bioengineering department and the lead author of the study. The release said people with osteoarthritis of the knee were twice more likely to have a fabella than people without osteoarthritis. But whether the fabella causes osteoarthritis in the knee is not known. The fabella can also cause pain and discomfort on its own and can get in the way of knee replacement surgery. Their findings could thus have implications for the treatment of patients with knee pain or osteoarthritis or those needing knee replacements.

In old world monkeys, the fabella perhaps acted as a kneecap, increasing the mechanical advantage of the muscle. But when the ancestors of great apes and humans evolved, it seemed to disappear. “As we evolved into great apes and humans, we appear to have lost the need for the fabella. Now, it just causes us problems. But the interesting question is why it’s making such a comeback,” Berhaume asks.