Fabella: Humans Were Losing Osteoarthritis-Linked Knee Bone, Now It’s Making Comeback

The fabella (Latin for ‘little bean’), a small bone in a tendon behind the knee, was once rare in humans. Now, a team of researchers from the Department of Bioengineering at Imperial College London, UK, has found that fabellae are becoming more common. Their results, published in the Journal of Anatomy, show that between 1918 and 2018, the rate of fabellae occurrence in humans increased more than threefold.

Large (left), medium (center), and small (right) fabellae (a small bone to the right of each image) in the right knees of three female subjects. Image credit: Berthaume et al, doi: 10.1111/joa.12994.

Large (left), medium (center), and small (right) fabellae (a small bone to the right of each image) in the right knees of three female subjects. Image credit: Berthaume et al, doi: 10.1111/joa.12994.

Fabella prevalence in humans ranges from 3 to 87%, making it a normal variant in human anatomy. The highest rates reported are in Asians and Australians, and the lowest rates in Europeans and South Americans.

Although its exact function is unknown, the fabella is more common in non-human mammals, which has prompted debates about the role of this bone in locomotion.

“We don’t know what the fabella’s function is — nobody has ever looked into it,” said Dr. Michael Berthaume, first author of the study.

“The fabella is a sesamoid bone, meaning it grows in the tendon of a muscle. It 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.”

“Having a fabella has its drawbacks. People with osteoarthritis of the knee are twice as likely to have a fabella than people without osteoarthritis — however, it is unknown if the fabella causes osteoarthritis in the knee, and if so, how. It can also cause pain and discomfort on its own, and can get in the way of knee replacement surgery.”

The earliest records Dr. Berthaume and colleagues looked at, which were from 1875, showed that fabellae were found in 17.9% of the population.

From this, they created a statistical model which predicted 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% of the world population, and by 2018, they were present in 39% — a 3.5-fold increase.

“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,” Dr. Berthaume said.

“The answer could lie in nutrition. We found evidence of fabella resurgence across the world, and one of the few environmental changes that have affected most countries in the world is better nutrition.”

The team found the fabella seems to be the only sesamoid bone in the human body to be increasing in prevalence. Sesamoid bones are known to grow in response to mechanical forces.

“The average human, today, is better nourished, meaning we are taller and heavier. 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,” Dr. Berthaume said.

“Our findings could have implications for how we treat patients with knee pain, or those who need knee replacements or treatment for osteoarthritis,” the scientists said.


Michael A. Berthaume et al. Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review. Journal of Anatomy, published online April 17, 2019; doi: 10.1111/joa.12994

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