Scientists are baffled by the resurgence of what they assumed was a defunct extra bone behind the knee.
At the turn of the century around one in 10 people, had a fabella—a bean-shaped bone that, like the kneecap, is tucked into the tendons.
Scientists had assumed that the rate in the population had remained steady, or had fallen. But research has now revealed that nearly four out of 10 people—close to four times the rate 100 years ago—now have the extra bone.
The researchers looked at over 21,000 records of knees spanning 150 years from over 27 countries.
“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.”
The fabella is a sesamoid bone—the term used for bones that grow within the tendon. The kneecap is the largest sesamoid bone in the human body.
Berthaume said: “We are taught the human skeleton contains 206 bones, but our study challenges this. The fabella is a bone that has no apparent function and causes pain and discomfort to some and might require removal if it causes problems.
(L-R): Large, medium, and small ossified fabellae in the right knees of three female subjects. (Imperial College London)
“Perhaps the fabella will soon be known as the appendix of the skeleton,” Berthaume said.
The study found a statistical correlation between the presence of the bone and arthritis. Those with osteoarthritis in the knee were twice as likely to have the extra bone, which can also get in the way during knee surgery. The interesting question is why it’s making such a comeback,” said Berthaume.
One theory is 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.”
Another theory is that the bone isn’t as useless as thought, but may provide some mechanical advantage, as it does in old world monkeys, where it acts as a kneecap.
Berthaume said, “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.”
Bones have long been known to develop and strengthen in response to mechanical forces.
The researchers speculate that a fabella may form under the influence of certain mechanical forces. However, they believe that there is likely to be a genetic element—not all people may have the capacity to form one of the extra bones.
