knee bone

The fabella, a small bone in the knee once lost to human evolution; has made a surprising resurgence over the last century. The new findings could help clinicians treating patients with knee issues and provide insight into human evolution over the past 100 years. The bone, linked to knee problems and pain; is in a tendon behind the knee and was once rare in humans.
The earliest records they looked at, which from 1875, showed that fabellae found in 17.9 % of the population. From this, the researchers 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.

The knee joint tendon

Their analysis showed that in 1918, fabellae present in 11.2 % of the world population; by 2018, they were present in 39 % a 3.5-fold increase. The tiny fabella bone is found in a knee joint tendon. Credit: Michael Berthaume/Imperial College London. 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 study authors say their findings could have implications for how they treat patients with knee pain or those who need knee replacements or treatment for osteoarthritis.

In old world monkeys, the fabella can act as a kneecap, increasing the mechanical advantage of the muscle. But when the ancestors of great apes and humans evolved, it seemed to disappear. Dr. Berthaume said: “As they evolved into great apes and humans, they appear to have lost the need for the fabella. Now, it just causes us problems but the interesting question is why it is making such a comeback.”

Evidence of fabella resurgence

The answer, says Dr. Berthaume, could lie in nutrition. “They 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.” Researchers found the fabella seems to 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 they 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.” Not all people have fabellae, however, and there is likely a genetic component controlling the ability to form one but for those who can form a fabella, this increased mechanical forces might drive their formation.
The researchers say they are now looking at how prevalence differs across genders, ages, and regions and the proportion of people who have fabella in one or both knees. They also hope to delve into the genetics of the bone and how its inclusion in the human skeleton has changed over time.