Fabella: the knee bone that's making a comeback – and could be a real pain

A bone that increases the risk of arthritis is increasingly prevalent among humans, scientists have found, as they reveal better nutrition could be to blame.

The fabella, a small bone in the knee once lost to human evolution, has made a surprising resurgence over the last century.

The bone, which linked to knee problems and pain, is buried in a tendon behind the knee, and was once rare in humans. It is thought of as pointless by doctors, yet those who have arthritis appear more likely to be in possession of a fabella.

However researchers from Imperial College London have now found that fabellae are becoming more common in humans.

Their results, which spanned more than 21,000 knee studies over 150 years and across 27 countries, showed that between 1918 and 2018, the rate of fabellae occurrence in humans increased more than threefold.

The scientists' analysis showed that in 1918, fabellae were present in 11% of the world population, and by 2018, they were present in 39 per cent. The analysis was published last week in the Journal of Anatomy.

Dr Michael Berthaume, from Imperial’s Department of Bioengineering, and the report’s lead author, said: “We don’t know what the fabella’s function is – nobody has ever looked into it!”

“The fabella is a sesamoid bone, meaning it grows in the tendon of a muscle – the kneecap, for instance, is the largest sesamoid bone in the human body.

“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,” Dr Berthaume added. “Or it could be doing nothing at all.”

The earliest records that researchers analysed, from 1875, showed that fabellae were found in 17.9 per cent of the population. A statistical model was then created which predicted its prevalence rate.

The researchers made their estimations using X-rays, medical scans and medical journals' findings from a growing world population.

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.

Dr 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.

“Perhaps the fabella will soon be known as the appendix of the skeleton.”

As humans evolved from great apes and monkeys, the fabella appeared to have been lost. However researchers believe it could be making a come back as a result of healthier diets.

“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,” Dr Berthaume.

“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.”