A tiny bone that scientists thought was being lost to evolution could be making a comeback and causing knee pain, experts at Imperial College London have said.
The fabella is found buried in the tendon behind the knee of some people.
Linked to arthritis?
Doctors believe the bone is entirely pointless, and many people live without it.
However, some who do have it get arthritis, and it is thought that the fabella could have something to do with this.
The findings were published in the Journal of Anatomy.
The word fabella means ‘little bean’. In medical terms it is a sesamoid bone, which means it grows in the tendon of a muscle. Other sesamoid bones include the kneecap and the patella.
Scientists studied medical records from 150 years
Dr Michael Berthaume led a team at Imperial College London which looked at medical literature on knees from over 150 years in 27 different countries.
It found that, between 1918 and 2018, reports of finding the bone in knees increased to the point where it is thought to be three times as common now as it was a century ago.
In 1918 the bone was believed to be present in around 11 per cent of the population, whereas now it is closer to, 39 per cent.
What is the bone for?
It is not known why the fabella is there, as experts do not know why it was ever needed.
It started to disappear over time as humans evolved. Now that it is returning, it is starting to cause problems.
People who have osteoarthritis of the knee are twice as likely to have the little bone, but there is no evidence it is actually causing the problem, or how.
It can get in the way of knee replacement surgery and can cause pain on its own.
Why is it coming back?
The reason that the bone is making a comeback is thought to be because humans now are taller and heavier than they were in the past.
This means that they have longer shin bones and calf muscles, and so the knee is placed under more pressure.
Sesamoid bones grow in response to force and movement, and this could explain their resurgence.