Every schoolchild knows the human skeleton has 206 bones.
But the textbooks may need to be rewritten – after a bone scientists thought had been lost to evolution has made a curious comeback.
The fabella, a tiny bone that sits inside the tendon behind the knee, is more than three times as prevalent as 100 years ago – and now two in five of us have one.
Less than half an inch in diameter, the bone – which was found in our ancestors – has been dubbed the “appendix of the skeleton” because it is apparently pointless.
Now scientists believe modern diets, which have made us taller and heavier, have placed more strain on our knees – leading us to grow the extra bone to relieve the pressure.
But there’s a catch. Having a fifth bone in the knee could wear away important cartilage, damage that can cause osteoarthritis.
In fact sufferers of the painful condition – which affects one in five over-45s in England – are twice as likely to have a fabella.
Some experts now say the fabella should be routinely removed if it is found.
Researchers from Imperial College London reviewed more than 21,000 scientific studies performed over 150 years, in which the fabella bone was identified in scans and dissections.
They found 11.2% of the world’s population had a fabella in 1918. But by last year this figure had more than tripled to 39%, according to the review published in the Journal of Anatomy.
Study lead Dr Michael Berthaume said: “This research is so exciting because it is extremely rare to have a change to the human skeleton which affects everyone around the world.
“We have no idea and can only guess at what it does, so it could well turn out to be the appendix of the human skeleton.
“We hope that by studying it we can help people struggling with the pain of osteoarthritis, and figure out if people with fabellas should have them removed.”
The fabella, which is Latin for “little bean”, seems to have no evolutionary reason to exist and was believed to have died out with our ancestors before it reappeared in medical reports in 1875.
People are taller and heavier than in the past, so have a larger gastrocnemius muscle behind the knee which creates stress and friction.
The fabella, which is found in the tendon attaching the muscle to the thigh bone, may remove some of this stress by providing a smooth surface for the tendon to slide across. This is certainly the advantage it has in dogs, cats and some monkeys, which all have the extra bone.
But experts believe the fabella may create uneven force on the knee, increasing damage to the cartilage. It could also cause pain after knee replacement surgery, as the extra bone stays in place by burrowing into the thigh bone. When that part of the bone is replaced and there is no longer a depression to hold it, the tendon “snaps” from left to right with every stride, causing pain.
Dr Berthaume said: “We appeared to have lost the need for the fabella. Now, it seems to just cause us problems – but the interesting question is why it’s making such a comeback.” — Daily Mail