There’s a good reason you’ve never heard of the fabella, a tiny bone embedded in the tendon of the knee. The bone, whose name means “little bean,” doesn’t seem to serve any purpose. Not everyone has one, and people do just fine without it.But now, it seems the fabella is showing up in humans far more often than before — and we’re not entirely sure why.The median prevalence rate of the fabella in humans increased 3.5 times in the 20th century, from 7.64 per cent in 1900 to 31 per cent in 2000, according to a study by researchers at Imperial College London and published in the Journal of Anatomy.The impact of the increase is unclear, mainly because the fabella’s impact on the human body is unclear, too. There seems to be a connection between fabellas and osteoarthritis: people who suffer from join pain are twice as likely to have a fabella. But the link between the two isn’t obvious.Dr. Michael Berthaume, the study’s lead, happened upon the surprising prevalence of the bone almost by accident. His work focuses on the design of medical devices for people in lower and middle income countries, and he started noticing the bone while looking at scans of knees from a sample of Koreans.After talking it out with some other academics and his adviser, Berthaume decided to look into why the bone, which he had rarely seen before, was so common in the sample.“It was supposed to be a quick prevalence rate study,” Berthaume said. “Famous last words.”When the team started to research, they found that the prevalence rates cited in previous research varied widely, so they were forced to do a systematic study to determine, as accurately as possible, what the real rates were across the whole human population.They were able to determine the dramatic increase in the general prevalence rate over time, and in their own scans the team found that over half of the Korean people sampled had fabellas.Oddly, they also found that the other 10 “sesamoid” bones — similar to the fabella — had not increased in prevalence over time.
The white arrows show where the fabella is situated around the knee joint.
Michael Berthaume/Imperial College London
That means the fabella is somehow unique, which was puzzling to the team.“There are a lot of different ways in which these changes could have occurred, and there are different environmental factors,” Berthaume said.First they thought about hormonal effects, and about epigenetics, which is how genes express themselves. But they concluded that if this were the cause, other bones would have been affected, too.Next, Berthaume’s team discarded the idea that genetics could be at play, since the increase was similar in the 27 countries they looked at. Though there may be a genetic component in whether someone has a fabella — they were more common in Asian countries, Berthaume said, than European ones — it didn’t account for the increase over time.So that left environmental factors, and specifically environmental factors that had changed in the last 100 years and had affected basically everybody.“One of the few things that’s affected everyone around the world, on average, is that people are better fed,” Berthaume said.Sesamoid bones like the fabella tend to form in areas of the body where there is muscle tension, in order to reduce friction, Berthaume explained. He theorized that as people got taller and heavier over the last century, these bones increasingly formed to help the knee deal with the greater pressure.“This is not evolution,” Berthaume pointed out. “The reason why it’s not evolution is that we don’t think there’s a genetic basis for this, we think it’s just an environmental factor. A similar environmental factor would be the old practice of Chinese foot binding.”Even this theory though, the study says, does not explain everything — particularly why the bones develop in fetuses and why there was no correlation between height and if someone had a fabella.Still, the discovery opens up some interesting areas of study. The research team currently has a paper under review looking at how the prevalence breaks down based on demographics, and they’re also exploring the connection between the bone and osteoarthritis.Then there’s just the more general question of what the fabella really does, because it’s still not clear why it exists, let alone why it’s becoming more and more common.“We have no idea what it does, because nobody has looked into it. Which is what makes this for me so incredibly cool,” Berthaume said.“We still have so much to learn about the human body.”