If you’re reading this and you’re American, there’s a slight chance you might have a tiny little bone on the back side of your knee. It’s a feature that your great, great grandparents may not have had.
Anatomical evolution in action!
It’s a little bean in Latin and it’s called the fabella. (Perhaps with an instructor’s help, you can recreate the fabella as you build your MANIKEN® with the Anatomy in Clay® Learning System.)
The fabella is an example of a sesamoid bone (sesamoid is sesame seed in Latin). Over time, sesamoid bones have evolved in areas of action, like the knee. The patella is the most common sesamoid bone, but sesamoids are often found in the tendons of hands and feet, too. They protect tendons from the rigors and stresses of human movement.
Fabellas are between five and 20 millimeters in diameter and can easily go unseen and undetected over the course of a person’s life.
These researchers, at the Department of Bioengineering, Imperial College, London, conducted a meta-analysis by looking at dozens of previous studies on the prevalence of fabellas in patients. Those many studies, dated from 1875 to present, were carried out all over the world.
The presence of fabellas was confirmed by a variety of methods including cadaver dissection, examination, CT scan, MRI, X-ray, ultrasound, and PET CT.
What they found is that the presence of fabellas has been increasing worldwide over the past 130 years. Dr. Michael Berthaume and his colleagues believe that better diets have led to larger humans, which in turn has led to bigger bones and more weight for us humans. Today’s humans have bigger tibias and larger calf muscles than their ancestors.
Sesamoid bones tend to develop in areas of “high mechanical stimuli, such as pressure, friction, or stress,” the scientists said. Heavier, taller humans might experience the mechanical stimuli necessary to initiate fabella formation as the body compensates and adjusts to these relatively new stressors.
The research group found that fabellas had an occurrence rate of 31 percent in the year 2000, compared with a less than 8 percent occurrence in 1900. That’s a three-and-a-half fold increase. After much review, the researchers rejected the possibilities that these increases might be due to different detection methods as well as genetic factors or other environmental factors, like an increase of plastic all around us.
Interestingly, today’s humans have an extremely varied likelihood of having fabellas. In South American and parts of Europe, as few as three percent of populations may have these bean-like bones. But in Asia, especially Japan and China, as many as 87 percent have them.
Got a fabella in one leg? You have a two-in-three chance of having it in your other leg, too.
One element that the researcher acknowledged was challenging was how, exactly, fabellas were defined by different studies. In China, for instance, more than half of the fabellas were actually cartilaginous. Cartilage may or may not be detected by those methods cited earlier in this post.
So what’s the big deal?
It could be that having or not having fabellas is as innocuous as having or not having an unattached ear lobe. But some scientists have pointed to the potential of an increased risk of knee osteoarthritis.
All the more reason to stay active and keep the stress off your little sesamoids!