Hi I'm a doctor from Australia and today we're going to be talking about the fabella, a bone
that's found just behind the knee in about 20% of people.
If it's present, it's more often than not bilateral meaning it's behind both your knees.
Fabella is Latin for little bean and you can see why when you look at an xray of it.
It might look like it's just floating there but it's actually sitting inside the tendon
of the lateral head of the gastrocnemius muscle.
The gastrocnemius is a calf muscle and it has two heads that originate on the femur.
There's a lot of things that make the fabella a unique bone; such as, how it forms, it's
function and the fact that it's prevalence has increased dramatically over the past 100
years.
The fabella forms through endochondral ossification.
Endo means inside, chondrol means cartilage and ossification means turning into bone.
So it literally means turning cartilage into bone.
And this is basically what happens with the fabella.
It starts as a bit of cartilage in the fetal stage of development and, some time after
birth, the cartilage is replaced by bone which is ossification.
The thing is not everyone will have a cartilagenous fabella as a fetus.
It's mainly controlled by your genetics.
People from Asia and Oceanic countries are much more likely to have the genes for a fabella
than people from North America or Africa.
Some places like Japan have a prevalence of up to 87%.
So the ability to form a cartilagenous fabella is controlled by your genes but the ability
to turn it into bone is actually controlled by your environment.
Ossification can happen at any age after birth and is stimulated by mechanical stress.
The tendon of the gastrocnemius rubs against the femoral condyles when the knee is flexed.
This friction and stress stimulates osteoblasts within the cartilage to lay down new bone.
So studies show that athletes or people who use their knees a lot will ossify their fabella
earlier in life, than say someone who is less active or works a desk job.
It's also interesting to note that kangaroos have a more developed fabella than humans
because of the high forces involved when jumping.
Anyway, the main point is that the ability to form a fabella is genetically controlled,
and the ability to ossify it is environmentally controlled, caused by mechanical stress to
the tendon.
Just remember this for later on because it holds the key to understanding their function
and the reason why it's making a comeback in human evolution.
By the way, I have another video explaining how mechanical stress causes bone growth,
so if you're interested, click on the link in the cards or description.
The fabella is a type of sesamoid bone, a type of bone which forms within the tendon
of a muscle.
The kneecap or patella bone is the largest and most well known sesamoid bone in the body.
If you think about it, the patella acts like a pulley, helping to redirect the forces produced
by the quadricep muscles to reach around the bend of the knee thereby increasing the moment
arm of the quads.
There's also a synovial joint containing synovial fluid between the patella and femur which
reduces friction between the tendon and the knee.
Without the patella, knee extension would be much weaker because a lot of the energy
from the quads would be lost to friction between the quads tendon and knee bones.
There are other sesamoid bones in the body that are present in varying degrees in the
population.
For example, almost everyone will have 2 sesamoid bones where the thumb meets the hand but only
about half the population have bones where the index finger and little finger meets the
hand.
They serve similar functions to the patella, in that they provide mechanical advantage
and reduce friction.
The fabella is no exception to this rule and is thought to provide mechanical advantage
like a pulley when the knee is flexed from a fully extended position.
When ossified, it also forms a tiny synovial joint with the femoral condyle reducing friction
between the gastrocnemius tendon and the femur.
As a result, you can think of it sort of like a kneecap for the back of the knee.
When the fabella is present, there's also a ligament called the fabellofibular ligament
which runs from the fabella to the fibula and serves to stabilise the knee joint.
That said, the actual advantage the fabella provides in terms of numbers is not known
and it's likely the benefit of a fabella is minimal.
In fact, as we'll learn later, having a fabella can cause problems of its own.
So as we mentioned earlier, the fabella is making a comeback in human evolution.
In old world monkeys, it was commonplace, and had a bigger mechanical role like the
patella.
However, for some reason it disappeared in great apes and early humans.
This coincided around the time when our ancestors transitioned from walking on 4 legs to 2 legs,
meaning it likely disappeared because we no longer needed the mechanical advantage it
provided.
However, a recent study found fabellae to be present in 11.2% of the population in 1918
and 39% of the population in 2018.
This is a three and a half fold increase over the past 100 years.
Meanwhile the rates of other sesamoid bones have remained constant.
So why would the prevalence of the fabella be increasing, if evolution deemed it unnecessary
in bipedal mammals?
No one knows for sure but it might be because our human diets are much more nutritious now
and accordingly, humans are bigger, taller and heavier.
As a result, our shinbones are longer and our calf muscles are larger, which could be
putting more stress on the gastrocnemius tendons leading to a higher rate of ossification.
As I alluded to before, having an extra bone behind the knee can cause problems of its
own.
For example, the cartilage on the fabella can soften and become inflammed in a condition
called chondromalacia fabellae, or the cartilage can wear down as a result of osteoarthritis.
Both these conditions can occur as a result of overuse of the knee and can cause the fabella
to become painful.
Additionally, the fabella, just like any other bone, can fracture if there's a big enough
force applied to it.
And finally, if the fabella is big enough or dislocates, it can compress structures
around it.
For instance, in rare cases the common peroneal nerve can be compressed, which can lead to
an involuntarily foot drop and loss of sensation over the top of the foot.
If the popliteal artery is compressed, it can cause cramps, numbness or tingling down
the back of the leg.
Just keep in mind that all these conditions are pretty rare so having a fabella shouldn't
be a reason for concern.
Overall, I found it interesting how this bone that was once lost in human evolution is making
a comeback.
And this might have something to do with how the forces around our knees are changing in
response to changes in our diet and body composition.
If you also think that's pretty cool, feel free to drop a like and hit subscribe.
And if you have any questions for me, leave a comment below.
Cheers.