An Anatomical Riddle.

Following a fascinating conversation with a Senior Orthotist, I thought I would re-count his comments and set this anatomical riddle: 

What is found only 80% of the time bilaterally, is flat and elliptical, measures 15mm across and is twice as likely to be found present alongside an OA knee?

Here is a clue below:

The answer of course is the fabella!!!

Let's be honest, you clicked and scrolled for the answer didn't you? Or at least I’m hoping you did to save my blushes. Personally I had never heard of it and thought I was quite well versed in my bony anatomy.

The Imperial College team, principally Dr Michael Berthaume and colleagues in the Department of Bioengineering put this little bone in the limelight these past weeks. However, they were not the first, as it was initially alluded to in the 17th century and then more specifically in 1875. My Orthotist colleague was shocked, believing he had read multiple radiography reports referring to it simply as a loose body! 

What is it?

Fabella (little bean) - is a sesamoid bone. Typically 10mm - 15mm in size on average, but can reach up to 20mm, is typically a flat elliptical shape and in some can be physically palpated.

Where is it located?

Initially a small fibrocartilage disc that can be seen in a developing foetus, it can be seen to ossify as early as 12 years old. It articulates with the posterior part of the articular surface of the lateral femoral condyle and is embedded in the muscular fibres of the gastrocnemius muscle. Anteriorly the fabella is bordered by the posterior capsule of the knee joint and posteriorly it is situated at the endpoint of the oblique popliteal ligament and the lateral gastrocnemius tendon. In addition, the fabellofibular ligament runs to its distal insertion at the fibular head and is found bilaterally in 80% of the population.

Prevalence

Dr Michael Berthaume and his colleagues at Imperial's Department of Bioengineering undertook a systematic review of all literature in the last 150 years in 27 countries, totalling over 21,000 knees and it is now thought to be three times as common as 100 years ago. In 1918 fabella were present in 11% of the world population and by 2018 they were present in 39%, whilst in areas like Asia and Australia it is almost every other knee.

It is thought to be by genetic neutral selection that we have evolved to have an increase in prevalence and that it is more likely to be environmental changes that has heavily influenced its increase in frequency. For example in Asia, increased tensile forces due to squatting and kneeling, and the fact we are now much better nourished resulting in greater height and weight resulting in additional stress and torque to the knee.

Why should the clinician take it more seriously?

For most the fabella does not hurt. If it does, we call it fabella syndrome characterized by a sharp pain, local tenderness and exacerbation of pain with extension of the knee. It can also cause pain during knee flexion, climbing stairs, cross-legged sitting and sporting activities.

Pain is associated with varus type of stress on the knee and passive and active internal rotation of the tibia. Occasionally the fabella is too close to the common fibular nerve and can cause altered neurology.

The fabella has been cited as a relevant cause of pain post-TKA due to mechanical irritation of the posterolateral tissues of the knee.

The most sensationalist headline outlined (correctly I may add); the fact that an OA knee is twice as likely to have a fabella present. This is likely an area of future research, as currently no proof exists that it is responsible for causation and it is more likely to be a predisposition to OA and a fabella simultaneously due to a genetic or environmental element. The tabloid hysteria and stress to the general public of worrying about this little bone bringing you to a theatre soon for your TKR can therefore be rejected.

We would like to hear of any clinical examples of the fabella you have come across? Also, do be honest and let us know if you had even heard of it?