Fabella is a sesamoid bone, present in posterior capsule of the knee joint, embedded in the tendon of lateral head of gastrocnemius muscle, articulated with posterior surface of lateral condyle of femur and accompanied by oblique popliteal ligament and fabello-fibular ligament.
In many cases fabella is ossified and can be seen radiologically; cartilaginous fabella can be seen on MRI.
It is generally a benign anatomical variant, but can sometimes cause posterolateral knee pain. Its size varies from a tiny dot to about 22mm.
Its presence is higher in Asian population.
Posterolateral knee pain can be associated with the presence of Fabella, referred to as Fabella syndrome.
Pain increases with extension of the knee which causes compression of Fabella onto lateral femoral condyle.
Pain is also exacerbated by climbing up and down stairs and deep squatting.
Compression of common fibular nerve by Fabella may cause palsy of the nerve, and size of common fibular nerve is reduced significantly, proximally in relation to Fabella, which can be seen ultra sonographically or in MRI.
Local anesthesia and steroid injection under ultrasound guidance can be tried initially which cures many patients.
Surgical treatment in resistant cases is by Decompression fabellectomy.
