A fabella is a little bone which sometimes forms at the back of the knee, within the tendon of the gastrocnemius muscle, like a tiny kneecap at the back of the knee.
Normal anatomical variant
It is a normal variant in 10-20% of people. The little bone is often wrongly diagnosed by the inexperienced doctor as a 'loose body' or even a 'foreign body' inside the knee. However, in rare cases the fabella causes posterolateral knee pain and surgeons may choose to excise it.
A Comprehensive Review of the Fabella Bone. Dalip D, Iwanaga J, Oskouian RJ and Tubbs RS. Cureus. 2018 Jun; 10(6): e2736.
The fabella syndrome - a rare cause of posterolateral knee pain: a review of the literature and two case reports. Driessen A, Balke M, Offerhaus C, White WJ, Shafizadeh S, Becher C, Bouillon B and Höher J. BMC Musculoskelet Disord. 2014; 15: 100.
Ultrasound Evaluation and Surgical Excision of a Fabella Causing Peroneal Neuropathy in a Track Athlete Dale KM, Boggess SB, Boggess B and Moorman CT. Case Rep Orthop. 2018; 2018: 2371947.