Symptomatic enlarged fabella.

To cite: Agathangelidis F, Vampertzis T, Gkouliopoulou E, et al. BMJ Case Rep Published online: [please include Day Month Year] doi:10.1136/bcr-2016218085 DESCRIPTION A male patient aged 20 years presented to our outpatient department reporting of a 2-year history of right knee discomfort associated with snapping episodes, especially when moving from a seated to a standing position. His medical history was unremarkable and there was no history of trauma. The oblique radiograph of the knee revealed a large ossified structure at the posterolateral corner (figure 1). Routine blood tests, erythrocyte sedimentation rate and C reactive protein were within normal limits. MRI of the knee showed the large ossified structure within the lateral head of the gastrocnemius, while the rest of the examination was normal (figure 2). The tendon of the lateral head of the gastrocnemius attached to its posterior aspect and the anterior surface articulated with the lateral femoral condyle. Based on the location and anatomical characteristics, the structure was identified as a fabella and its large size explained the patient’s symptoms. Following informed consent, the patient agreed to surgical excision of the fabella which promptly relieved him from his symptoms. There was histopathological confirmation that the core was an osseous tissue with a periphery of ossified cartilage (figure 3). A fabella is a sesamoid bone located in the anterior gliding surface of the lateral head of the gastrocnemius muscle and its substance can be either bony or cartilaginous. 2 The bony fabella can be easily distinguished from the surrounding tissue and is easily identified in plain lateral radiographs of the knee. On the other hand, a cartilaginous fabella is characterised by a partly ossified cartilage in the centre and it is more difficult to capture in imaging studies. There is a wide range of incidence reported in the literature ranging from 8.7% to 85.8%. The reported length is 4–22 mm with a diameter of about 10 mm, while in our case, it measured 30×16×8 mm. To the best of our knowledge, this is the largest fabella described in the literature and we believe that its size was the main reason of the mechanical symptoms.