Correct answer to Q2 is: Accessory bone – sesamoid bone
Discussion
There are no changes in the superficial tissues. No change in the cartilage. The imaging findings correspond with an echogenic structure, with increased sound attenuation, indicating bone. In this location it corresponds to the fabella.
The fabella is a sesamoid bone located in the posterolateral part of the knee (1), associated with the lateral head of the gastrocnemius muscle articulating with the lateral femoral condyle. It is an articular sesamoid bone occurring in 20 to 30% of the population (2). Rarely it can be found in the medial head of the gastrocnemius.
The patient said she occasionally had similar symptoms in her right knee, and the ultrasound examination revealed similar images.
Image 3: Right popliteal region, transverse and longitudinal view.
In view of the findings, and to confirm the diagnosis of fabella, an x-ray of both knees was done.
Image 4: x-ray of both knees, lateral view. The fabella is marked with an arrow.
The fabella pain syndrome (3) should be considered as a differential diagnosis when a patient presents with persistent posterolateral knee pain, which could also be due to meniscal tears, lateral ligament instability, Baker’s cyst, and proximal tibiofibular joint hypomobility. Patients with fabella pain syndrome usually complain that the posterolateral knee pain is worse on fully extending the the knee joint, which is due to compression of the common peroneal nerve.
Conclusion
The fabella can be mistaken for loose bodies or osteophytes which are usually asymptomatic in patients. Ultrasound in a useful tool to see the relation to anatomical structures when an incidental finding occurs.
References
- Dalip D, Iwanaga J, Oskouian RJ, Tubbs RS. A Comprehensive Review of the Fabella Bone. Cureus. 2018 Jun 5;10(6):e2736. doi: 10.7759/cureus.2736. PMID: 30087812; PMCID: PMC6075638
- Kawashima T, Takeishi H, Yoshitomi S et-al. Anatomical study of the fabella, fabellar complex and its clinical implications. Surg Radiol Anat. 2007;29 (8): 611-6. doi:10.1007/s00276-007-0259-4
- Driessen, Arne; Balke, Maurice; Offerhaus, Christoph; White, William; Shafizadeh, Sven; Becher, Christoph; Bouillon, Bertil; Höher, Jürgen (2014). The fabella syndrome – a rare cause of posterolateral knee pain: a review of the literature and two case reports. BMC Musculoskeletal Disorders, 15(1), 100–.doi:10.1186/1471-2474-15-100
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