Hannah Akre, DO
PGY-2
MedStar/Georgetown
Washington DC, District of Columbia, United States
Michael Lu, MD
Assistant Professor at Georgetown University School of Medicine
MedStar Health/Georgetown University
Washington DC, District of Columbia, United States
Ultrasound Evaluation for Fabella Syndrome and Common Peroneal Neuropathy
Case Description:
68-year-old female with a history of left knee meniscectomy presented with 4 months of focal left lateral knee pain with associated tingling along the lateral lower leg. She had pain with flexion beyond 120 degrees, as well as positive Tinels over the left lateral femoral condyle. MRI knee showed a degenerative meniscal tear, moderate osteoarthritis, and a baker’s cyst. She previously tried meloxicam, physical therapy (PT), and a knee injection without improvement. Limited diagnostic ultrasound was performed demonstrating a fabella over the lateral femoral condyle with notable subluxation of the common peroneal nerve (CPN) with dynamic knee flexion concordant with her symptoms. A diagnostic injection at the fabella with hydrodissection around the CPN provided 100% relief immediately. However, the effects were short-lived. EMG showed mildly reduced velocity/amplitude of the CPN at the fibular head. A fabellectomy was performed and she obtained 95% improvement 1-month post-surgery.
Discussions:
The fabella is a sesamoid bone typically located in the posterior-lateral head of the gastrocnemius muscle and is present in 10-30% of the population. Its presence is usually asymptomatic; however, it can cause mechanical pain over the lateral femoral condyle as well as impinge on the CPN, which is termed Fabella Syndrome. This syndrome is most commonly resolved with conservative management, including PT and medications. Rarely fabellectomy is pursued. The use of ultrasound for visualization, as well as for interventional diagnostic injections in this case helped with diagnosis.
Conclusions:
Fabella Syndrome is an uncommon pathology that can cause functionally limiting posterolateral knee pain and CPN neuropathy. Fabellectomy was previously only discussed in case reports and journals for rare diseases. This case highlights the value of ultrasound evaluation in establishing this rare diagnosis and instances when surgery is necessary.
