Large Ganglion Cyst of the Flexor Hallucis Longus (FHL) Tendon

🧠 Teaching Case: Large Ganglion Cyst of the Flexor Hallucis Longus (FHL) Tendon with Tenosynovitis on MRI

📌 Clinical Context: A 60-year-old patient presents with pain along the foot. No history of acute trauma. Pain is exacerbated by plantarflexion, especially during activities

📸 MRI Findings:

👉 Sagittal and axial T2-weighted fat-saturated images reveal:

A multiloculated, well-defined hyperintense cystic lesion(Blue circle) along the course of the FHL tendon, centered in the forefoot and mid foot region

Tenosynovitis: Surrounding high signal fluid within the tendon sheath with mild tendon sheath thickening.(Red arrow)

The FHL tendon is intact but slightly flattened within the tunnel, indicating mass effect from the cyst.

🧬 Diagnosis: 🔹 Ganglion cyst of the flexor hallucis longus tendon sheath

🔹 Associated tenosynovitis

🧠 Pearls for Practice:

✅ Ganglion cysts are benign mucin-filled synovial cysts, often arising from joint capsules or tendon sheaths. In the ankle, FHL is a common site due to repetitive motion and mechanical stress.

✅ Tenosynovitis may be reactive or secondary to chronic friction from the cyst. In athletes and dancers, FHL involvement is especially common due to overuse.

✅ MRI is the modality of choice:

T1: Hypointense

T2/STIR: Hyperintense, well-circumscribed

Look for communication with joint or tendon sheath

✅ Clinical Correlation: Symptoms may mimic tarsal tunnel syndrome or posterior ankle impingement.

🩺 Management:

Conservative: NSAIDs, rest, activity modification

Interventional: Ultrasound-guided aspiration

Surgical: Excision if symptomatic or recurrent

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