1004 - When a Cyst Gets on Your Nerves: Peroneal Compression from a Posterior Knee Ganglion

Posterior knee ganglion cyst with peroneal nerve compression, initially misidentified as Baker’s cyst.

Case Description:

A 76-year-old man with hypertension, aortic aneurysm, metastatic prostate cancer, osteoarthritis, and ACL reconstruction presented with 1.5 months of posterior-lateral knee pain, swelling, and paresthesia radiating down the shin to the dorsum of the right foot. The exam revealed a small, soft, non-tender posteromedial mass and a larger, tender posterolateral mass reproducing his symptoms. Ultrasound confirmed a small medial cyst and a larger septated lateral cyst, both avascular. Aspiration was attempted but unsuccessful.

CT showed severe tricompartmental osteoarthritis and a cystic lesion atypical for Baker’s cyst. On follow-up, the patient developed new right foot weakness and persistent numbness. MRI demonstrated a small Baker’s cyst with an intra-articular loose body and a large multiloculated cyst deep to the popliteus, extending to the proximal tibiofibular joint, with mass effect on the peroneal nerve. Findings were most consistent with a ganglion cyst.

Discussions:

Baker’s cysts are common and usually posteromedial, while ganglion cysts of the proximal tibiofibular joint are rare (0.1–0.8% on MRI). Ganglion cysts atypical location predisposes them to compress the common peroneal nerve, leading to sensory loss and foot drop. Neurologic symptoms such as numbness or foot drop should raise suspicion for nerve compression rather than a simple Baker’s cyst. Ultrasound is useful for initial characterization but may not delineate extent or nerve involvement. MRI remains the gold standard for differentiating cysts and confirming neurovascular compression. Management differs significantly. Baker’s cysts often respond to conservative care or aspiration, whereas ganglion cysts compressing the peroneal nerve typically require surgical excision to prevent irreversible deficits. 

Conclusions:

This case underscores the importance of distinguishing Baker’s cysts from ganglion cysts when location or symptoms are atypical. Early recognition of peroneal nerve compression and timely referral for surgical management are essential to optimize neurologic recovery.