Extraneural ganglion cyst causing foot drop.

Ganglion cysts around the knee are uncommon and rarely cause compressive neuropathy of the common peroneal nerve. When present, they may result in foot drop and sensory disturbances, making early recognition crucial. We report a male in his late 50s presenting with a swelling over the lateral aspect of the right proximal lower leg, progressive weakness of ankle dorsiflexion and sensory loss over the dorsum of the foot. Examination revealed a firm, fluctuant swelling posterolateral to the fibular neck. Ultrasound and MRI confirmed a multiloculated cystic lesion compressing the common peroneal nerve. Nerve conduction studies demonstrated conduction block across the fibular neck. The patient underwent surgical excision of the cyst, which was found intraoperatively to arise from the posterolateral joint line causing compression of the nerve. Postoperatively, recovery was uneventful. The patient at the 6 month follow-up showed complete neurological recovery.