Lateral Meniscal Cyst Diagnosed After Acute Onset of Common Peroneal Nerve Palsy.

Common peroneal nerve palsy is often caused by compression at the fibular head, with ganglion cysts being the most common etiology. Although rare, meniscal cysts can also compress the nerve. We report a case of common peroneal nerve palsy due to a lateral meniscal cyst, successfully treated with surgery. A 38-year-old man presented with progressive pain and numbness in the right lateral knee and foot, along with difficulty dorsiflexing the ankle without any apparent history of trauma. Examination revealed a palpable mass on the lateral knee and sensory deficits in the superficial peroneal nerve distribution. Magnetic resonance imaging demonstrated a cystic lesion originating from the lateral meniscus. Nerve conduction studies showed reduced conduction velocity, confirming a diagnosis of common peroneal nerve palsy secondary to a lateral meniscal cyst. The patient underwent surgical excision of the cyst and arthroscopic meniscal repair. Intraoperatively, the cyst was compressing the common peroneal nerve. Postoperatively, motor function recovered over 1 year, with minor residual sensory deficits. No recurrence was observed at 1-year follow-up. Lateral meniscal cysts are a rare cause of common peroneal nerve palsy. Early diagnosis and surgical intervention can prevent permanent nerve damage and ensure functional recovery.