Intraneural ganglion cyst of the peroneal nerve at the lateral knee: A case report and literature review

Introduction: Intraneural ganglion cysts can arise from the peroneal nerve at the lateral knee secondary to synovial fluid tracking along the articular branch and transforming within the nerve into a mucinous cyst, resulting in nerve compression. Case Report: A 17-year-old right-handed male presented with a four-month history of right foot drop. He is physically active and attributed the foot drop to a sprained ankle. EMG/NCS showed a right common peroneal neuropathy distal to the innervation of the biceps femoris short head with active denervation. MRI showed an intraneural ganglion cyst in the common peroneal nerve starting at the level of biceps femoris. On exam, he had right foot drop and sensory deficits referable to the peroneal distribution, along with a right steppage gait. He had successful decompression of the ganglion cyst, excision of the articular branch and resection of the proximal tibiofibular joint, with clinical improvement. Conclusion: Early recognition and surgical treatment leads to better outcomes for patients when an intraneural ganglion cyst results in neurologic deficits. Physical activities and trauma, which increase stress on the knee joints, may predispose ganglion cyst formation within peroneal nerves. Fibers of the deep peroneal nerve may be preferentially affected when compared to the superficial peroneal nerve. Disconnection of the articular branch and proximal tibiofibular joint resection may decrease risk of recurrence.