A case of intraneural cyst of the common peroneal nerve.

Intraneural cysts of the common peroneal nerve, as of other peripheral nerves, are rare. Their etiopathogenesis is unknown, though theories involving a tumor of synovial origin, mucoid degeneration or injury have all been proposed. An ex-amateur footballer with paralysis of the left CPN but unaffected peroneus longus proved, on surgical exploration of the CPN at the level of the head of the fibula, to have an intraneural cyst, which was removed. As it was a true intraneural cyst with no cleavage plane, no endothelial lining and no articular peduncle, and given the footballing history, the pathogenesis was considered to be traumatic. A year after cysts removal, the patient has fully regained the strength of the left foot.