To describe US and MR imaging findings in patients with peroneal intraneural ganglia.

Seventeen consecutive patients with a palpable mass in the fibular neck area and foot-drop suggesting dysfunction of the common peroneal nerve were prospectively evaluated with 12-5MHz and 17-5MHz US and 1.5TMR imaging. All patients had abnormal nerve conduction studies

In all but one patients, US and MR imaging identified ganglia of variable size and shape in relation to the superior tibiofibular joint. Based on their relationship with the peroneal nerve, US and MR imaging divided these cysts in extraneural ganglia (n=9/16), which developed outside the nerve, and intraneural ganglia (n=7/16), developing within the nerve. Intraneural ganglia had a stereotypical appearance: US found the bulk of the ganglion on the anterolateral aspect of the superior tibiofibular joint, located remotely from the position of the peroneal nerve. The ganglion was invariably associated with a dilated articular branch of this nerve. This branch appeared as a long tubular process coursing along the anterolateral aspect of the fibula to join the bulk of the ganglion with the nerve. It was markedly enlarged and assumed a cystic appearance without detectable fascicles. More proximally, the fascicles of the deep and common peroneal nerve were displaced eccentrically by the cyst growing within the epineurium. The overall size of the ganglion and the entity of nerve deficit varied with time with fluctuating phases of worsening and recovery of symptoms. Two intraneural ganglia regressed at 6-months follow-up.

US and MR imaging are promising for evaluating patients with peroneal neuropathy caused by superior tibiofibular joint ganglia. By providing unique information on the intraneural location of the cyst and the status of the involved nerve, US and MR imaging have potential for major impact on treatment planning.