Foot drop caused by a movable cystic mass during knee motion

BACKGROUND: Nerve injury due to a movable cystic mass during knee motion is a rare neuropathy, which can be diagnosed precisely using a dynamic ultrasonogram. OBJECTIVE: To present a case with foot drop and common peroneal neuropathy that involved a movable cystic mass during knee flexion adjacent to the proximal tibiofibular joint. METHODS: Case report. RESULTS: A 47-year-old female was referred to our institute for a right foot drop. Common peroneal nerve (CPN) injury was confirmed by an electrophysiologic study. Upon a dynamic ultrasonographic study during knee flexion, a cystic mass was found that was not scanned on the static images. The mass displaced the lateral head of the gastrocnemius muscle and CPN. A cystic mass that was adjacent to the proximal tibiofibular joint was confirmed by magnetic resonance imaging. CONCLUSION: Ultrasonography is a convenient first-line diagnostic method of peripheral nerve disorder by virtue of its higher spatial resolution, cost-effectiveness, and prompt diagnosis. The real-time and dynamic scanning attributes are the discriminative merits of this imaging tool. This case report suggests that CPN compression occurred due to a movable cystic mass during knee flexion. Dynamic ultrasonographic evaluations could be helpful to diagnose pathologic musculoskeletal conditions.