A complex cyst characterized into its individual components: a shared pathogenesis from the superior tibiofibular joint.

In a patient with a peroneal neuropathy, magnetic resonance imaging (MRI) allowed characterization of a complex para-articular cyst into three different types of cysts, all derived from the superior tibiofibular joint: 1) an intraneural cyst extending along the articular branch to the common peroneal nerve; an interconnected intraneural component extending within the extensor digitorum muscle neural branch, penetrating the fascia of the anterior compartment, and reaching the subcutaneous tissues; 2) an intraosseous cyst isolated to the fibular head and neck, and 3) an extraneural cyst heading toward the tibial nerve and vessels. Joint resection and articular branch disconnection led to excellent functional recovery; an MRI confirmed no cyst recurrence. This case illustrates that different types of cysts can derive from a single joint of origin and extend in various locations and that the articular (synovial) theory is versatile for demonstrating a joint connection, even in unusual appearing combinations of cysts.