Magnetic resonance imaging of a deep peroneal intraneural ganglion cyst originating from the second metatarsophalangeal joint: a pattern of propagation supporting the unified articular (synovial) theory for the formation of intraneural ganglia.

UNLABELLED: A deep peroneal intraneural cyst of the first web space of the foot is presented. Analysis of the magnetic resonance image scans revealed not only a connection with the medial aspect of the second metatarsophalangeal joint, but also the presence of an interconnected cyst within the lateral digital branch of the hallux. These characteristic magnetic resonance image findings are consistent with those previously described for a peroneal intraneural ganglion cyst that arose from the superior tibiofibular joint, and include (1) origin (ascent) from the second metatarsophalangeal joint with propagation along the articular branch and into the dorsal digital branch of the second toe, (2) cross-over within the shared epineurial sheath of the deep peroneal nerve, and (3) further propagation (descent) within the dorsal digital branch of the hallux. The analogous features between intraneural ganglion cysts affecting small and large-caliber nerves support the fundamental principles of the unified articular (synovial) theory for the formation of intraneural ganglia, including (1) a connection to a synovial joint, (2) dissection of joint fluid through a capsular rent along the articular branch into the parent nerve, and (3) intra-epineurial, pressure-dependent propagation of cyst fluid along paths of least resistance. LEVEL OF CLINICAL EVIDENCE: 4.