The mechanism underlying combined medial and lateral plantar and tibial intraneural ganglia in the tarsal tunnel.

Intraneural ganglion cysts in the tarsal tunnel are rare. We present a patient who had an intraneural ganglion cyst involving the medial and lateral plantar and distal tibial nerves. Magnetic resonance imaging revealed evidence to support the joint-related (i.e., subtalar) origin of the cyst. Careful reinterpretation of the imaging supported a phasic mechanism (i.e., cross-over) to explain the interrelated pathogenesis of the intraneural cyst within the three nerves. This mechanism is analogous to that described for the prototypes-the peroneal, tibial and sciatic nerves in the knee region-and can be generalized to other nerves in the foot and ankle region. We believe that understanding the pathogenesis sheds light on the effective treatment.