Occult Isolated Articular Branch Cyst of the Lateral Plantar Nerve.

We present the first known case of cyst fluid localized to an articular branch without involvement of the larger parent nerve. During a routine tarsal tunnel decompression in a patient with fluctuating plantar foot symptoms and "normal" magnetic resonance imaging findings, we identified cyst fluid within an articular branch of the lateral plantar nerve to the subtalar joint. Our incidental intraoperative discovery was corroborated by retrospective review of the magnetic resonance images. Although we do not know whether this cyst was responsible for the patient's complaints, we believe this finding represents a snapshot into the life cycle of intraneural ganglion cysts: either the "birth" of an ultra-early one or the remnant of a once larger one. Both interpretations are consistent with the unifying articular theory and add further insight into the dynamic phases of the progression of intraneural ganglion cysts.