Extreme subparaneurial ganglion cysts. Part 2: The geyser theory as a mechanistic explanation for crossover.

Objective: Extreme subparaneurial ganglion cysts are poorly understood. In Part 1, the authors demonstrated that patterns of distribution in the subparaneurial space are consistent with the principles of the articular theory for intraneural ganglion cysts (INGCs). How a cyst transfers from the subepineurial to the subparaneurial compartment remains to be determined. In this part, the authors provide evidence supporting that cyst transfer occurs at the sciatic nerve bifurcation based on MRI and histological analysis, and introduce the geyser theory as an explanation for the redistribution of cysts.

Methods: The authors analyze MR images obtained in patients with extreme subparaneurial cysts as well as newly identified cases of nearly extreme subparaneurial cysts (i.e., cysts visible in the subparaneurial compartment but not to the extreme degree as defined in Part 1) for potential crossover sites. They also analyze histological sections from 10 cadavers around the sciatic nerve bifurcation for the presence of openings in the epineurium and paraneurium.

Results: MRI studies performed in 8 patients in Part 1 and 8 newly identified examples with nearly extreme subparaneurial INGCs showed evidence to support communication between different compartments at the sciatic nerve bifurcation (subepineurial-to-subparaneurial and subparaneurial-to-neighboring soft tissues). Openings in the epineurium of the common peroneal and tibial nerves at the sciatic nerve bifurcation were consistently found in all 10 cadaver specimens, as were openings in the paraneurium.

Conclusions: The authors expand on the evidence to support cyst redistribution at the sciatic nerve bifurcation for INGCs in the knee region. They put forth a logical explanation for the development of a spectrum of extreme INGCs (ranging from a faint amount of cyst around the epineurium [wedding ring sign] to extreme subparaneurial INGCs [owl eyes]) based on principles seen in a geyser. The authors believe that radiological occurrence of crossover can be explained by the presence of fenestrations in both the epineurium and paraneurium at the sciatic nerve bifurcation, allowing cyst crossover in two steps: from the subepineurial to the subparaneurial space ("cross") and between a shared subparaneurial space of different nerves ("over"). Finally, the frequent occurrence of extraneural rupture ("cross-out") is due to cyst transfer into the soft tissue compartment via an opening in the outermost circumneurial layer.