Posterior interosseous nerve intraneural ganglion cyst from the scapholunate joint.

Abstract

To the editor: During the course of a routine cadaveric dissection of the wrist, an intraneural ganglion cyst was incidentally identified at a unique site. The posterior interosseous nerve (PIN) and artery were identified with loupe magnification in the floor of the 4th extensor compartment overlying the distal interosseous membrane. Just distal to the point of arborization at the level of the radiocarpal joint, an intraneural ganglion cyst was noted within one of the branches of the PIN (Figure 1A). This branch, along with the grossly visible cyst, was carefully dissected through the dorsal wrist capsule. The intraneural cyst originated from the scapholunate joint and extended through a fenestration in the scapholunate ligament (Fig. 1B–D). It propagated proximally for approximately 2 cm. A pseudocyst with adjacent nerve fascicles was confirmed histologically, and was traced to the level of the joint in serial sections (Fig. 2). Pathology findings. A: The longitudinal histological sections from formalin-fixed paraffin-embedded tissue stained with hematoxylin and eosin (H&E). The fibrocollagenous tissue forming the ligament is to the right of the figure, and the joint is not seen here. The wall of the cystic structure is composed of dense fibrocollagenous tissue, and no epithelial lining (20×). An internal fibrocollagenous septa is also present. B: Inset from A. Small peripheral nerve fascicles were identified adjacent to the pseudocyst wall, showing mild myxoid change (200×). C: Nerve fascicles were highlighted by the S-100 immunostain (200×, polyclonal, Dako). D: Neurofilament staining did not demonstrate any nerve fibers in the pseudocyst wall (200×, clone 2F11; Dako). [Color figure can be viewed at wileyonlinelibrary.com] Operative findings. A: At the level of the wrist, an intraneural ganglion cyst (arrow) can be seen within one of the two terminal branches of the posterior interosseous nerve (PIN) [blue background]. B: The intraneural cyst (arrow) specimen and the scaphoid and lunate bones were removed. C: Disarticulation has been performed. The cyst (probe) and its origin to the scapholunate ligament are seen. S, scaphoid; L, lunate. D: A defect in the scapholunate ligament (probe) near the articular branch of the PIN is present. S, scaphoid; L, lunate. [Color figure can be viewed at wileyonlinelibrary.com] This rare anatomic finding has broad clinical implications. A posterior interosseous intraneural ganglion cyst has not been reported at the wrist (Desy et al., 2017). The PIN intraneural cyst described herein communicated with the scapholunate joint by an articular branch. The scapholunate joint/ligament is the most common site for extraneural ganglion cysts at the wrist. Extraneural cysts in this location arise from non-neural pedicles; some have thought that they can cause occult dorsal wrist pain because of the consistent proximity of the ganglion cyst and the PIN (Dellon and Seif, 1978; Steinberg and Kleinman, 1999). Our group has put forth a unifying articular theory to explain the formation and propagation of intraneural and extraneural ganglion cysts (Spinner et al., 2009). Mounting anatomic and clinical evidence supports this theory (Spinner et al., 2011). Intraneural and extraneural ganglion cysts form from synovial surfaces. Intraneural cysts originate from a synovial joint, egress through a capsular rent and dissect along an articular branch typically extending to the parent nerve; extraneural cysts would egress separate from the nerve. Both may present with findings of neuropathy, the former by intrinsic, and the latter by extrinsic compression. The shape and dimensions of these cysts would obey basic principles of fluid dynamics such as the path of least resistance and pressure fluxes. A posterior interosseous intraneural cyst has been described in one case in the elbow region (Hashizume et al., 1995); despite its not being described as having a joint connection by the original authors, our group's reinterpretation of the published imaging supported an elbow joint connection (Wang et al., 2009). This is the third known example of a joint-related intraneural ganglion cyst found in a cadaver (Spinner and Wang, 2016); the others being a deep ulnar nerve cyst connecting to a carpal bone and a tibial nerve cyst connecting to the knee joint. This specimen extends (and further unifies) our understanding of the pathogenesis of intraneural and extraneural ganglion cysts at the wrist and elsewhere.