Superficial ulnar nerve articular branch: More than arcane trivia.

Abstract

Dear Editor, Our group has shown the robustness of Hilton's law and its application to understanding the joint-related origin of intraneural ganglion cysts as explained by the unifying articular theory (Spinner et al., 2003; Hebert-Blouin et al., 2014; Spinner et al., 2016). Because of the lack of known superficial ulnar nerve joint innervation in the palm in common anatomic references, two cases of superficial ulnar intraneural ganglion cysts in the palm could not be supported by the articular theory and challenged our current understanding-until now (Netter, 2014; Standring, 2016; Grant, 2017). The first description of a superficial ulnar intraneural ganglion was limited to a few sentences in a Japanese article. The diagnosis was confirmed at operation without any preoperative imaging or operative photographs (Hirooka et al., 1998). Due to the limitations and inability to further study the case, it was easily dismissed. A similar, more recent case of a superficial ulnar intraneural ganglion cyst includes images from the preoperative magnetic resonance imaging (MRI) study and intraoperative photographs (Ozturk et al., 2017). Neither of the two cases had an identified joint connection. Convinced by the second case of the existence of this entity, the inability to harmonize this entity with the articular theory demanded an explanation. Thus, we sought an anatomic explanation based on the current theory. Our close inspection of the figures in the second case revealed evidence to suggest the possibility of a cystic articular branch (Fig. 1). Disappointingly, we were unable to obtain the original images from the authors of the second study. (A) Axial T1-weighted MR image showing intraneural cyst within the superficial ulnar nerve (asterisk) and fatty atrophy within the palmaris brevis muscle (arrowhead). (B) Axial T2-weighted MR image showing intraneural cyst within the superficial ulnar nerve (asterisk) with additional cyst within a possible articular branch that was not previously noted (arrow). Note the subacute denervation changes within the palmaris brevis muscle (arrowhead). (C) Intraoperative photograph showing the intraneural ganglion cyst within the superficial ulnar nerve (asterisk) with a small nerve branch that is thought to represent an articular branch (arrow). (D) Enlarged intraoperative image of box from panel C showing the intraneural ganglion cyst within the superficial ulnar nerve (asterisk), articular branch (arrow; corresponding to arrow in Panel B), and the putative path of cyst propagation into the superficial ulnar nerve (dashed line). Annotated and adapted from Ozturk et al. (2017). A relatively obscure reference provides the simple answer. Contrary to common anatomic sources, there does appear to be joint innervation by the superficial ulnar nerve to the 5th metacarpophalangeal joint and/or carpal joints (Murakami, 1969). The innervation to these joints is described as being a twig arising from the superficial ulnar nerve near the division into the 4th common palmar digital nerve and the proper palmar digital nerve or arising from one of these branches. In 8 of 40 cadavers, this innervation was identified (Murakami, 1969). Our own cadaveric dissection confirmed the presence of this articular innervation from the superficial ulnar nerve (Fig. 2). Photograph of cadaveric dissection showing an articular branch (AB) arising from the superficial ulnar nerve (SUN) to the 5th metacarpophalangeal joint. DUN, deep branch of the ulnar nerve; UN, ulnar nerve. These two cases of superficial ulnar intraneural ganglion cysts have wrestled the knowledge of the articular branch of the superficial ulnar nerve out of the realm of arcane, esoteric trivia, and into clinical relevance. A lapse in anatomy that once challenged Hilton's Law and the unifying articular theory, now further supports both. The clinical relevance of even small nerve twigs should not be forgotten: omnia ad unguem (everything to the finger nail).