The UN passes through Guyon’s canal as it makes its way from the forearm to the wrist, where it is prone to entrapment syndromes [11]. The goal of this study was to gather cadaveric data on the prevalence of variations of the UN and its branches within Guyon’s canal, its histological presentation, and any correlations of the nerve and the canal with the gross morphometrics of the hand.
We found that males generally had longer canals; however, in women, the UN travelled further before branching. No significant differences were noticed when comparing right to left sides. Likewise, in the histological study, no significant differences were noticed. The surgical significance of the findings above means that risk of injury in the area is not increased on either side, since findings are symmetrical. Since the success of nerve grafts greatly depends on the histological compatibility [12], our findings suggest that a nerve similar to the parameters described above can be used as a guide to picking a suitable graft. Women, however, would have a higher risk of a more complicated (sensory/motor) lesion if injury occurred, since there is a higher chance of encountering the combined, unbranched UN.
Branching patterns of the UN in this area commonly varied, with the most common bifurcation into superficial and deep branches constituting only 85% of the subjects. Trifurcation was the second most common with 13%. These branching patterns were found to be symmetrical in most cases (78%) suggesting that if surgery is needed on both hands in one patient, there is a strong chance that a similar pattern will be found. It was also noted that in 70% of specimens, the UN gave a cutaneous branch within Guyon’s canal, providing another nervous structure to be aware of during surgery so as to avoid accidental transection. Other common variations included anastomoses between the ulnar and median nerves at the level of the digital nerves to fingers four and five, which was observed in 57% of specimens. These findings are similar to findings in the previous literature on an anastomosis in the same area named the Berretini anastomosis which has a prevalence of 60.9% [13]. We would like to acknowledge that other variations in the anatomy of the UN are possible; however, they were not seen in the samples studied.
The contribution of the UN to the motor functions of the palmaris brevis and the muscles of the hypothenar eminence in the context of Guyon’s canal also varied greatly. Only 52% of the specimens had a branch to the palmaris brevis from the superficial branch of the UN, and 34% did not contribute to the motor innervation of the palmaris brevis. Though there were patterns of supplying the muscles of the hypothenar eminence, in 78% of cases, the UN did not contribute at all.
The limitations encountered in this study were mostly the small number of specimens studied. Furthermore, dissection did not spare the ulnar artery. Thus, estimation of anatomical relationship between the ulnar artery and the UN is not possible. Studies done in the past on Guyon’s canal have been performed using many different points of reference for measurement, making comparisons difficult. However, recent studies have shown similar findings in branching and anastomosing patterns. Murata et al. reported that in 86% of their samples, the UN bifurcated in Guyon’s canal, with trifurcation making up the remaining 14% [14]. They also reported that in 8.6% of the specimens, there was an anastomosis between the UN’s sensory branches to the fingers, compared to our 6% rate of occurrence [14].
The clinical importance of variations in the neurovasculature contained in Guyon’s canal is the role it plays in entrapment and UN neuropathies. Bozkurt et al. reported that the most common place for entrapment/compression within the canal is the distal end, and the most common cause is an anomalous slip of muscle [3]. He also suggests other causes, including lipomas, ganglia, and overuse of the hand; however, no such examples were seen in his study. Murata et al. also mentioned the importance of the existence of a fibrous fascial arch over the deep UN as a cause of entrapment. Though a majority of the specimens they studied included such an arch (30/35), in those that did not, they noted that further exploration and care should be taken to find the source of compression in this variant [14]. Most reports are individual case studies; however, in a retrospective study, Murata et al. found that a majority of cases of UN compression are idiopathic, with trauma being the second most common [15].
Further studies have described the space known as Guyon’s canal divided into three zones. Francisco and Agarwell [
16] describe the three zones and their deficits as follows:
Zone 1—the space past, the bifurcation of the UN into deep and superficial branches, presents with sensory, motor, or both types of deficits.
Zone 2—the space surrounding the deep motor branch of the UN can present with paralysis of the intrinsic muscles and/or the hypothenar muscles.
Zone 3—the space surrounding the superficial branch of the UN presents with only sensory deficits.
Such divisions have been described in the past as well; however, the terms proximal middle and distal were used to describe zones 1, 2, and 3, respectively [11]. The symptoms and zones, however, do correlate, and it was reported that just over half of the lesions observed were found in zone 2 [11].
Due to the variety of the UN within Guyon’s canal, the zones described above may not encompass all patients presenting with symptoms of UN entrapment. Certain anastomoses like those with the median nerve that was found in our study could present with more extensive deficits than those described above. For example, sensory deficits are common in the area between the middle and ring fingers in traumatic lacerations of communicating branches between the median and UN communicating branches [13]. Incidence of trifurcations would present with different zone patterns. Keeping the prevalence of these variations in mind, we would like to suggest as Ombaba et al. does that when a patient is considered for decompression surgery of Guyon’s canal that all zones be explored, regardless of patient presentation, to ensure success of the procedure [17]. Current literature for exploration of Guyon’s canal suggests exploring the canal from lateral to medial, or starting from zone 3 and proceeding up to zone 1 [1]. However, with such high prevalence of variation within the canal, we would like to stress that these suggestions should be followed to include all three zones, and not simply stop once the compression is thought to be found in a more distal zone. This way there can be little doubt as to whether the compression has been entirely eliminated.
