The Guyon’s Canal, also known as the ulnar canal, is a fibro-osseous tunnel located on the anteromedial side of the wrist, extending from the proximal end of the pisiform to the level of the hook of hamate [1]. This canal was first described in 1861 by Guyon [2] as an intra-aponeurotic compartment with its anterior wall being formed by a fibrous layer and its posterior wall being formed by the anterior carpal ligament.

Guyon described the canal as having a medial wall formed by the pisiform, coated with aponeurotic tissue proximally and fascia covering the hypothenar eminence distally [3]. Cobb et al. [4] described the lateral boundary of the Guyon’s canal as extending to, but not attaching to the hook of hamate, thus allowing for the ulnar artery and sensory components of the ulnar nerve (UN) to take a radial course in relation to the hook of hamate. The roof of the Guyon’s canal is formed by the distal extension of the antebrachial fascia, also known as the palmar carpal ligament, and adipose tissue. However, the hook of hamate does not form a true lateral wall for the canal [3, 4]. The lateral border of the canal is also formed partially by the insertion of the palmaris brevis muscle into the flexor retinaculum [4].

Although the terminology “Guyon’s canal” is widely accepted, there have been several proposals of alternate names of the canal in the literature. McFarlane et al. [5], who observed the palmaris brevis more distal but in the same position as Guyon, along with Enna et al. [6] suggested the term piso-hamate tunnel. Denman [7] noted that the ulnar carpal space passes beyond the level of the hook of hamate and that the palmaris brevis muscle forms the radial boundary of the space upon joining the flexor retinaculum. Therefore, he concluded that this region should be referred to as the piso-retinacular space and the passage of the deep branch of the UN be called the piso-hamate tunnel [4].

The Guyon’s canal and its anatomy are important in understanding the diagnosis and treatment of ulnar tunnel syndrome, also known as Guyon’s canal syndrome [3]. The compression of the UN within the Guyon’s canal produces a wide range of symptoms, including wrist pain radiating to the ulnar two digits associated with motor and sensory deficits [3]. Accessory muscles are the most common anatomical variations within the Guyon’s canal which might contribute to the symptoms of ulnar tunnel syndrome [3]. The symptoms might also be associated with the piso-hamate hiatus located between the piso-hamate ligaments and fibrous arch at the origin of the hypothenar eminence, a site where the deep branch of the UN might be compressed [3]. Furthermore, anatomical knowledge of the canal and the UN can be critical in surgical procedures of the hand [8–10].

Taking into account the clinical importance of UN compression in the Guyon’s canal and the anatomical variability of the canal, this paper aimed to: (1) identify where the UN splits into its superficial and deep branches and the distance from the pisiform to its point of branching; (2) identify the distribution and variations in the branches of the UN; and (3) evaluate where the palmar cutaneous branch leaves the Guyon’s canal; and (4) histologically evaluate cross sections of the UN.

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].

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.

The UN in Guyon’s canal has been found to have a significant variance in its anatomy. The most common variation is the UN branching into a deep and superficial branch within Guyon’s canal. Common variants include trifurcation of the UN, anomalous small motor, and sensory branches, all of which give surgeons more structures to be aware of when operating in the area, particularly during UN decompression surgery. These variants can also alter the typical three-zone division of Guyon’s canal and the symptoms associated with each zone. A proper understanding of the possible variations can help surgeons to understand patients presenting with UN entrapment and ensure that proper planning and execution of decompression surgery resolves the patient’s symptoms.