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 . This canal was first described in 1861 by Guyon  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 . Cobb et al.  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 .
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. , who observed the palmaris brevis more distal but in the same position as Guyon, along with Enna et al.  suggested the term piso-hamate tunnel. Denman  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 .
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 . 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 . Accessory muscles are the most common anatomical variations within the Guyon’s canal which might contribute to the symptoms of ulnar tunnel syndrome . 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 . 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.