19.1.1 Guyon’s Canal (▶Fig. 19.2, ▶Fig. 19.3, ▶Fig. 19.4, ▶Fig. 19.5)

The anatomic space adjacent to the hypothenar eminence was described by Jean Casimir Félix Guyon, a French surgeon and urologist, as “loge de Guyon” in 1861. ² As the term loge indicates, this space is a hiatus starting at the distal edge of the palmar carpal ligament proximally and ending at the proximal fibrous arch of the hypothenar fascia distally. However, textbooks sometimes defined it to include the more proximal area where the palmar carpal ligament (PCL) exists. This area is also on the risk of nerve compression. The floor of the tunnel is the flexor digitorum profundus proximally, the flexor retinaculum (FR) distally (▶Fig. 19.4b). The radial boundary of the tunnel is the underlying flexor retinaculum (FR). The pisiform and the flexor carpi ulnaris form the ulnar boundary of the tunnel. The roof of the tunnel is composed of PCL, the palmaris brevis, and hypothenar fat and fibrous tissue. Within this tunnel, the ulnar nerve is accompanied by the ulnar artery radially (▶Fig. 19.2, ▶Fig. 19.3, ▶Fig 19.4) and divides into its deep motor branch and two superficial sensory branches. The superficial branch of the ulnar artery and the arterial branches to the palmaris brevis and the overlying skin pass through the fat tissue at the hiatus. In some hands, the motor branch to the abductor digiti minimi (ADM) is bifurcated from the main trunk of the ulnar nerve within this tunnel. Guyon’s canal continues distally to the pisohamate tunnel, through which the deep branch of the ulnar nerve (DBUN) passes (▶Fig 19.2, ▶Fig. 19.3, ▶Fig. 19.4, ▶Fig. 19.5). ^{1 , 3 – 5}

19.1.2 The Pisohamate Tunnel (▶Fig. 19.6)

The pisohamate tunnel ⁶ is the middle segment of the ulnar carpal tunnel (▶Fig. 19.6, ▶Fig. 19.7, ▶Fig. 19.8, ▶Fig. 19.9) continuing from Guyon’s canal proximally and the opponens tunnel distally. The entrance of the tunnel is formed by the fascial arcade or intermuscular space, consisting of the bony attachment of the hypothenar muscles. Ganglions and other masses in this tunnel can compress the DBUN and cause pure motor ulnar palsy (▶Fig. 19.7). The floor of the tunnel is formed by the pisohamate and pisometacarpal ligament and joint capsule between these bones (▶Fig. 19.9 and ▶Fig. 19.10). The hook of the hamate and the pisiform form the radial and ulnar boundary of the tunnel, respectively. ^{1 , 3 – 5} The structural patterns of the entrance (arcade) of the pisohamate tunnel can be classified into three types (▶Fig. 19.11). ⁷ In type 1, it consists of the intermuscular space between the ADM and the flexor digiti minimi brevis (FDMB). In this type, the fascial continuity between the ADM and the FDMB forms the arched structure (fascial arch) across the hiatus and the DBUN passes under this fascial arch. This type is the most common. In type 2, it consists of the fascial arch between the two origins of the FDMB, the pisiform and the hook of the hamate. In type 3, it consists of the intermuscular space between the ADM and the opponens digiti minimi (ODM). There is no fibrous arcade at the entrance. In this type, the FDMB may be absent. The DBUN passes through this tunnel and then turns radially around the hook of the hamate. In some cases, an artery accompanies the DBUN in this tunnel.

19.1.3 The Opponens Tunnel (▶Fig. 19.6)

The opponens tunnel is the third segment of the ulnar carpal tunnel continuing from the pisohamate tunnel distally. The ODM originates from the hook of the hamate, pisohamate ligament, and distal margin of the FR. This muscle splits into two fibromuscular lamellae (superficial and deep), which form a slitlike tunnel. The DBUN passes between the superficial and the deep layers of the ODM and continues radially and dorsally, branching several motor branches to the ODM. ^{1 , 3 – 5}

19.1.4 Anatomical Zones of the Ulnar Carpal Tunnel

The ulnar carpal tunnel has been divided in three zones based on the topography of the nerve and its relationship to the surrounding structures (▶Fig. 19.12). ⁸ Zone I consists of the portion of the tunnel proximal to the bifurcation of the DBUN and superficial branches of the ulnar nerve (SBUN). Zone II encompasses the DBUN and surrounding structures. Zone III includes the SBUN and adjacent distal and lateral tissues.

19.2.1 Palmaris Brevis

The palmaris brevis is a thin and small quadrangular muscle located between the hypothenar fascia and the skin at the ulnar side of the palm. The muscle fibers of this muscle are perpendicular to the axis of the upper extremity. ^{4 , 9 , 10}

Origin : The muscle arises from the FR and the medial border of the central part of the palmar aponeurosis. ^{4 , 9 , 10}

Insertion : The muscle fibers insert into the dermis on the ulnar border of the hand.

Function : It provides wrinkling of the skin on the ulnar side of the palm of the hand and deepens the hollow of the palm by accentuating the hypothenar eminence. ^{9 , 10}

Innervation : The nerve supply arises from the SBUN. ^{4 , 9 , 10} Blood supply : The branch from the superficial palmar arch (SPA) gives blood supply to this muscle. ^{4 , 9 , 10}

Variations : The muscle varies greatly in size. This muscle may be absent or doubled. It sometimes inserts into the FDMB or the pisiform. ^{9 , 10} The palmaris brevis profundus muscle has been found to arise transversely from the FR and the palmar aponeurosis to the pisiform and may be located within Guyon’s canal; therefore, it may cause ulnar nerve compression. ¹¹