19 The Hypothenar Area: Anatomy of the Ulnar Carpal Tunnel



Keichi Murata

19 The Hypothenar Area: Anatomy of the Ulnar Carpal Tunnel



19.1 Anatomy of the Ulnar Carpal Tunnel


The ulnar carpal tunnel 1 is composed of three distinct parts from a proximal to distal direction: Guyon’s canal, the pisohamate tunnel, and the opponens tunnel (▶Fig. 19.1). The ulnar nerve passes through these tunnels, accompanying the ulnar artery.

Fig. 19.1 Anatomic location of the ulnar carpal tunnel. (Right hand). ADM: Abductor digiti minimi; FDMB: Flexor digiti minimi brevis; ODM: Opponens digiti minimi; UA: ulnar artery; SBUN: Superficial branch of ulnar nerve; DBUN: Deep branch of ulnar nerve.


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

Fig. 19.2 Volar aspect of the distal forearm with the flexor carpi ulnaris (FCU) on the ulnar border of the ulnar artery and the ulnar nerve at entrance to the Guyon’s canal.
Fig. 19.3 Deeper dissection with structures of the carpal tunnel and FCU removed sahowing the entrance to Guyon’s canal.
Fig. 19.4 (a) Different perspective at the entrance to Guyon’s canal showing the relationship of the tortuous ulnar artery on the radial side of the ulnar nerve. (b) End on perspective showing the relationship of Guyon’s canal to the carpal tunnel. The flexor retinaculum forms the roof of the carpal tunnel and the floor of Guyon’s canal. FCR: flexor carpi radialis; P: pisiform; Tra: trapezium; Tri: triquetrum
Fig. 19.5 The ulnar nerve divides into the deep branch (DBUN) and the superficial branch (SBUN). The DBUN enters the pisohamate tunnel.

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)

Fig. 19.6 The three parts of the ulnar carpal tunnel: 1. Guyon’s canal; 2. the pisohamate tunnel; 3. the opponens tunnel. ADM: Abductor digiti minimi; DBUN: Deep branch of ulnar nerve; SBUN: Superficial branch of ulnar nerve; UA: Ulnar artery; UN: Ulnar nerve.

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.

Fig. 19.7 Ganglion cyst compressing the deep branch of the ulnar nerve in the pisohamate tunnel, causing motor ulnar palsy without sensory symptoms.
Fig. 19.8 The elevator shows the entrance to the pisohamate tunnel.
Fig. 19.9 The floor of the Guyon’s canal and the pisohamate tunnel. ADM: abductor digiti minimi; FCU: flexor carpi ulnaris; P: Pisiform; FR: flexor retinaculum; PH: pisohamate ligament; PM: pisometacarpal ligament; UA: ulnar artery; UN: ulnar nerve.
Fig. 19.10 (a, b) Pisohamate and pisometacarpal ligaments that form the floor of the pisohamate tunnel. (P: Pisiform; MC5: 5th metacarpal; 5MC: 5th metacarpal.)
Fig. 19.11 The structural patterns of the arcade of the pisohamate tunnel. (Right hands) a-1,2: Scheme and photograph of Type 1 arcade, respectively. b-1,2: Scheme and photograph of Type 2 arcade, respectively. c-1,2: Scheme and photograph of Type 3 arcade, respectively. *Arcade, ADM: Abductor digiti minimi, FDMB: Flexor digiti minimi brevis, ODM: Opponens digiti minimi, DBUN: Deep branch of the ulnar nerve, UN: Ulnar nerve, UA: Ulnar artery.


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.

Fig. 19.12 Scheme of the three zones of the ulnar carpal tunnel.


Zone I

Zone I extends approximately 3 cm from the proximal edge of the palmar carpal ligament (PCL) to the bifurcation of the ulnar nerve into the DBUN and the SBUN. The ulnar nerve, along with the ulnar artery, passes deep to the PCL to enter the ulnar carpal tunnel. The artery is located slightly superficial and radial to the nerve at this level. In Zone I, the ulnar nerve carries both motor and sensory fibers; the palmar–radial fibers contain the fascicles to become the SBUN, whereas the dorsal–ulnar fibers contain the fascicles to become the DBUN. Therefore, a lesion in Zone I will have a high likelihood of producing both motor and sensory deficits.



Zone II

Zone II encompasses the portion of the ulnar carpal tunnel distal to the bifurcation of the ulnar nerve, in the region where the DBUN passes. Zone II is consistent with combination of the pisohamate and the opponens tunnels. The DBUN in Zone II is purely motor; therefore, lesions in this zone produce only motor deficits.



Zone III

Zone III originates just distal to the bifurcation of the ulnar nerve. This zone encompasses the SBUN. This zone is located in superficial layer between the hypothenar fascia dorsally and subcutaneous tissue palmarly. As the SBUN continues distally, it gives off small branches that innervate the palmaris brevis. The SBUN in Zone III contains mostly sensory fibers, along with motor fibers to the hypothenar muscles. Lesions in this zone will produce primarily sensory deficits, with possible motor weakness of the hypothenar muscles.



19.2 Muscle Anatomy



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

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Jan 25, 2021 | Posted by in ORTHOPEDIC | Comments Off on 19 The Hypothenar Area: Anatomy of the Ulnar Carpal Tunnel

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