Applied Anatomy

The ulnar nerve gives off the dorsal ulnar cutaneous branch 6 to 8 cm proximal to the ulnar styloid, to innervate the skin over the ulnar side of the dorsum of the hand and the dorsal aspect of digit V and half of digit IV. Then, the ulnar nerve enters Guyon canal (distal ulnar tunnel) at the level of the distal wrist crease where it divides into superficial (primarily sensory) and deep (pure motor) palmar branches. The deep branch enters the hand through the pisohamate hiatus while the superficial branch travels subcutaneously passing over the hypothenar muscles.

Guyon canal is formed proximally by the pisiform bone and distally by the hook of the hamate. Its floor is formed by the triquetrum and hamate bones along with the thick transverse carpal ligament, while its roof is composed of a loose connective tissue (Figure C9-1). In the distal portion of Guyon canal lies the pisohamate hiatus (PHH). This aperture is bounded anteriorly by a fibrous arch formed by the two musculotendinous attachments of the flexor brevis digiti minimi (or quinti), a hypothenar muscle, to the hook of hamate and the pisiform bone (Figure C9-2). The posterior boundary of the PHH, is formed by a thick pisohamate ligament which extends from the pisiform bone to the hook of the hamate. The origin of the major motor branch to the ADM is proximal to this hiatus in the majority of hands.

Figure C9-1 Anatomy of the ulnar nerve within Guyon canal at the wrist. 1 = ulnar artery, 2 = superficial branch of the ulnar nerve, 3 = hamulus, 4 = fibrous arch of the hypothenar muscles (see also Figure C9-2), 5 = pisiform, 6 = transverse carpal ligament, 7 = palmaris brevis, 8 = palmar carpal ligament.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Gross MS, Gelberman RH. The anatomy of the distal ulnar tunnel. Clin Orthop 1985;196:238–247, with permission.)

Figure C9-2 The pisohamate hiatus (PHH) in the distal portion of Guyon canal. ABD.D.Q. = abductor digiti quinti (or minimi), F.B.D.Q. = flexor brevis digiti quinti (or minimi), F.C.U. = flexor carpi ulnaris, Opp.D.Q. = opponens digiti quinti, U. = ulnar.

(Modified from Uriburu IJF, Morchio FJ, Marin JC. Compression syndrome of the deep motor branch of the ulnar nerve [pisohamate hiatus syndrome]. J Bone Joint Surg 1976;58A:145–147, with permission.)

The deep palmar motor branch innervates the hypothenar muscles (the ADM, flexor brevis digiti minimi, and opponens digiti minimi) while in Guyon canal and often gives these muscles an additional branch after it enter the PHH. Then, the deep palmar motor branch travels through the palm and innervates all four dorsal and three palmar interossei, the third and fourth lumbricals, the adductor pollicis, and the deep head of the flexor pollicis brevis. The superficial cutaneous branch innervates the palmaris brevis muscle, as well as the ulnar side of the palm and palmar aspect of digit V and half of digit IV.

Clinical Features

Patients with ulnar neuropathy at the wrist often presents with painless unilateral hand atrophy. These ulnar nerve lesions pose a diagnostic challenge, particularly when the weakness is not associated with sensory loss. It is useful in sorting out the cause of hand weakness or atrophy to distinguish between atrophy of all intrinsic hand muscles from atrophy that is restricted to the thenar or hypothenar muscles. Table C9-1 lists the various causes of wasting and weakness of the hand.

Table C9-1 Causes of Unilateral Atrophy and Weakness of Intrinsic Hand Muscles