Ulnar Nerve

Normal Anatomy

The ulnar nerve continues directly from the medial cord of the brachial plexus, and contains motor and sensory fibers arising from the C8 and T1 roots. The ulnar nerve crosses from the anterior to the posterior compartment at mid-arm level, piercing the intermuscular septum. The ulnar nerve may pass under the arcade of Struthers, present in 70% of individuals, approximately 8 cm proximal to the medial epicondyle [35] . The arcade of Struthers, made of fibers from

Mri Parsonage Turner

Fig. 6. Parsonage-Turner syndrome. Oblique sagittal fat-suppressed, T2-weighted (A) and oblique coronal proton density (B) images demonstrate denervation edema and fatty infiltration of the infraspinatus (IS) and teres minor (TMi) muscles.

the deep fascia of the distal arm, connects the medial intermuscular septum to the medial head of the triceps muscle.

The ulnar nerve is quite superficial at the elbow, where it descends posterior to the medial epicondyle within the cubital tunnel. The floor of the cubital tunnel is formed by the elbow capsule and portions of the ulnar collateral ligament, and the roof of the tunnel is formed by the arcuate ligament (Osborne ligament) or cubital tunnel retinaculum, extending from the medial epicondyle to the medial olecranon process (Fig. 7). It has been postulated that the cubital tunnel retinaculum is a remnant of the anconeus epitrochlearis muscle [36].

The ulnar nerve enters the anterior compartment of the forearm between the humeral and ulnar heads of the flexor carpi ulnaris muscle. It then continues in the forearm between the flexor carpi ulnaris and the flexor digitorum profundus muscles, and splits into superficial and deep ulnar branches in the region of Guyon's canal at the wrist.

In the elbow and forearm, the ulnar nerve supplies the elbow joint, the flexor carpi ulnaris muscle, and the ulnar half of the flexor digitorum profundus to the fourth and fifth fingers. The superficial motor branch of the ulnar nerve innervates the palmaris brevis, whereas the deep motor branch supplies the hypoth-enar muscles, third and fourth lumbricals, all interossei, adductor pollicis, flexor pollicis brevis-deep head, flexor digiti minimi, abductor digiti minimi, and opponens digiti minimi muscles. The ulnar nerve and its branches also provide sensation to the medial palm, and to the palmar and distal dorsal skin of the fifth finger and medial one half of the fourth finger.

MRI Anatomy

The ulnar nerve is clearly highlighted by fat throughout its course in the elbow and proximal forearm. The nerve is easily visualized on axial images as it traverses the cubital tunnel behind the medial epicondyle, roofed by the arcuate ligament (see Fig. 7) [37-39]. Lateral to the nerve, the accompanying recurrent ulnar vessels can often be identified. When imaging the elbow at 90° of flexion,

Arcuate Ligament Osborne
Fig. 7. Normal MRI anatomy of the ulnar nerve at the cubital tunnel. Axial proton density image depicts the ulnar nerve (open arroW) behind the medial epicondyle (m). The arcuate ligament (arrow) is seen.

reduction in the size of the cubital tunnel may render identification of the ulnar nerve difficult. Distal to the elbow joint, the ulnar nerve is commonly highlighted by fat as it travels in between the ulnar and humeral heads of the flexor carpi ulnaris muscle (Fig. 8). More distally, the nerve is found anterior to the flexor digitorum profundus muscle. The normal nerve is low in signal on T1-weighted images, but can demonstrate mild hyperintensity on fluid-sensitive images.

Pathology

Compressive neuropathy of the ulnar nerve at the elbow is the second most common neuropathy in the upper extremity, exceeded only by carpal tunnel syndrome. There are several potential sites of compression of the ulnar nerve at the distal arm and elbow. Entrapment at the cubital tunnel is the most common, likely related to the superficial course of the nerve rendering it more susceptible to direct trauma. The cubital tunnel experiences dynamic changes during flexion and extension of the elbow. Reduction in the size of the cubital tunnel during flexion is in part secondary to progressive tightening of the overlying arcuate ligament [40]. At 90° of elbow flexion, the greatest tightness of the arcuate ligament is achieved. Further decrease in the volume of the cubital tunnel and medial displacement of the ulnar nerve during elbow flexion are secondary to medial bulging of both the ulnar collateral ligament and the medial head of the triceps.

The arcade of Struthers, the edge of the medial intermuscular septum, thickened arcuate ligament, and the deep flexor pronator aponeurosis (4 cm distal to the medial epicondyle) are other potential sites of compressive ulnar neuropathy.

Mri Elbow Ulnar Nerve

Fig. 8. Normal MRI anatomy of the nerves at the elbow joint. Axial proton density image shows the ulnar nerve (arrowhead) traveling in between the ulnar (u) and humeral (h) heads of the flexor carpi ulnaris muscle. The median nerve (open arroW) is identified in the interfascial plane between the brachialis (Br) and pronator-flexor (P) muscle groups. The superficial, sensory branch of the radial nerve (black arrow) and the posterior interosseous nerve (white arroW) are visualized.

Fig. 8. Normal MRI anatomy of the nerves at the elbow joint. Axial proton density image shows the ulnar nerve (arrowhead) traveling in between the ulnar (u) and humeral (h) heads of the flexor carpi ulnaris muscle. The median nerve (open arroW) is identified in the interfascial plane between the brachialis (Br) and pronator-flexor (P) muscle groups. The superficial, sensory branch of the radial nerve (black arrow) and the posterior interosseous nerve (white arroW) are visualized.

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