Dorsal forearm muscles: US anatomy Pictorial Essay

Abstract

THE DORSAL COMPARTMENT OF THE FOREARM CONTAINS NINE MUSCLES: four belong to the superficial group (extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris and anconeus) and five to the deep group (supinator, abductor longus, extensor pollicis brevis, extensor pollicis longus, and extensor indices). Of these nine muscles the following details are considered: origin, course, distal insertion and their anatomical connection with those structures which are most often affected by pathologies. The radiologist must have a thorough knowledge of this complex topographic anatomy in order to perform ultrasound (US) and magnetic resonance imaging (MRI) examinations and correctly interpret the images.

Keywords: Epitroclear muscles; Extensor muscles; Forearm; Posterior interosseous nerve; Ultrasound.

Fig. 1

Proximal portion of the forearm, dorsal compartment: MRI (a) and US (b) evidence from lateral to medial: extensor carpi radialis longus and extensor carpi radialis brevis on the outside of radius; extensor digitorum communis, the extensor digiti minimi and extensor carpi ulnaris in the superficial layers; abductor pollicis longus and extensor pollicis brevis in the deep layers.

Fig. 2

The supinator muscle arises from the epicondyle, the radial collateral ligament of the elbow, the radial annular ligament and the supinator crest of the ulna. It encloses the third proximal of the radius and is inserted to the anterior and lateral profile of the radius. The muscle belly is traversed by the posterior interosseous nerve, a branch of the radial nerve, which perforates it proximally passing through the fibrous arch of Frohse to emerge in the deep layers and reach the back of the elbow (a: MRI; b: US).

Fig. 3

Middle-distal third forearm, dorsal compartment: MRI (a) and US (b) show from lateral to medial: extensor carpi radialis longus and extensor carpi radialis brevis on the outside of the radius (a, b); abductor pollicis longus and extensor pollicis brevis originate in the deep layers and rise to the superficial layers (a, c); extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris in the upper layers and extensor pollicis longus in the deep layers (a, c).

Fig. 4

In the proximal forearm abductor pollicis longus and extensor pollicis brevis are located in the deep layers on the ulnar side; to reach the first osteofibrous tunnel they travel on the radial side rising to the upper layers and then cross over extensor carpi radialis brevis and extensor carpi radialis longus (proximal intersection) (a: MRI, b: US).

Fig. 5

Extensor pollicis longus arises from the middle third of the posterior surface of ulnar diaphysis and the interosseous membrane; it travels laterally and gives rise to a tendon which traverses the third osteofibrous tunnel and crosses over extensor carpi radialis longus and extensor carpi radialis brevis (distal intersection) (a: MRI, b: US) and inserts to the basis of the distal phalanx of the thumb.