Forearm and Wrist Joint: Anatomy and Techniques

Flexor Compartment

The two main muscles of the flexor compartment are flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), which occupy the central part of the compartment and make up most of its bulk. Overlying these are three muscles, flexor carpi ulnaris (FCU), palmaris longus and flexor carpi radialis (FCR) ( Fig. 10.1 ). The final member of the compartment, flexor pollicis longus (FPL), arises from the radius and lies alongside flexor digitorum profundus in the mid- and distal forearm.

The three principal nerve trunks, median, ulnar and radial, all lie in the anterior, though strictly the radial nerve is still in the lateral component of the posterior compartment, albeit in a very anterior location. Ulnar and radial nerves are accompanied by their corresponding arteries. The median nerve lies more centrally between flexor digitorum superficialis and profundus. Occasionally, the median nerve is also accompanied by a persistent median artery. The anterior interosseous nerve and its accompanying artery are located between flexor digitorum profundus and flexor pollicis longus in close proximity to the interosseous membrane.

Extensor Compartment

In the extensor compartment, three separate layers are discernible: two posterior (superficial and deep) and one lateral. The muscles in the lateral compartment are brachioradialis, and more distally extensor carpi radialis longus (ECRL) and brevis (ECRB) ( Fig. 10.2 ), whose tendons will ultimately form extensor compartment 2. In the posterior section, the superficial layer contains extensor digitorum (ED), extensor digiti minimi (EDM) and extensor carpi ulnaris (ECU) (extensor compartments 4, 5 and 6) ( Fig. 10.3 ). The deep component contains abductor pollicis longus (APL) on the radial side of extensor pollicis longus (EPL), forming extensor compartments 1 and 3 respectively. More distally, abductor pollicis longus will receive an additional component from extensor pollicis brevis (EPB) to complete compartment 1. In the proximal forearm, the extensor compartments from radial to lateral are therefore 2, 1, 3, 4, 5 and 6. It is clear that 1 and 2 are not optimally arranged and are going to have to cross each other to reach their ultimate destination. The crossover occurs just proximal to the wrist.

Although it is not particularly large, the main nerve in the extensor compartment is the posterior interosseous nerve (PIN). This is the major motor division of the radial nerve, formed below the elbow joint and passing into the posterior compartment between the two heads of supinator. Unlike its anterior namesake, the posterior interosseous nerve does not directly lie on the interosseous membrane, but between the superficial (EDC, EDM and ECU) and deep (APL and EPL) muscle groups. It can be followed distally where it is located deep to extensor digitorum communis at the level of the wrist.

Which Nerve Innervates Which Muscle?

Neural compression syndromes manifest on imaging in two ways. At the level of the compression, the nerve is usually enlarged and in many cases the lesion causing the compression can be seen. Distal to the compression, the innervated muscle may atrophy. The ultrasound features of this are usually late. MRI is more sensitive as it can also pick up the oedematous stage of denervation, which ultrasound does not. It is helpful, therefore, to know the innervation of the forearm muscle so that the correct pattern of atrophy may be recognized.

Overview

The most commonly employed position to examine the wrist and hand is to have the patient seated opposite the examiner. Both wrists can be placed on the examination couch, allowing full access and side-to-side comparison. Plenty of coupling jelly is required due to the varied contours of the hand, consequently an absorbent pad placed beneath the hand is essential.

As with other joints, the patient's symptoms will direct the examination and the efficacy of ultrasound is highest when there is a specific localizing symptom. If pain is more diffuse, MRI is more likely to be helpful. The wrist examination, therefore, will be separated into specific areas with specific imaging goals in each area.

The examination of the patient with pain on the dorsal aspect of the wrist begins with an assessment of the extensor tendons. These are arranged in six compartments numbered accordingly from radial to ulnar. Each compartment is associated with specific pathology. As with tendons elsewhere, the extensor tendons are best assessed in the axial plane.

The tendons on the flexor side of the wrist are also arranged in compartments. The largest compartment is the carpal tunnel. This contains the superficial and deep flexor tendons, flexor pollicis longus tendon and the median nerve. The finger flexors share a single large synovial compartment. The flexor pollicis longus tendon has its own synovial sheath. The other three flexor tendons are flexor carpi radialis, flexor carpi ulnaris and palmaris longus. Flexor carpi radialis has its own compartment on the medial aspect of the scaphoid and tripezium tubercle and inserts on the base of the second metacarpal. Flexor carpi ulnaris inserts into the pisiform.

Position Variations

For some interventional procedures, particularly where the patient is anxious and there is a risk of fainting, an alternative position is to have the patient lying prone with their arm extended above their head, the so-called Superman position. This also allows nearly as much access to all parts of the wrist and hand. In addition, if an interventional procedure is required then, the patient's anxiety will be allayed by being recumbent and not having to sit up and ‘observe’ the procedure.