Fasciae of the upper limb

The superficial fascia

The superficial fascia of the upper limb shows regional differences between, for example, the shoulder region and the hand. In the shoulder region and arm, it contains a variable amount of fat. In the female there is a deposition of fat in this region – a secondary sexual characteristic, the amount of which tends to increase after middle age. At the elbow, a subcutaneous bursa is present between the skin and the olecranon process. This may become enlarged in people who often tend to lean on their elbows, giving rise to a condition known as “student’s elbow”.

There is nothing particularly noteworthy about the superficial fascia in the forearm. However, in the hand there are several specializations, most of which enhance the hand’s tactile or prehensile capabilities. On the dorsum of the hand the fascia is loose and thin, and can be readily lifted away from the underlying tissue. It is in the palm of the hand, as well as the palmar surface of the digits where specializations of the fascia can be seen. In the centre of the palm, strong bands of connective tissue connect the skin to the palmar aponeurosis, which is a thickening of the deep fascia. Overlying the thenar and hypothenar regions the fixation of the skin to the deep fascia is less well marked, but here the superficial fascia is thicker and less fibrous to facilitate the gripping action of the hand. This is because it can adapt to the contours of the object being held. Palmaris brevis lies in the superficial fascia over the hypothenar eminence. By wrinkling the skin, it improves the grip. Similar less fibrous pads of tissue are also found opposite the metacarpophalangeal joints, where the superficial transverse metacarpal ligament(a band of transverse fibres) connects to the palmar surfaces of the fibrous flexor sheaths of the fingers.

The pads on the palmar surfaces of the distal phalanges are highly specialized regions which house numerous tactile nerve endings. Here the skin is firmly attached to the latter two-thirds of the distal phalanx. However, the blood supply to the distal phalanx itself runs through this highly specialized pad. If the pad becomes infected there may be compression of the artery with death of this part of the bone. On the dorsum of the distal phalanx is the nail and there is no superficial fascia deep to it.

The deep fascia

The deep fascia of the upper limb is continuous with that of the upper back, and consequently can be traced superiorly to the superior nuchal line on the occipital bone, to the ligamentum nuchae centrally in the cervical region, and to the supraspinous and interspinous ligaments in the thoracic region. In the shoulder region, the deep fascia is extremely strong over infraspinatus and teres minor, being firmly attached to the medial and lateral borders of the scapula. Superiorly, a sheath is formed for deltoid, which attaches to the clavicle, and the acromion process and spine of the scapula.

The deep fascia covering pectoralis major attaches above to the clavicle, and may be traced, via the clavicle, to the neck. Inferiorly, it is continuous with the fascia of the anterior abdominal wall. Medially, the fascia is firmly attached to the sternum, whereas laterally it becomes thickened as the axillary fascia, which forms the floor of the axilla. Further laterally it becomes continuous with the deep fascia of the arm.

Deep to pectoralis major is the clavipectoral fascia. Medially, this is attached to the first costal cartilage and passes to the coracoid process and coracoclavicular ligament laterally. The clavipectoral fascia splits to surround subclavius superiorly, and thus attaches to the undersurface of the clavicle. It also splits to enclose pectoralis minor inferiorly. An extension of the fascia from the lateral border of pectoralis major passes into the axilla and attaches to the axillary floor. This is often referred to as the suspensory ligament of the axilla. The deep surface of the clavipectoral fascia is connected to the axillary sheath surrounding the axillary vessels and brachial plexus.

In the arm, the deep fascia forms an investing layer around the muscles. It attaches at the elbow to the medial and lateral epicondyles of the humerus and the olecranon process, becoming continuous with the deep fascia of the forearm. Two intermuscular septa arise from the deep surface of this investing layer and pass to attach to the supracondylar ridges of the humerus. Both the medial and lateral intermuscular septa are found only in the lower half of the arm. Besides separating the arm into flexor and extensor compartments they also give attachment to muscles in each compartment. Of the two, the medial intermuscular septum is said to be the stronger.

In the forearm, the deep fascia of the elbow is very strong because many of the muscles arising from either the common flexor or extensor tendons also arise from the overlying fascia. The bicipital aponeurosis helps to strengthen the fascia anteriorly, while the triceps insertion does so posteriorly. The deep fascia is also strong and thick where it attaches to the posterior border of the ulna, because it gives attachment to flexor digitorum profundus, and flexor and extensor carpi ulnaris. However, the fascia becomes thinner as it approaches the wrist, although at the wrist there are two thickenings of the transverse fibres forming the flexor and extensor retinaculae. These serve to hold the tendons entering the hand in place and prevent “bowstringing”.

The flexor retinaculum

The flexor retinaculum is found anterior to the carpus where it acts as a strong band for retention of the long flexor tendons, converting the carpal sulcus into a tunnel. It attaches laterally to the tubercle of the scaphoid and to both lips of the groove on the trapezium, and medially to the pisiform and the hook of the hamate. Flexor carpi radialis passes below the flexor retinaculum in its own lateral compartment surrounded by its synovial sheath. Medial to it, the single tendon of flexor pollicis longus is present, lying within its synovial sheath. All eight tendons of flexor digitorum superficialis and profundus run below the retinaculum in a common synovial sheath. The median nerve also enters the hand by passing below the flexor retinaculum, where it lies in front of the superficial tendons. It is here that it may become compressed if the synovial sheaths become inflamed, thus giving rise to the “carpal tunnel syndrome”.

In the palm of the hand there are two layers of fascia. The deeper layer covers the interossei and encloses adductor pollicis. The more superficial layer is strong in its central part forming the palmar aponeurosis. On each side of this, the fascia thins out to cover the thenar and hypothenar muscles. The palmar aponeurosis strengthens the hand for gripping, yet also protects the underlying vessels and nerves. It is a dense, thick triangular structure bound to the overlying superficial fascia. The apex is at the wrist and its base at the webs of the fingers. From the base four slips pass into the fingers to become continuous with the digital sheaths of the flexor tendons. Each slip further divides and has attachments to the deep transverse metacarpal ligament, the capsule of the metacarpophalangeal joint and the sides of the proximal phalanx. The slips cross in front of the lumbricals, and the digital vessels and nerves. 

Fibro-osseous canals, retinaculae and synovial sheaths of the flexors of the wrist and fingers

The tendons of the digital flexors are held in close proximity to the phalanges by fibrous sheaths. These act to prevent “bowstringing” of the tendons and ensure that their pull produces immediate movement at the interphalangeal joints. The canals are formed by a shallow groove on the anterior surface of the phalanges and by a fibrous sheath which attaches to the raised lateral and medial edges of the palmar surfaces of the proximal and middle phalanges and the palmar surface of the distal phalanx. The canal is closed distally by attaching to the distal phalanx, but is open proximally deep to the palmar aponeurosis. Most of the fibres of this sheath are arranged transversely, but at the interphalangeal joints they have a criss-cross arrangement to allow flexion to occur. All five of the fibroosseous canals are lined with a synovial sheath which surrounds the enclosed tendons. In the fingers, the synovial sheath surrounds the tendons of superficialis and profundus and is connected to them by the vinculae. The sheath of the thumb contains only the tendon of flexor pollicis longus within a synovial covering.