Midcarpal joint - part II

PART I

Relations

All of the structures entering or leaving the hand have to cross the region of the wrist. Some of these will lie directly against the carpal bones, others will be separated by intervening soft tissues. The nature of the arrangement of the carpal bones, in which they form part of a fibro-osseous canal, makes the anterior aspect of the wrist an extremely important region.

The carpal bones of each row form a transverse arch with a palmar concavity(figure a). The principal structure maintaining the bones in this position is the flexor retinaculum. Consequently, it is considered by some to be an accessory ligament to the intercarpal joints. The flexor retinaculum attaches medially to the pisiform and the hook of the hamate, and laterally to the scaphoid tubercle and to both lips of the groove on the trapezium so forming a small lateral compartment separate from the rest of the canal. Passing through this lateral compartment is the tendon of flexor carpi radialis enclosed within its own synovial sheath(b). Through the larger main part of the canal pass:

1. the tendon of flexor pollicis longus most laterally, deep in the concavity of the carpal bones;
2. the four tendons of flexor digitorum profundus lying side-by-side directly over the capitate;
3. the four tendons of flexor digitorum superficialis overlying those of profundus, with the tendons to the third and fourth digits anterior to those for the second and fifth; the tendons of flexors digitorum superficialis and profundus being enclosed within the same synovial sheath;
4. the median nerve lying lateral to the superficialis tendons(b).

Inflammation of the synovial sheaths within the so-called carpal tunnel may lead to compression of the median nerve, and gives rise to carpal tunnel(or median nerve) syndrome. This leads to paraesthesia and diminution of sensory acuity in the region of the median nerve’s sensory distribution, loss of power and limitation of some thumb movements, together with some wasting of the thenar eminence. Passing anterior to and blending with the flexor retinaculum is the tendon of palmaris longus. Also passing superficial to the retinaculum on the medial side is the ulnar nerve with the ulnar artery lateral to it. In addition, the palmar cutaneous branches of the median and ulnar nerves, and the superficial palmar branch of the radial artery enter the hand by crossing the retinaculum.

On the posterior aspect of the carpal bones, the extensor tendons pass into the hand. They are bound down by fibrous septa passing from the deep surface of the extensor retinaculum to ridges on the radius, ulna and capsular tissues of the joint(upper picture b).

The six longitudinal compartments so formed separate and transmit the tendons of the nine muscles of the extensor compartment. Most laterally, over the lateral surface of the radial styloid process and continuing over the scaphoid and trapezium, pass the tendons of abductor pollicis longus and extensor pollicis brevis within the same synovial sheath(a).  In the adjacent compartment over the radius lateral to the dorsal tubercle, and then over the scaphoid and the most medial part of the trapezium and the trapezoid, run the tendons of extensor carpi radialis longus and brevis(a). In the third compartment, in a groove on the medial side of the dorsal tubercle, is the tendon of extensor pollicis longus(a). However, because this tendon uses the dorsal tubercle as a pulley, it deviates laterally towards the thumb and so crosses the scaphoid and trapezium between the tendons of the two previous compartments. Running over the most medial part of the dorsum of the radius, and then over the adjacent parts of the scaphoid and lunate, and the capitate are the four tendons of extensor digitorum, with the tendon of extensor indicis deep to them(b). All five tendons are enclosed with a common synovial sheath. Crossing the posterior surface of the inferior radioulnar joint, the lunate and the adjacent surfaces of the capitate and hamate is the tendon of extensor digiti minimi(b). Finally, the tendon of extensor carpi ulnaris passes in a groove on the back of the ulna and onto the triquetral before attaching to the base of the fifth metacarpal(b). The dorsal branch of the ulnar nerve and the terminal branches of the superficial radial nerve cross the extensor retinaculum to enter the dorsum of the hand on its medial and lateral sides respectively. The other major structure to enter the hand is the radial artery, and it does so by a convoluted route(a). In the forearm proximal to the flexor retinaculum, the radial artery can be palpated lateral to the tendon of flexor carpi radialis. It then turns laterally to cross the radial collateral carpal ligament, the scaphoid and trapezium, being crossed by the tendons of abductor pollicis longus and extensors pollicis brevis and longus before passing into the palm of the hand between the two heads of the first dorsal interosseus muscle. The hollowed region between the tendons of abductor pollicis longus and extensor pollicis brevis laterally, and extensor pollicis longus medially when the thumb is extended is known as the “anatomical snuffbox”(a). The radial artery crosses the floor of this hollow, which is formed from proximal to distal by the radial styloid process, scaphoid, trapezium and base of the first metacarpal; its pulsations can be felt radially by applying firm pressure between the tendons.

Stability

Because of the attachment of the flexor retinaculum and the numerous tendons crossing the joint both anteriorly and posteriorly, the wrist is a relatively stable region. Nevertheless, abnormal stresses applied to it may result in dislocation or fracture.

A fall on the outstretched hand may result in a dislocation at the radiocarpal and/or midcarpal joints, involving anterior dislocation of the lunate. Usually this can be reduced by manipulation. Care must be taken however in not confusing a dislocation with a Colles’ fracture. A fall on the hand is more likely to result in the force being transmitted through the trapezium and trapezoid to the scaphoid, which tends to fracture across its waist. Persistent pain on applying pressure in the anatomical snuffbox is characteristic of scaphoid fracture. Care should be taken when setting the fracture that the two fragments are aligned and in contact, otherwise non-union and/or a vascular necrosis may result if viable blood vessels reach only one of the fragments. (The blood supply to the scaphoid is from distal to proximal.)

Movements

The movements which occur at the radiocarpal and midcarpal joints take place at the same time. The total range of flexion and extension is therefore 85° in each direction. Flexion is limited by tension in the extensor tendons and is greatly reduced if the fingers are fully flexed. The main muscles providing flexion are the flexors carpi radialis and ulnaris. The main muscles producing extension are the extensors carpi ulnaris and radialis longus and brevis. Radiographic film shows that flexion and extension movements at the wrist occur about a single transverse axis through the head of the capitate.

The total range of abduction and adduction which occurs at the wrist is also the sum of the ranges possible at the radiocarpal and midcarpal joints. Consequently, abduction has a range of 15° and adduction a range of 45°. The movements occur about a single anteroposterior axis passing through the head of the capitate slightly more distal to the axis for flexion and extension. Abduction is more limited than adduction primarily because the radial styloid process projects further distally than the ulnar styloid process. Abduction is brought about by flexor carpi radialis and extensors carpi radialis longus and brevis, while adduction is achieved by flexor and extensor carpi ulnaris.

Biomechanics

The lines of action of the muscles of the wrist are always oblique with respect to the axes of movement. By using only one muscle the movement produced would not be a pure movement. For example, contraction of flexor carpi radialis would produce flexion and abduction at the wrist. To produce pure flexion the unwanted abduction has to be cancelled; this is achieved by contracting flexor carpi ulnaris. Thus by combining various forces acting in different directions any desired movement can be produced within the complete range of motion of the joint.

The carpal flexors and extensors fix the wrist during extension or flexion of the fingers, thereby preventing the digital muscles from losing power and efficiency, which would occur if they also acted on the radiocarpal and midcarpal joints. When powerful movements of the fingers are required, both the flexors and extensors of the wrist contract simultaneously. The importance of such actions is obvious when attempts are made to grip strongly with the finger flexors when the wrist is already flexed. Extending the wrist stretches these muscles so that they can now exert considerable power. Slight extension of the wrist is the position naturally adopted when the hand is used for gripping: look at your own wrist when writing or picking up a mug. If the wrist is likely to become fixed through disease, it should be secured in a position of slight extension so that a powerful and precise grip can still be achieved.

The extrinsic finger flexors are the major force-producing muscles during exertions of the hand. Deviation of the wrist causes these tendons to move against the adjacent walls of the carpal tunnel. When the wrist is flexed the tendons are supported by the flexor retinaculum, and when extended by the carpal bones. The force between the tendons and the retinaculum may compress the median nerve and be an important factor in carpal tunnel syndrome: such compression has been confirmed by direct pressure measurement. As well as the median nerve being compressed, the synovial sheaths surrounding the flexor tendons are also compressed, both in flexion and extension. This may lead to their inflammation and subsequent swelling, leading to further compression of the median nerve. Taking into account wrist size, the loading of the flexor retinaculum in flexion is 14% greater in females than in males. This may be one of the reasons why carpal tunnel syndrome is between 2 and 10 times more prevalent in women than men.

In some cases of limitation or absence of movement at the wrist, often associated with persistent pain, total wrist arthroplasty can relieve pain and improve mobility.