The joints of the fingers - part I

In general, the fingers act in one plane to close around an object and so form a pincer action with the opposed thumb. The size of the object being grasped will determine whether, and to what extent, two dimensional movement at the metacarpophalangeal joints occurs.

The articulation of the second to fifth metacarpals with the distal row of carpal bones has already been considered; consequently, this section considers the metacarpophalangeal joints and the two interphalangeal joints of each finger.

Metacarpophalangeal joint

The metacarpophalangeal joint of the fingers is structurally and functionally similar to that of the thumb.

Articular surfaces

The articular surface of the metacarpal head is biconvex, with unequal curvatures transversely and anteroposteriorly. It is broader anteriorly than posteriorly, with the hyaline cartilage extending further proximally on its anterior aspect(figure a).

The base of the proximal phalanx is biconcave, but has a smaller articular surface than the metacarpal head(figure a). The surface area is increased by the presence of the palmar ligament attached to the anterior margin of the articular surface.

Joint capsule and synovial membrane

The fibrous capsule surrounding the joint is loose, and is attached closer to the articular margins on the posterior aspects of the bone than anteriorly. The capsule is strengthened on each side by collateral ligaments and replaced anteriorly by the palmar ligament. Posteriorly the extensor hood of the long extensor tendon replaces the capsule, blending at the sides with the collateral ligaments. The posterior part of the capsule also receives fibres from the distal slips of the palmar aponeurosis(figure a).

The capsule is lined by synovial membrane, which also covers all non-articular surfaces. Synovial-lined anterior and posterior recesses of the capsule permit freedom of movement, particularly during flexion.

Ligaments

In addition to the collateral and palmar ligaments associated with each metacarpophalangeal joint, the heads of the second to fifth metacarpals are united by the deep transverse metacarpal ligaments.

The collateral ligaments pass from the tubercle and adjacent depression on the side of the head of the metacarpal to the palmar aspect of the side of the base of the proximal phalanx(figure a). They are strong and tend to fan out in passing from metacarpal to phalanx. Anteriorly they blend with the palmar ligament, while posteriorly the extensor expansion joins them.

The palmar ligament is a dense fibrocartilaginous plate firmly attached to the anterior margin of the base of the proximal phalanx(figure a). Proximally it is loosely attached to the neck of the metacarpal by the joint capsule. On each side it receives some fibres from the collateral ligaments. The palmar ligament acts as a mobile articular surface facilitating flexion at the joint.

The deep transverse metacarpal ligaments are a series of short ligaments connecting the palmar ligaments of the four metacarpophalangeal joints of the fingers(figure b). They are continuous with the palmar interosseus fascia and blend with the fibrous flexor tendons. Consequently, they act to bind the heads of the four medial metacarpals together, and so limit their movement apart. (There is no ligament between the first and second metacarpal, hence the independence and freedom of movement of the thumb). The deep transverse metacarpal ligaments also receive fibres from the distal slips of the palmar aponeurosis, as well as part of the extensor expansion as it passes forwards on each side of the head of the metacarpal.

Passing behind the deep transverse metacarpal ligaments are the tendons of both the dorsal and palmar interossei, while in front of them pass the tendons of the lumbricals.

Blood and nerve supply

The arterial supply to the joints is by branches from the adjacent digital arteries, while the nerve supply is by twigs from the median, and possibly the radial nerve for the index and middle finger, and the ulnar nerve for the ring and little fingers. Root value of the nerve supply is C7.

Relations

On the posterior aspect of the joint is the expansion of the long extensor tendon(picture a below), part of which passes around the sides of the metacarpal head blending with the deep transverse metacarpal ligament. The tendon of the lumbrical muscle passes lateral to the joint(a,c), anterior to the deep transverse metacarpal ligament, before attaching to the base of the proximal phalanx and the dorsal digital expansion. Exactly which interosseus tendons pass medial and lateral to each metacarpophalangeal joint depends on which finger is being considered:

1. for the index finger, the first dorsal interosseus is lateral and the first palmar interosseus is medial;
2. for the middle finger, the second and third dorsal interossei are lateral and medial respectively;
3. for the ring finger, the second palmar interosseus is lateral and the fourth dorsal interosseus is medial(figure a);
4. for the little finger, the third palmar interosseus is lateral and the tendon of abductor digiti minimi is medial.

Immediately anterior to the joint is the tendon of flexor digitorum profundus, and anterior to that, the tendon of flexor digitorum superficialis, which at the level of the joint splits into two(b). Flexor digiti minimi brevis is situated on the anterolateral aspect of the joint of the little finger.

Digital branches from the dorsal and palmar metacarpal arteries, together with digital branches from the median, ulnar and radial nerves, depending on the finger in question, pass either side of the metacarpophalangeal joint.

Stability

The metacarpophalangeal joint is stabilized primarily by the long flexor and extensor tendons crossing the joint, as well as by the lumbricals and interossei. Dislocations of the joint do occur; however they can often be reduced by manipulation.

Movements

Active movement at the metacarpophalangeal joint takes place about two axes, each of which is located in the metacarpal head approximately nine-tenths of the midline length of the metacarpal from its base. These are the movements of flexion and extension, and abduction and adduction. To these can be added axial rotation, which occurs passively.

Flexion and extension occur about a transverse axis through the head of the metacarpal(figure a). The geometry of the articular surfaces dictates that the intersection of the longitudinal axes of the proximal phalanx and the metacarpal moves distally during flexion. In extension the anterior surface of the metacarpal head articulates with the palmar ligament(figure a). During flexion the ligament moves past the head, turning upon itself to glide along the palmar surface of the shaft of the metacarpal(figure a). As this is occurring, the capsule and its synovial lining is being unfolded so as not to limit movement prematurely. The range of flexion is slightly less than 90° for the index finger, but progressively increases towards the little finger. Flexion of one joint in isolation is limited by tension developed in the deep transverse metacarpal ligaments; flexion is ultimately resisted by tension in the collateral ligaments. The range of active extension is variable between subjects but may reach 50°. Passive extension may reach as much as 90° in individuals with the lax ligaments. A study of over 400 hands gave the following total ranges of active/passive flexion and extension for each of the fingers: index, 148° - 155°; middle, 145° - 151°, ring, 149 - 159° and little, 152° - 172°.

Flexion at the metacarpophalangeal joint is brought about primarily by the lumbrical muscles, aided by the tendons of flexor digitorum profundus and superficialis, as well as the interossei. In the little finger, flexor and abductor digiti minimi also contribute to the movement. Extension is achieved at all of the metacarpophalangeal joints by extensor digitorum, with the addition of extensor indicis in the index finger and extensor digiti minimi in the little finger.

Abduction and adduction at the joint occur about an anteroposterior axis through the metacarpal head; the movement occurs away from or towards the middle finger respectively(figure a). The movement is easier and has a greater range when the finger is extended, being as much as 30° in each direction(figure b). Tension developed in the collateral ligaments in flexion of the joint severely limits the side-to-side movement, so much so that at 90° flexion the total range may be no more than 10°. The total range of active/passive abduction and adduction movements, with the fingers in a neutral position, has been reported as follows: index, 50°-62°; middle, 40°-53°; ring, 38°-55° and little, 57°-68°.

Abduction at the joint is brought about by the dorsal interossei for the index, middle and ring fingers, and by abductor digiti minimi for the little finger. At the index and middle fingers, the movement may be assisted by the first and second lumbricals respectively via their attachment to the extensor hood. If the joint is hyperextended, then extensor digitorum will also aid abduction. Adduction towards the middle finger is achieved by the palmar interossei and can be assisted by the third and fourth lumbricals for the ring and little fingers. If the joint is being flexed simultaneously, then adduction is assisted by flexor digitorum superficialis and profundus.

Active rotation is not possible except in little finger; however, because of the shape of the joint surfaces and the relative laxity of the associated ligaments, a certain degree of passive rotation can occur(figure a) having a maximum range of 60°. In the index finger, the range of medial rotation is of the order of 45°, while lateral rotation is negligible. Medial and lateral rotation in the remaining fingers have approximately equal ranges of movement.