The joints of the thumb - part II

PART I

Metacarpophalangeal joint

The metacarpophalangeal joint of the thumb is of similar design to those of the fingers, being a synovial condyloid joint between the head of the first metacarpal and the base of the proximal phalanx.

Articular surfaces

The articulation is between the rounded head of the metacarpal and the shallow, oval concavity of the base of the proximal phalanx(picture below). Both surfaces are covered with hyaline cartilage. The biconvex metacarpal head is wider anteriorly than posteriorly; indeed the articular surface does not extend very far on its posterior surface. In addition, the curvature of the metacarpal head is greater in the transverse plane than in the anteroposterior plane. The base of the proximal phalanx has a much smaller articular area than the metacarpal head. It is increased anteriorly however, by the presence of a fibrocartilaginous plate known as the palmaer ligament, which is attached to the anterior surface of the base of the phalanx by a small fibrous band which functions like a hinge.

Joint capsule and synovial membrane

A loose fibrous capsule surrounds the joint, being attached closer to the articular margins posteriorly than anteriorly(picture above). It is strengthened at the sides by collateral ligaments. Anteriorly the capsule is mainly replaced by the palmar ligament, which has a weak attachment to the neck of the metacarpal. Posteriorly the capsule is strengthened by or may be entirely replaced by the expansion of extensor pollicis longus.

Synovial membrane lines all non-articular surfaces of the joint and presents anterior and posterior recesses when the joint is extended.

Ligaments

The palmar ligament is a dense fibrocartilaginous pad that increases the phalangeal articular surface anteriorly. It is firmly attached to the anterior surface of the base of the proximal phalanx, and loosely attached to the anterior aspect of the neck of the metacarpal(picture above). The collateral ligaments of the joint blend with the sides of the palmar ligament. The ligament contains two small sesamoid bones which are attached to the phalanx and the metacarpal by straight and cruciate fibres. It is grooved on its anterior aspect by the tendon of flexor pollicis longus.

The strong collateral ligaments on either side of the joint are attached proximally to the tubercle and adjacent depression on the side of the metacarpal head, and pass to the palmar aspect of the neck of the metacarpal(picture above). Although cord-like in appearance, they do fan out slightly from proximal to distal gaining attachment to the margins of the palmar ligament. The collateral ligaments are relatively lax during extension, becoming increasingly taut with flexion of the joint.

Blood and nerve supply

The arterial supply to the joint is by branches from the princes pollicis artery, while its nerve supply is by twigs from the median nerve, root value C7.

Stability

The metacarpophalangeal joint of the thumb is stabilized by the collateral ligaments, as well as the tendons of flexor and extensor pollicis longus as they pass in front of and behind the joint to their insertion on the distal phalanx. Flexor and extensor pollicis brevis and abductor pollicis brevis also cross the joint to insert into the base of the proximal phalanx.

Movements

Being a condyloid joint, the metacarpophalangeal joint has, according to its shape, two degrees of freedom of movement, these being flexion and extension, abduction and adduction. However, as in the carpometacarpal joint, there is a third movement of axial rotation which occurs passively and is allowed because of a small degree of elasticity of the associated ligaments.

Flexion and extension. At the metacarpophalangeal joint this occurs about a single, fixed axis which passes transversely through the metacarpal at approximately nine-tenths of its midline length from its base. In passing from flexion to extension, the area of contact shifts from the palmar end of the phalangeal base to its distal end.

Flexion has a range of 45°, while extension is zero under normal circumstances, both actively and passively(picture above). It is in full extension that the anterior part of the metacarpal head articulates with the palmar ligament. As flexion proceeds, the palmar ligament gradually loses contact with the metacarpal head. At the same time the synovial recesses progressively become unpleated.

Flexion at the joint is brought about primarily by flexor pollicis brevis, aided by flexor pollicis longus. Similarly, extension from the flexed position is due primarily to extensor pollicis brevis, with some help from extensor pollicis longus.

Abduction and adduction. These are limited due to the width of the metacarpal head(picture above). The 15° of abduction and negligible adduction occurs about an anteroposterior axis through the head of the metacarpal. As well as the bony limitations to the movements, the collateral ligaments also become taut adding to the restriction.

Abduction is caused by contraction of abductor pollicis brevis. Although adductor pollicis attaches to the base of proximal phalanx, because of the severe limitation of adduction at the metacarpophalangeal joint, its action is seen principally at the carpometacarpal joint.

A certain amount of axial rotation is possible at the metacarpophalangeal joint of the thumb, which is of importance during opposition(picture above). The movement can be produced actively by the co-contraction of flexor and abductor pollicis brevis, or passively as when pressing the thumb against the index finger. The actively produced rotation is always medially directed, while that produced passively can be in either direction depending which side of the thumb comes into contact with the finger.

Movements at the joint during opposition. These include a secondary flexion at the metacarpophalangeal joint following that at the carpometacarpal joint during opposition. At the same time there is abduction at the joint, which continues after the metacarpal becomes adducted. The degree of abduction is largest when contact is made with the pad of the little finger. The flexion and abduction movements at the metacarpophalangeal joint cause, initially, an active axial rotation at the joint. After contact has been made the degree of rotation may be augmented passively.

Interphalangeal joint

Because the thumb only contains two phalanges, there is only one interphalangeal joint. Like those of the fingers, however, it is a synovial hinge joint permitting movement in one direction only.

Articular surfaces

The articulation is between the pulley-shaped head of the proximal phalanx and the base of the distal phalanx which has a median ridge separating two shallow facets(figure a below). As in the metacarpophalangeal joint, a fibrocartilaginous plate(palmar ligament) is attached to the anterior margin of the base of the distal phalanx.

Joint capsule and synovial membrane

A fibrous capsule completely surrounds the joint, being replaced by the palmar ligament anteriorly and strengthened at the sides by the collateral ligaments(figure a).

Ligaments

The collateral ligaments are attached to the sides of the head of the proximal phalanx, and pass to the palmar aspect of the base of the distal phalanx. They blend with the lateral margins of the palmar ligament.

As in the metacarpophalangeal joint, the palmar ligament is a fibrocartilaginous plate attached to the anterior margin of the base of the distal phalanx. It is loosely attached to the front of the neck of the proximal phalanx via the joint capsule.

Blood and nerve supply

The blood and nerve supply is the same as that for the metacarpophalangeal joint.

Movements

Being a hinge joint supported by strong collateral ligaments, movement is only allowed in one plane(figure b). Flexion and extension occur about a transverse axis passing approximately through the neck of the proximal phalanx. The range of flexion is in excess of 90°, while extension is normally no more than 10°. However, passive hyperextension may be marked in some individuals who apply large forces using the thumb, for example butchers and physiotherapy manipulators.

Accessory movements of joints of the thumb

Carpometacarpal joint. Gripping the trapezium between the thumb and the index finger of one hand, and the base of the first metacarpal with the other, the metacarpal base can be moved in both anteroposterior and mediolateral directions. With the same grip longitudinal gapping and rotation can be achieved.

Metacarpophalangeal and interphalangeal joints. If the principle of stabilizing the proximal bone and moving the distal one is employed, then anteroposterior gliding movements can be demonstrated at both the metacarpophalangeal and interphalangeal joints. Once again, best results are obtained when each bone is held firmly between the thumb and index finger. A good range of rotation, as well as longitudinal gapping, is also possible at the metacarpophalangeal joint.