The joints of the thumb - part I

The thumb is an extremely mobile and specialized digit, both of which are important prerequisites for the movement of opposition, and for the normal prehensile functioning of the hand.

Carpometacarpal joint

The carpometacarpal joint of the thumb, although extremely mobile, nevertheless provides a stable base from which it can effectively and efficiently work. It plays a vital role in movements of the thumb allowing movement in three dimensions.

Articular surfaces

The joint is between the trapezium and the base of the first metacarpal(picture a). It provides the best example in the body of a synovial saddle joint, with the two surfaces being reciprocally concavoconvex and covered with hyaline cartilage.

The articular surface of the trapezium is concave more or less in an anteroposterior direction, and convex perpendicular to this. The base of the first metacarpal has reciprocal curvatures. The concavities and convexities of the surfaces do not lie strictly within the transverse and anteroposterior planes, as will become evident when the axes about which movement occurs are considered.

Joint capsule and synovial membrane

A loose but strong fibrous capsule compeletely encloses the joint, attaching to the articular margins of both bones(b). It is lined with synovial membrane. The capsule is thickened laterally by the radial carpometacarpal ligament, and anteriorly and posteriorly by anterior and posterior oblique ligaments.

Ligaments

The radial carpometacarpal ligament passes between the adjacent lateral surfaces of the trapezium and first metacarpal(b).

The anterior and posterior oblique ligaments pass from their respective surfaces of the trapezium to the medial side of the first metacarpal, converging as they do so(b). The posterior oblique  ligament becomes taut in flexion of the thumb, while the anterior becomes taut during extension.

Blood and nerve supply

The arterial supply to the joint is by branches from the palmar and dorsal carpal networks, while its nerve supply is by twigs from the anterior and posterior interosseus nerves, root value C7, 8.

Relations

The carpometacarpal joint of the thumb lies deep to the thenar muscles, which cover its anterior aspect. Medially is the tendon of flexor pollicis longus and laterally the tendons of extensor pollicis longus and brevis.

Stability

The joint is principally stabilized and the surfaces kept in opposition by the tone of the muscle whose tendons cross the joint. The shape of the articular surfaces and the looseness of the fibrous capsule allow the joint its large degree of mobility, and hence play only a minor role in its stability.

Movements

Because of the thumb is rotated approximately 90° with respect to the plane of the hand, the terminology used to describe its movements appears at first to be confusing. The terms flexion, extension, abduction and adduction are used as if the thumb were in line with the fingers. That this is not so can be clearly seen when observing your own hand; the thumbnail faces almost laterally while the finger nails face posteriorly. Thus flexion and extension occur in a coronal plane, as does abduction and adduction of the remaining four digits; and abduction and adduction occur in a sagittal plane, as do flexion and extension of the other digits. Due to the nature of the joint surfaces and the looseness of the capsule, a certain amount of rotation is also possible at the joint. Because movements at the joint are brought about in the main by muscles whose tendons lie parallel to the metacarpal, compression across the two opposing surfaces always accompanies the movements. Consequently, the joint surfaces tend to grind against each other, instead of there being simple rolling or gliding.

Flexion and extension. This occurs in the plane of the palm of the hand, so that flexion in thumb moves medially and in extension it moves laterally(figure below). The axis about which the movement occurs passes through the base of the metacarpal at the centres of curvature of the concave trapezium and the convex metacarpal. It does not lie exactly in an anteroposterior plane but is set slightly obliquely from posterior, lateral and proximal to anterior, medial and distal.

The total range of flexion and extension is between 40° and 50°. Towards the end of full flexion, tension developed in the posterior oblique ligament results in a medial rotation of the metacarpal causing the palmar surface of the thumb to face posteriorly. Conversely, towards the end of full extension, lateral rotation of the metacarpal occurs because of tension developed in the anterior oblique ligament. Flexion at the joint occurs as a secondary movement of flexor pollicis longus and brevis acting on the interphalangeal and metacarpophalangeal joints respectively. Similarly extension is due to the actions of extensor pollicis longus and brevis as they act on the interphalangeal and metacarpophalangeal joints respectively.

Abduction and adduction. Both occur at right angles to the palm so that in abduction the thumb is carried forwards away from the palm and in adduction it is moved back towards the palm(figure above). The axis about which movement occurs is perpendicular to the axis of flexion and extension. It passes through the trapezium at the centers of curvature of the concave metacarpal and convex trapezium, and runs slightly obliquely from medial, posterior and distal to lateral, anterior and proximal.

The range of abduction and adduction is about 80°, with adduction being brought about by adductor pollicis pulling on the proximal phalanx. Abduction is brought about by the direct action of abductor pollicis longus on the joint, and by the secondary action of abductor pollicis brevis acting on the metacarpophalangeal joint.

Opposition. This is described as a movement in which the distal pad of the thumb is brought against the distal pad of any of the remaining four digits(picture above). It is an essential movement of the hand, and its loss markedly reduces the functional capacity of the hand. The movement is complex involving flexion, abduction and rotation followed by adduction at the carpometacarpal joint, as well as movements at other joints of the thumb.

Essentially, opposition consists of three elementary movements. Initially, flexion and abduction of the thumb occur simultaneously at the carpometacarpal joint due to the action of flexors pollicis longus and brevis and abductor pollicis longus. This produces a certain amount of passive axial rotation of the metacarpal, possible because of the looseness of the joint capsule – the rotation is directed medially due to the posterior oblique ligament becoming taut. At some point during this movement, opponens pollicis contracts to produce an active rotation of the metacarpal – this is the second elementary movement. Finally, adduction occurs at the carpometacarpal joint, produced by adductor pollicis, to bring the metacarpal back towards the plane of the palm of the hand.

Movements of the thumb at the metacarpophalangeal joint contribute significantly to the overall movement of opposition. At the same time as the carpometacarpal joint is being flexed and abducted, so is the metacarpophalangeal joint. Again the simultaneous movments of flexion and abduction of the proximal phalanx bring about a degree of axial rotation at this joint. Consequently, the pad of the thumb comes to face posteromedially.

Returning the thumb to the anatomical position has no specific name. It is brought about mainly by the contraction of the extensor muscles of the thumb. Perhaps it should therefore be referred to as exposition.