Articular capsules of metacarpophalangeal joints

At a glance

English

Articular capsules of metacarpophalangeal joints

Latin (TA2)

Capsulae articulares articulationum metacarpophalangearum

System

Joint

Region

Hand

Side

Paired, left and right

About the articular capsules of metacarpophalangeal joints

Each metacarpophalangeal joint, the knuckle joint at the root of a finger or the thumb, is wrapped in its own articular capsule. The capsule has two layers: an outer fibrous membrane and an inner synovial membrane that lines it and secretes the fluid lubricating the cartilage. The fibrous layer is fixed to the metacarpal close to the edge of its rounded head and to the margin of the shallow, concave base of the proximal phalanx.

The capsule is loose, which is what allows the joint its wide range. It is not of even strength all round. On the palmar side it is replaced and reinforced by a thick plate of fibrocartilage, the palmar ligament or volar plate, which is firmly bound to the phalanx and more loosely to the metacarpal. At the sides it is strengthened by the collateral ligaments. On the back it is thin, and the extensor expansion of the finger lies over it and blends with it, so the extensor tendon effectively completes the dorsal wall. The palmar plates of the second to fifth fingers are tied to one another by the deep transverse metacarpal ligament.

Within this envelope the fingers bend and straighten, spread apart and come together, and combine these into circumduction. The head of the metacarpal is wider on its palmar side, so the collateral ligaments tighten as the finger flexes; side-to-side movement is free when the finger is straight and almost lost when it is bent. The thumb joint behaves more like a hinge, and two sesamoid bones sit in its palmar plate.

The capsule matters clinically because its synovial lining is a common target of rheumatoid arthritis. Swollen synovium stretches the capsule and its supports, and the fingers may then drift towards the ulnar side. When a hand is splinted, the joints are usually held flexed so that the collateral ligaments stay at full length and do not stiffen short.

Attachments and function

Formed by

Margins of the articular surface of each metacarpal head.

Joins

Margins of the articular surface on the base of the matching proximal phalanx.

Movements

Encloses and seals the joint, holds its synovial fluid, and with its reinforcing ligaments allows flexion, extension, abduction, adduction and circumduction while limiting overextension.

Nerve supply

Articular twigs from the digital nerves that cross the joint.

Blood supply

Articular branches from the neighbouring palmar digital and metacarpal arteries.