The midcarpal joint - part I

The midcarpal joint is the articulation between the proximal and distal rows of carpal bones, each of which can be considered to act as a single functional unit. The lateral part of the joint consists of two plane surfaces which are arranged to form a slight convexity directed distally. The larger medial part of the joint is concave distally in all directions.

Articular surfaces

Laterally, plane joint surfaces on the trapezium and trapezoid articulate with the slightly rounded distal surface of the scaphoid. The head of the capitate articulates with the scaphoid and lunate in the central part of the joint. The apex of the hamate also articulates with the lunate, while its ulnar surface articulates with the triquetral.

Joint capsule

The midcarpal joint is surrounded by a fibrous capsule, composed in the main of irregular bands of fibres running between the rows of bones. Anteriorly and posteriorly these bands constitute the palmar and dorsal intercarpal ligaments. At the sides of the midcarpal joint the capsule is strengthened by collateral ligaments.

Intercarpal synovial cavity

The intercarpal joint cavity is large and complex. It extends from side-to-side between the two rows of carpal bones; however this may be partially or completely interrupted by an interosseus ligament between the scaphoid and capitate. Extensions of the cavity pass proximally between the scaphoid, lunate and triquetral as far as the interosseus ligaments between them. Rarely is there communication with the radiocarpal joint cavity. Further extensions of the cavity pass distally between the trapezium, trapezoid, capitate and hamate. If the interosseus ligaments connecting these bones do not extend the full depth of the articulation, or one is missing(usually that between trapezium and trapezoid) then the intercarpal joint cavity communicates with the carpometacarpal joint and is prolonged between the bases of the medial four metacarpals. The intercarpal cavity does not, however, communicate with the first carpometacarpal or the pisiform-triquetral joint spaces.

Synovial membrane lines the capsule and all non-articular surfaces, attaching to the margins of all joint surfaces.

Ligaments

Palmar intercarpal ligament

The palmar intercarpal ligament passes from the bones of the proximal row predominantly to the head of the capitate. This ligament is sometimes reffered to as the radiate capitate ligament.

Dorsal intercarpal ligament

The dorsal intercarpal ligament merely passes from the bones of one row to those of the other.

Radial collateral ligament

The radial collateral ligament is a strong distinct band passing from the scaphoid to the trapezium(picture above). It is a continuation of the radial collateral ligament of the radiocarpal joint.

Ulnar collateral ligament

The ulnar collateral ligament connects the triquetral and the hamate, and is a continuation of the ulnar collateral carpal ligament of the radiocarpal joint.

Interosseus ligament

Occasionally, a slender interosseus ligament passes from the lateral side of the capitate to the scaphoid near its trapezoid articular surface(picture one).

Blood and nerve supply

The arterial supply to all the intercarpal joints is by branches from the palmar and dorsal carpal networks.

The nerve supply to the joints is by twigs from the anterior and posterior interosseus nerves, and the deep and dorsal branches of the ulnar nerve; root value C7, 8.

Movements

Movements at the intercarpal joints, except the midcarpal joint, are small, accompanying and facilitating movements at the radiocarpal and midcarpal joints. Movements possible at the midcarpal joint are flexion and extension, and abduction and adduction. These occur about transverse and anteroposterior axes passing through the head of the capitate.

Flexion and extension

In flexion the hand moves towards the front of the forearm, while in extension it moves towards the back of the forearm. Extension is freer than flexion, having a range of 50°; flexion has a range of only 35°. In these movements the head of the capitate rotates within the concavity formed by the scaphoid and lunate, while the hamate rotates against the triquetral. Accompanying these movements is a compensatory swing of the scaphoid on the lunate in order to receive the head of the capitate.

Abduction and adduction

During adduction the capitate rotates so that its distal part moves medially; the hamate approaches the lunate and separates from the triquetral. In abduction the capitate comes close to the triquetral separating the hamate from the lunate. Accompanying abduction and adduction is a complex movement of torsion between the two rows of carpal bones. During abduction the distal row of carpal bones undergoes a “rotation” in the direction of supination and extension, while the proximal row “rotates” in the direction of pronation and flexion. The twisting of the scaphoid delays its impact on the radial styloid process by bringing its tubercle forwards; it also makes the tubercle more easily palpable. In adduction a reverse twisting motion occurs so that the proximal row “rotates”  in the direction of supination and extension, while the dorsal row moves in the direction and flexion. It must be emphasized that these movements are of extremely small magnitude. It is debatable whether they contribute much to the normal functioning of the wrist.

The range of abduction and adduction are 8° and 15° respectively. The principal limit to abduction is a closing of the lateral part of the joint space between the scaphoid and the trapezium.

Accessory movements

Anteroposterior gliding movements of any two adjacent carpal bones can be produced if one is stabilized while the other is moved. This can be achieved by gripping each bone between the thumb and index finger.

Anteroposterior movement at the midcarpal joint can be elicited using a similar technique to that described for the radiocarpal joint. A firm circular grip is applied around each carpal row. While the proximal row is stabilized, the distal row can be moved anteroposteriorly. Applying the same grip, a longitudinally applied force separates the two joint surfaces.