The Carpal Joints

These joints taken together constitute the composite articulatio carpi, or what is popularly termed the “knee-joint” in animals. This consists of three chief joints, viz., (1) The radio-carpal joint, formed by the distal end of the radius and the proximal row of the carpus; (2) the intercarpal joint, formed between the two rows of the carpus; (3) the carpo-metacarpal joint, formed between the distal row of the carpus and the proximal ends of the metacarpal bones. The proximal and middle joints may be regarded as ginglymi, although they are not typical or pure examples of hinge-joints. The distal joint is arthrodial. In addition there are arthrodial joints formed between adjacent bones of the same row (Articulationes interosseæ). All these constitute a very composite joint, with numerous ligaments. The articular surfaces have been described in the Osteology.

The joint capsule may be regarded, so far as the fibrous part is concerned, as being common to all three joints. It is attached close to the margin of the articular surface of the radius above and the metacarpus below; its deep face is also attached to a considerable extent to the carpal bones and to the small ligaments. Its anterior part, the dorsal or anterior common ligament, is rather loose, and assists in forming the fibrous canals for the extensor tendons. Its posterior part, the volar or posterior common ligament, is very thick and dense, and is closely attached to the carpal bones. It levels up the irregularities of the skeleton here, and forms the smooth anterior wall of the carpal canal. It is continued downward to form the subcarpal or inferior check ligament, which blends with the tendon of the flexor perforans about the middle of the metacarpus.

The synovial membrane forms three sacs corresponding to the three joints. The radio-carpal sac is the most voluminous; it includes the joints formed by the accessory carpal bone, and also those between the proximal carpal bones as far as the interosseous ligaments. The intercarpal sac sends extensions upward and downward between the bones of the two rows as far as the interosseous ligaments; it communicates between the third and fourth carpal bones with the carpo-metacarpal sac. The latter is very limited in extent, and is closely applied to the bones; it incloses the carpo-metacarpal joint, and lubricates also the lower parts of the joints between the distal carpal bones and the intermetacarpal joints.

The external lateral ligament (Ligamentum carpi collaterale ulnare) is attached above to the external tuberosity of the distal end of the radius. Its long superficial part is attached below to the proximal end of the external small metacarpal chiefly, but some fibers end on the large metacarpal bone. A canal for the lateral extensor tendon separates a short deep band which ends on the ulnar carpal bone. Other deep fibers connect the latter with the fourth carpal bone, and the fourth carpal with the metacarpus.

The internal lateral ligament (Ligamentum carpi collaterale radiale) resembles the preceding in general, but is stronger and wider distally. It is attached above to the internal tuberosity of the distal end of the radius and ends below on the proximal ends of the large and inner small metacarpal bones. Deep fasciculi are detached to the radial and second carpal bones. The first carpal bone, when present, is usually embedded in the posterior part of the distal end of the ligament. The posterior part of the ligament is fused with the posterior annular ligament (Ligamentum carpi transversum), and concurs in the formation of a canal for the tendon of the flexor carpi internus.

A number of special short ligaments connect two or more adjacent bones; only the most distinct of these will be described here.

The accessory carpal bone is connected with adjacent bones by three ligaments (Fig. ). The proximal one is a short band which extends from the accessory carpal in front of the groove on its outer face and is inserted into the distal end of the radius behind the groove for the lateral extensor tendon. A middle band connects the accessory with the ulnar carpal. The distal ligament consists of two strong bands which pass from the lower margin of the accessory to the fourth carpal and the proximal end of the outer metacarpal bone; these bands transmit the action of the muscles, which are inserted into the accessory carpal bone. The other bones of the proximal row are connected by two anterior or dorsal ligaments, which are transverse in direction, and two interosseous ligaments. An oblique ligament passes from an eminence on the posterior surface of the radial carpal bone to a small depression on the radius internal to the facet for the accessory carpal bone.

Two ligaments connect the proximal and distal rows posteriorly. The inner one joins the radial to the second and third carpal, and the outer one attaches the ulnar to the third and fourth carpals.

The bones of the distal row are connected by two strong transverse anterior or dorsal ligaments and two interosseous ligaments.

There are four carpo-metacarpal ligaments. Two oblique anterior bands connect the third carpal with the large metacarpal. Two interosseous ligaments pass downward from the interosseous ligaments of the distal row to end in depressions in the interstices between the proximal ends of the metacarpal bones.

Movements.—Taking the joint as a whole, the chief movements are flexion and extension. In the standing position the joint is extended. When the joint is flexed, slight lateral movement and rotation can be produced by manipulation. The anterior part of the capsule is, of course, tense during flexion, the posterior part in extension.

The movement practically all occurs at the radio-carpal and intercarpal joints, the articular surfaces of which are widely separated in front during flexion, but remain in contact behind. The distal row remains in contact with the metacarpus. The intermediate and ulnar carpals move together as one piece, but the radial does not move so far as the intermediate, so that the anterior and interosseous ligaments connecting these bones become tense and oblique in direction.