The radius is firmly united to the ulna by two joints, and an intermediate fibrous union, viz.:

• The superior radio-ulnar - whereat the head of the radius rotates within the radial notch and annular ligament.
• The union of the shafts - the mid radio-ulnar union.
• The inferior radio-ulnar - whereat the lower end of the radius rolls round the head of the ulna.

The Superior Radio-ulnar Joint

Class. - Diarthrosis. Subdivision. - Trochoides.

The bones which enter into this joint (which is often included with the elbow- joint) are, the ulna by its radial notch and the radius by the smooth vertical border or rim on its head. There is but one ligament special to the joint, viz.:

Annular.

The annular ligament consists of bands of strong fibers, somewhat thicker than the capsule of the elbow-joint, which encircle the head of the radius, retaining it against the side of the ulna. The bulk of these fibers forms about three- fourths of a circle, and they are attached to the anterior and posterior margins of the radial notch; some few are continued round below the radial notch, and form a complete ligamentous circle.

The ligament is inseparably connected along its upper edge and lateral (i. e., its non-articular) surface with the anterior, posterior, and lateral portions of the capsule of the elbow, a few of the fibers of these portions, especially of the lateral, descending to be attached to the neck of the radius. The lower part of the articulation is covered in anteriorly, posteriorly, and laterally by a thin independent membranous layer, which passes from the lower edge of the annular ligament to the neck of the radius, strengthened on the lateral side by those fibers passing down from the capsule. They are loose enough to allow the bone to rotate upon its own axis. Medially and below the cavity is closed in by a loose membrane, the ligamentum quadratum, which passes from the lower border of the radial notch to the neck of the radius.

The synovial membrane is the same as that of the elbow-joint, and, after lining the annular ligament, passes on to the neck of the radius, and thence up to the lower margin of the articular cartilage.

Vessels and nerves

The arterial and nerve -supply are the same as those to the lateral part of the elbow-joint. Relations. - Behind lies the anconeus and in front the lateral border of the brachialis.

The Mid Radio-ulnar Union

Class. - Synarthrosis. Subdivision. - Syndesmosis.

There are two interosseous ligaments which pass between the shafts of the bones and unite them firmly together, viz.:

Oblique cord. Interosseous membrane.

The oblique cord [chorda obliqua] is a fairly strong, narrow band, which passes from the lower end of the rough lateral border of the coronoid process, downward and laterally to be attached to the posterior edge of the lower end of the tuberosity of the radius and the vertical ridge running from it to the medial border of the bone.

Some of its fibers blend with the fibers of insertion of the biceps tendon; behind, it is in close contact with the supinator; below, a thin membrane passes off from it to the upper edge of the interosseous membrane; the posterior interosseous vessels pass in the space between it and the interosseous membrane; occasionally a slip is continued into the annular ligament of the superior radio-ulnar articulation.

The interosseous membrane is attached to the ulna at the lowest part of the ridge in front of the depression for the supinator, and along the whole length of the interosseous border as far as the inferior radio-ulnar articulation, approaching the front of the bone in the lower part of its attachment. To the radius, it is attached along the interosseous border, from an inch (2.5 cm.) below the tuberosity to the ulnar notch for the lower end of the ulna.

It is strongest and broadest in the center, where the fibers are dense and closely packed; it is also well marked beneath the pronator quadratus, and thickens considerably at the lower end, forming a strong band of union between the two bones. Its fibers pass chiefly downward and medially, from the radius to the ulna, though some take the opposite direction; at the lower end, some are transverse. On the posterior surface are one or two bands, which pass downward and laterally from the ulna to the radius, and frequently there is a strong bundle as large as the oblique cord; this, which may be called the inferior oblique ligament, stretches from the ulna, an inch and a half above its lower extremity, downward and laterally to the ridge above and behind the ulnar notch of the radius.

Relations

At its attachment to the bones, the interosseous membrane blends with the periosteum. Its upper border is connected with the oblique cord by a thin membrane, which is pierced by the posterior interosseous vessels; and the lower border, which stretches across between the two bones just above the inferior radio-ulnar articulation, assists in completing the capsule of that joint. Its anterior surface is in relation with the flexor digitorum profundus and flexor pollicis longus in the upper three-quarters, the lower fourth being in relation with the pronator quadratus.

Vessels and nerves

The anterior interosseous vessels and nerve descend along the middle of the membrane, the artery being bound down to it. About an inch from the lower end it is pierced by the anterior interosseous artery. The posterior surface is in relation with the supinator, abductor pollicis longus (extensor ossis metacarpi pollicis), extensor pollicis longus and brevis, and the extensor indids proprius; at its lower part, also with the posterior branch of the anterior interosseous artery, and the deep branch of the radial nerve (posterior interosseous).

The Inferior Radio-ulnar Joint

Class. - Diarthrosis. Subdivision. - Trochoides.

This is, in one respect, the reverse of the superior; for the radius, instead of presenting a circular head to rotate upon the facet on the ulna, presents a concave facet which rolls round the ulna. The articulation may be said to consist of two parts at right angles to each other; one between the radius and ulna, and the other between the ulna and the articular disc (triangular fibro-cartilage).

The ligaments are:

•  Articular disc.
•  Anterior radio-ulnar.
•  Posterior radio-ulnar.

The articular disc (triangular fibro-cartilage) assists the radius in forming an arch under which is received the first row of carpal bones. Its base is attached to the margin of the radius, separating the ulnar notch from the articular surface for the carpus, while its apex is fixed to the fossa at the base of the styloid process of the ulna. It gradually and uniformly diminishes in width from base to apex, becoming rounded where it is fixed to the ulna; it is joined by fibers of the ulnar collateral ligament of the wrist.

The articular disc is about three-eighths of an inch (1 cm.) wide, and the same from base to apex; thicker at the circumference than in the center; smooth and concave above to adapt itself to the ulna, and smooth and slightly concave below to fit over the triquetral bone. Its anterior and posterior borders are united to the anterior and posterior radio-ulnar and radio- carpal ligaments. It is the most important structure in the inferior radio-carpal articulation, as it is a very firm bond of union between the lower ends of the bones, and serves to limit their movements upon one another more than any other structure in either the upper or lower radio- ulnar joints. Its structure is fibrous at the circumference, while in the center there is a preponderance of cells. It differs from all other fibro-cartilages in entering into two distinct articulations; and separates entirely the synovial membrane of the radio-ulnar joint from that of the wrist.

The lower end of the interosseous membrane extends between the ulna and radius immediately above their points of contact. Transverse fibers between the two bones form a sort of arch above the concave articular facet of the radius, and, joining the anterior and posterior radio-ulnar ligaments, complete the articular capsule of the inferior radio-ulnar joint. The ligaments represent merely thickenings of the capsule.

The anterior radio-ulnar ligament is attached by one end to the anterior edge of the ulnar notch of the radius, and by the other to the rough bone above the articular surface of the ulna as far medially as the notch, as well as into the anterior margin of the triangular cartilage from base to apex.

The posterior radio-ulnar ligament is similarly attached to the posterior margin of the ulnar notch at one end, and at the other to the rough bone above the articular surface of the extremity of the ulna as far medially as the groove for the extensor carpi ulnaris, with the sheath of which it is connected, as well as into the whole length of the posterior margin of the articular disc. Both the radio-ulnar ligaments consist of thin, almost scattered, fibers.

The synovial membrane, sometimes called the membrana sacciformis, is large and loose in proportion to the size of the joint. It is not only interposed between the radial and ulnar articular surfaces, but lines the terminal articular surface of the ulna and the upper surface of the articular disc.

Vessels and nerves

The arterial supply is derived from the volar interosseous artery and branches of the volar carpal rate.

The nerve-supply comes from the volar interosseous of the median, and the deep branch of the radial (posterior interosseous).

Relations

Behind lies the tendon of the extensor digiti quinti proprius and in front the flexor digitorum profundus.

The movements of the radius

The upper end of the radius rotates upon an axis drawn through its own head and neck within the collar formed b}' the radial notch and the annular ligament, while the lower end, retained in position by the articular disc, rolls round the head of the ulna. This rotation is called pronation, when the radius from a position nearly parallel to the ulna turns medialward so as to lie obliquely across it; and supination, when the radius turns back again, so as to uncross and lie nearly parallel with the ulna. In these movements, the radius carries with it the hand, which rotates on an axis passing along the ulnar side of the hand; thus, the hand when pronated lies with its dorsum upward, as in playing the piano, while when supinated, the palm lies upward - the attitude of a beggar asking alms. Ward thus expresses the relations of the two extremities of the radius in pronation and supination: 'The head of the radius is so disposed in relation to the sigmoid cavity (ulnar notch) at the lower end that the axis of the former if prolonged falls upon the center of the circle of which the latter is a segment;' the axis thus passes through the lower end of the ulna at a point at which the articular disc is attached, and if prolonged further, passes through the ring finger. Thus, the radius describes, in rotating, a blunt-pointed cone whose apex is the center of the radial head, and whose base is at the wrist; partial rotation of the bone being unaccompanied by any hinge-like or antero-posterior motion of its head, and pronation and supination occurring with- out disturbance to the parallelism of the bones at the superior radio-ulnar joint. Associated with this rotation in the ordinary way, there is some rotation of the humero-ulnar shaft, which causes lateral shifting of the hand from side to side; thus, with pronation there is some abduction, and with supination some adduction combined, so that the hand can keep on the same superficies in both pronation and supination. The power of supination in man is much greater than pronation, owing to the immense power and leverage obtained by the curve of the radius, and by the attachment of the biceps tendon to the back of the tuberosity. For this reason, all our screw-driving and boring tools are made to be used by supination movements.

In the undissected state, the amount of rotation it is possible to obtain is about 135°, so that neither the palm nor the fore part of the lower end of the radius can be turned completely in opposite directions; yet in the living subject this amount can be greatly increased by rotation of the humero-ulnar shaft at the shoulder-joint.

Pronation is checked in the living subject by (a) the posterior inferior radio-ulnar ligament, which is strengthened by the connection of the sheath of the extensor tendons with it; (b) the lowermost fibers of the interosseous membrane; (c) the back part of the ulnar collateral and adjacent fibers of the posterior ligament of the wrist, and (d) the meeting of the soft parts on the front of the forearm.

Supination is checked mainly (a) by the medial ulnar collateral ligaments of the wrist, but partly also by (b) the oblique cord; (c) the anterior inferior radio-ulnar ligament, and (d) the lowest fibers of the interosseous membrane.

The interosseous membrane serves, from the direction of its fibers downward and medially from the radius to the ulna, to transmit the weight of the body from the ulna to the radius in the extended position of the elbow, as in pushing forward with the arms extended, or in supporting one's own weight on the hands, the ulna being in intimate contact with the humerus, but not at all with the carpus; while the area of contact of the radius with the humerus is small, and that of the radius with the carpus large. Hence the weight transmitted by the ulna is communicated to the radius by the tightening of the interosseous membrane. Conversely, in falls upon the hand with the arm extended, the interosseous membrane acts as a sling to break the violence of the shock, and prevents the whole force of the impact from expending itself directly upon the capitulum.

Muscles which act upon the radio-ulnar joints.

Pronators.

• Pronator teres,
• pronator quadratus,
• flexor carpi radialis,
• palmaris longus.

Supinators

• Biceps,
• supinator,
• extensor pollicis longus.

The brachio-radialis is chiefly a flexor of the elbow-joint, but it takes part in the initiation of the movement of supination when the hand is fully pronated and of pronation when the hand is fully supinated.