The radius and the ulna are connected at their upper and lower extremities by synovial joints, termed the superior and inferior radio-ulnar joints. In addition, the bodies of the bones are connected by an interosseous membrane and a ligament, which together constitute a middle radio-ulnar union.
A. THE SUPERIOR RADIO-ULNAR JOINT
This articulation forms a pivot-joint between the circumference of the head of the radius and the osseofibrous ring formed by the radial notch of the ulna and the annular ligament.
B. THE MIDDLE RADIO-ULNAR UNION
The bodies of the radius and ulna are connected by the oblique cord and the interosseous membrane of the forearm.
The oblique cord is a small, flattened band, extending from the lateral side of the tuberosity of the ulna to the radius a little below the radial tuberosity. Its fibers run at right angles to those of the interosseous membrane. It is sometimes wanting.
The interosseous membrane of the forearm is a broad and thin sheet, the fibers of which slant obliquely downwards and medially from the interosseous border of the radius to that of the ulna; the lower part of the membrane is attached to the posterior of the two lines into which the interosseous border of the radius divides. Two or three bands are occasionally found on the posterior surface of this membrane; their fibers descend obliquely from the ulna towards the radius, i.e. at right angles to the other fibers. The membrane is deficient above, commencing about 2 or 3 cm. below the tuberosity of the radius; is broader in the middle than at either end; and presents an oval aperture a little above its lower margin, for the passage of the anterior interosseous vessels to the back of the forearm. Between its upper border and the oblique cord there is a gap, through which the posterior interosseous vessels pass. The membrane connects the bones, and increases the extent of surface for the attachment of the deep muscles of the forearm. It also transmits to the ulna and thence to the humerus any force acting upwards through the hand and radius. It is relaxed in complete pronation or supination, and is tense when the hand is midway between the prone and supine positions. In front the membrane is in relation, in its upper three-fourths. with the flexor pollicis longus on the radial side, with the flexor digitorum profundus on the ulnar aide and between these muscles with the anterior interosseous vessels and nerve; in its lower one-fourth with the pronator quadratus; behind, with the supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis; and, near the wrist, with the anterior interosseous artery and posterior interosseous nerve.
C. THE INFERIOR RADIO-ULNAR JOINT
This is a pivot-joint formed between the head of the ulna and the ulnar notch of the lower end of the radius; the surfaces are enclosed in a capsular ligament and held together by an articular disc.
The capsular ligament is slightly thickened in front and behind; above it is lax, and, lined with the synovial membrane, projects upwards as a pouch (recessus sacciformis) in front of the lower part of the interosseous membrane.
The articular disc (fig. 542), triangular in shape, binds the lower ends of the ulna and radius together. Its periphery is thicker than its center, which is occasionally perforated. It is attached by its apex to a depression between the styloid process and the head of the ulna; and by its base, which is thin, to the prominent edge which separates the ulnar notch from the carpal articular surface of the radius. Its margins are united to the ligaments of the wrist-joint. Its upper surface, smooth and concave, articulates with the head of the ulna. Its lower surface, also smooth and concave, forms a part of the radiocarpal joint and articulates with the medial part of the lunate bone; when the hand is adducted, it articulates with the triquetral bone. Each of its surfaces is clothed with svnovial membrane : the upper. with that of the inferior radio-ulnar articulation : the lower, with that of the radiocarpal joint.
Movements.-The movements which take place at the radio-ulnar joints result in pronation and supination of the hand. In pronation the radius, carrying the hand with it, is thrown obliquely across the front of the ulna, its upper end being lateral, and its lower end medial to that bone. If the forearm be semiflexed when this movement occurs the palm of the hand is directed downwards if the forearm be extended the palm looks backwards. In supination the movement is reversed, the radius lies lateral to and parallel with the ulna, and the palm is directed upwards-forwards if the forearm be extended. The power of supination is greater than that of pronation, and thus all screw-driving instruments are made to be used in this movement.
Muscles producing the movements.—These muscles may be grouped as (a) those acting on the elbow-joint, and (b) those acting on the radio-ulnar joints:
(a) Muscles acting on the elbow-joint:
Flexion.-Brachialis, Brachioradialis, Biceps, Pronator teres.
(b) Muscles acting on the radio-ulnar joints:
Pronation.-Pronator teres, Pronator quadratus.
Applied Anatomy- From the breadth of the elbow-joint, and the manner in which the articular surfaces are interlocked, and also on account of the strong collateral ligaments and the support which the joint derives from the mass of muscles attached to the epicondyles of the humerus, lateral displacement of the bones is very uncommon; whereas anteroposterior dislocation, on account of the shortness of the anteroposterior diameter, the weakness of the anterior and posterior ligaments, and the want of muscular support, occurs much more frequently. Dislocation of the elbow-joint is common in children. In lesions of this joint it is often difficult to ascertain the exact nature. of the injury except by x-ray examination.
The elbow-joint is occasionally the seat of acute synovitis. The joint-cavity then becomes distended with fluid, the bulging showing itself principally around the olecranon consequence of the laxness of the articular capsule. Again, there is often some swelling, just above the head of the radius, in the line of the humeroradial joint, or the whole elbow may assume a fusiform appearance,
Dislocation of the head of the radius alone is a not uncommon accident, and occurs most frequently in young persons from falls on the hand when the forearm is extended and supinated, the head of the bone being displaced forward. It is attended by rupture of the annular ligament. Occasionally a peculiar injury, which is supposed to be a subluxation, occurs in young children. It is believed that the head of the radius is displaced downwards in the annular ligament, the upper border of which becomes folded over the head of the radius, between it and the capitulum of the humerus; the small size of the head of the radius in the child predisposes to this injury. The forearm becomes fixed in a position of semiflexion, midway between supination and pronation, and great pain is complained of when any attempt is made to move the joint.
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