This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Fractures of the shaft of the radius are not common. They are produced by both direct and indirect injury. The hand is attached to and articulates mainly with the radius, so that in falls on the hand the force is transmitted to the radius, and the shaft of the bone is not infrequently fractured in this manner.
These fractures are of interest from an anatomical point mainly on account of the influence of rotation and muscular action in displacing the fragments. The forearm possesses the movement of rotation; the radius is the movable bone and rotates around the ulna, hence when it is broken its fractured ends are readily displaced. Fractures of this bone are to be treated with the hand in half or full supination because in these positions the interosseous space is preserved. In pronation the radius crosses the ulna obliquely and lies close upon it and is then most liable to be bound to it by callus. A certain amount of callus or deformity may occur without interfering with the ulna opposite.
It should also not be forgotten that most muscles have more than one action. The biceps is both a flexor and supinator. The brachioradialis flexes, supinates, and exerts a directly upward traction on the outer surface of the lower end of the radius.
The fractures of the shaft of the radius may be divided into those above and those below the insertion of the pronator radii teres. This muscle inserts by a comparatively small tendon into the outer and posterior surface of the middle of the radius.
When the bone is fractured above the pronator radii teres insertion, and below the tubercle, the upper fragment is drawn forward and rotated outward by the biceps. If the fracture is down close to the upper edge of the insertion of the pronator radii teres the supinator (brevis) will assist in the supination. The lower fragment will be pronated by the pronator radii teres and quadratus. It will be drawn toward the ulna by the teres, quadratus, and also by the action of the brachioradialis. The pronator radii teres will also tend to draw the lower fragment anteriorly. The injury is to be treated with the elbow flexed to relax the biceps and in a fully supinated position (Fig. 332).
When the fracture is below the insertion of the pronator radii teres and above the pronator quadratus we have the lower fragment drawn toward the ulna by the pronator quadratus and the brachioradialis. The quadratus also tends to pronate the hand (Fig. 333).
The upper fragment is displaced anteriorly by the flexing action of both the biceps and pronator radii teres. The supinator (brevis) and biceps both tend to supinate it and the pronator radii teres to pronate it. This tends to place the upper fragment midway between pronation and supination. All fractures of the radius are to be treated with the elbow flexed to relax the biceps muscle. It is to be marked that the position of the lower fragment follows the position of the hand in pronation and supination. Also that by bending the hand toward the ulnar side the lower fragment tends to be tilted away from the ulna and thus the interosseous space is increased.
Fig. 332. - Fracture of the shaft of the radius above the insertion of the pronator radii teres muscle. The upper fragment is rotated outward by the biceps and supinator muscles.
Fig. 333. - Fracture of the radius just below the insertion of the pronator radii teres muscle. The upper fragment is displaced directly forward in a position midway between pronation and supination.
Fig. 334. - Fracture below the middle of the shaft of the ulna, the lower fragment drawn toward the radius by the pronator quadratus muscle.
On account of the upper fragment assuming a middle position the fracture is dressed in this position with the thumb upward - an internal angular splint is used. Some surgeons prefer using the position of full supination.
The difference in the width of the interosseous space when the hand is in full supination and when it is in semisupination, though it may be slightly in favor of the latter position, is too little to give it any preference on that account.