The radius revolves on the ulna about an axis which passes through the centre of the head of the radius above and the styloid process of the ulna below, which line if prolonged would pass through the ring finger (Fig. 319). In pronation, the hand lies with the palm down and the radius is crossed diagonally over the ulna; the bones are close together (Fig. 320). In supination the hand lies with the palm up, the bones lie parallel to one another and widely separated (Fig. 321). In the midposi-tion the radius lies above the ulna and the space between them is at its maximum. The difference in this respect between midpronation and complete supination is slight. The head of the radius rotates in the orbicular ligament, the lower end of the radius revolves around the head of the ulna and rests on the interarticular triangular fibrocartilage. The range of movement is from 140 degrees to 160 degrees. The radius is pronated by the pronator teres and pronator quadratus muscles. It is supinated by the brachioradialis, supinator (brevis), and biceps muscles. Some of the other muscles also aid slightly in these movements, especially the flexor carpi radialis in pronation. In fractures the preservation of the interosseous space is essential for the proper performance of pronation and supination; hence anything which tends to encroach on it, as displacement of the fragments or their position as influenced by the position of the hand, is to be guarded against.

Fig. 319.   The axis of rotation in pronation and supination.

Fig. 319. - The axis of rotation in pronation and supination.

Fig. 320.   Position of the bones of the forearm when the hand is in the position of pronation.

Fig. 320. - Position of the bones of the forearm when the hand is in the position of pronation.

Fig. 321.   Position of the bones of the forearm when the hand is in the position of supination.

Fig. 321. - Position of the bones of the forearm when the hand is in the position of supination.

The muscles of supination are much stronger than those of pronation; for this reason instruments intended to be used in a rotary manner turn from the inside toward the outside; that is, in the direction of supination. The screw-driver is an example.