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.
The ulna alone is rarely luxated (when displaced it would practically be a backward and inward luxation of the elbow) but the head of the radius is not infrequently pulled out of place (Fig. 30S). The accident occurs in children, particularly young ones who, in walking with their elders, are frequently lifted or helped along by a pull on the hand. The pull, accompanied by hyperextension of the elbow and some adduction of the hand, draws the head of the radius from beneath the orbicular ligament and then the tension of the biceps drags it forward. The displacement may be either marked or slight. A marked displacement in the well-developed arm of an adult is readily recognized, but in the fat, chubby, undeveloped arm of an infant it is easily overlooked.
Pain attracts attention to the part. There is apt to be inability to flex the arm beyond a right angle, due to the radius impinging on the lower end of the humerus. Careful palpation reveals a hollow below the lateral (external) condyle which should be normally occupied by the head of the radius. The outer side of the forearm at the bend of the elbow may be abnormally full and pressure here may detect the head of the radius displaced forward (Figs. 309 and 310).
The forearm is to be extended almost to a straight line. Pressure is to be made with the thumb to force the head of the radius back into place. While this is done the forearm is to be flexed on the arm and if the head is replaced the elbow can be bent to its normal acute angle. On extension being made the radius frequently again jumps forward, hence the injury is to be subsequently treated with the arm in a flexed position.
Fig. 310. - Anterior luxation of the head of the radius.