322 APPLIED ANATOMY. 



the bones are intact they rest on one another at their ends, leaving a space between 

 across which stretches the interosseous membrane. The action of this membrane in 

 preventing a separation of the fragments has already been pointed out, and the 

 influence on the fragments of pronation and supination will be discussed further on. 

 The two bones, radius and ulna, traverse the forearm from the elbow to the 

 wrist like two bridges, when they are broken they naturally fall inward toward one 

 another. This approximation of the fragments is aided by the muscles, particularly 

 the pronators and the brachioradialis. 



The pronator quadratus and teres both pass from the ulna to the radius, the 

 one at the lower and the other at the upper portion of the forearm. When they 

 contract they naturally tend to draw the bones toward one another. The brachio- 

 radialis, arising from the lateral (external) supracondylar ridge of the humerus 

 and inserting into the base of the styloid process of the radius, by its contraction 

 tends to tilt the upper end of the lower fragment toward the ulnar side. 



Pressure on the bones by bandages wound around the part likewise causes them to 

 encroach on the interosseous space, hence the desirability of splints which are wider 

 than the forearm so that lateral pressure on the bones by the bandages is prevented. 



3. The fragments may be rotated on one another in the direction of pronation 

 or supination and, becoming united in this misplaced position, render the normal 

 movements of rotation either much restricted or altogether impossible. 



This axial rotary displacement is due either to the lower fragments being 

 dressed in a position of pronation or to muscular action. As has already been 

 pointed out (see movements of pronation and supination, page 314), in performing 

 the movements of pronation and supination the ulna is the fixed bone and the radius 

 is the movable one. When the hand is pronated the radius crosses the ulna 

 obliquely and -lies almost or quite in contact with it, thus obliterating the inter- 

 osseous space. When the hand is in a position of middle or full supination the bones 

 are widely separated. When fractures are treated in the prone position it is recog- 

 nized that the callus may bind the bones together in their approximated condition 

 and a loss of motion will result. 



This is one reason why it is always required to treat these fractures with the 

 hand midway between supination and pronation or in complete supination, in which 

 position the bones are widely separated. The influence of the supinator muscles, as 

 was pointed out by Lonsdale, is also important. As has already been stated, the 

 supinators are stronger than the pronators. When the fracture occurs above the in- 

 sertion of the pronator radii teres the upper fragment is rotated outward by the 

 biceps and supinator (brevis). There are no muscles to oppose them. On this 

 account it is necessary to dress the fracture with the hand supinated. When the bones 

 are broken below the middle of the forearm the pronator radii teres remains attached 

 to the upper fragment and tends to oppose the supinating action of the biceps and 

 supinator (brevis). Therefore the fracture is treated with the hand midway 

 between pronation and supination. A diminution or loss of the power of pronation 

 and supination is a common sequel of fractures of the forearm and is due either to 

 an interference with the movement of the bones by callus or displaced fragments or 

 by supination of the upper fragment. It is favored by treating the arm in an 

 unfavorable position. 



4. The fragments may be inclined toward one another, producing an angular 

 deformity. Simple bending at the site of injury produces this displacement. It is 

 liable to occur if a narrow band or sling is used to support the injured member. If 

 the hand is supported by the sling the arm sags at the seat of fracture. If the fore- 

 arm is supported at the site of fracture the hand falls and an angular deformity 

 again occurs. Treatment of the fracture with the hand in a supine position on a 

 splint with a long sling reaching and supporting the entire length of the forearm will 

 obviate and prevent the deformity. 



Fractures of the Shaft of the Radius. 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. 



