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 forearm may involve either the radius or ulna, or both. The radius is the bone most often broken. The preservation of the interosseous space and functions of pronation and supination are prominent points in treatment.
These fractures occur either from a direct blow on the part or are due to violence in falling on the outstretched hand. They usually occur in the middle or lower third. The character of the displacement depends more on the manner in which the injury is produced than on the action of the muscles, though in some cases they also have some influence.
The main function of the forearm in addition to that of serving as a pedestal or support for the hand is to perform the movements of pronation and supination. It is these movements that are most apt to be impaired in cases of fracture. When both bones are fractured the interosseous membrane still remains, running transversely from one bone to that of the opposite side. Therefore, while it is common enough, to find the fractured ends displaced toward one another, thus narrowing or obliterating the space between them, one never sees a displacement of the fragments producing a widening of the interosseous space. In fracture of both bones four types of deformity or combinations of these types are found.
1. The fractured ends of the distal or proximal fragments may preserve approximately their normal position to one another but be displaced either anteriorly or posteriorly or else to one side. When this is the case the displacement is one simply of overlapping. If the fragments are displaced laterally from one another then the tension of the muscles draws the fragments together and causes them to overlap. There is no special direction which this displacement may take. The lower fragments may be either in front or behind or to either side of the upper ones. The position of the fragments varies according to the direction of the fracturing force.
This displacement is to be remedied by traction on the hand to overcome the muscles and bring the broken ends opposite one another, and then by direct pressure pushing them as completely as possible back into their normal position.
The shafts of both bones have muscles arising from them on both their anterior and posterior surfaces and the sharp fractured ends of the bones not infrequently get stuck in the muscular fibres and so prevent proper approximation; non-union may be produced by this cause.
2. The fractured ends of the distal or proximal fragments may be displaced toward one another, thus lessening or even obliterating the interosseous space. When 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 brachioradialis, 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 interosseous 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 recognized 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 insertion 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 forearm 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.