Fractures Of The Shaft Of The Ulna

The shaft of the ulna is more often broken by direct violence than is the shaft of the radius. When the arm is raised to ward off a threatened blow the thumb is toward the body and it is the ulna which is presented externally to receive the impact of the blow, hence its more frequent injury. There are two main sites of injury, one just below its middle and the other a short distance below the elbow-joint, about at the junction of its middle and upper thirds. The former results from the fact that the bone below the middle is smaller and weaker than it is above and is not so well covered by muscles.

Fractures Just Below The Middle Of The Shaft Of The Ulna

The bones of the forearm act as props to separate the hand and elbow. The hand is attached to the radius and the radius rests on the capitellum of the humerus, therefore even when the ulna is fractured as long as the radius and attachments of the hand are intact there is usually but little overlapping of the fragments.

The lower fragment is most often displaced to the radial side. This is due to the action of the pronator quadratus muscle (Fig. 334).

The upper fragment articulating with the humerus by a pure hinge-joint cannot be displaced laterally, but the radius and hand can move bodily toward the ulna, being favored in so doing by the pronator radii teres. Thus it is seen that both upper and lower fragments have a tendency to incline toward the radius and so obliterate the interosseous space and interfere with rotation.

Fig. 335.   Fracture through the upper third of the ulna viewed from the outer side.

Fig. 335. - Fracture through the upper third of the ulna viewed from the outer side.

As to whether the lower or upper fragment will be nearer to the radius will depend upon the direction of the line of fracture. If this is from within downward and outward, as is the more usual, then the lower fragment will be to the radial side of the upper one.

The treatment of fractures in this locality should be with the hand placed in the position of full supination. Hamilton ("Fractures and Dislocations," page 319) stated that he had three times seen supination lessened in this injury but never pronation. The ulna is to be pushed away from the radius by pressure made between them with the thumb and finders and the hand bent toward the radial side.

Fracture At The Upper Third

The radius articulates with the upper end of the ulna in the lesser sigmoid cavity. Immediately below this is a depression in the ulna called the bicipital hollow, intended to accommodate the bicipital tubercle when the forearm is pronated. At this point the bone is slightly narrowed and then widens again toward the middle. This constricted part is 7 or 8 cm. (3 in.) below the tip of the olecranon process and the spot where fracture is likely to occur. When fracture does occur here, if displacement is marked, it produces characteristic lesions. The upper fragment may be displaced either posteriorly or anteriorly.

The carrying angle (page 282) formed by the line of the arm with the line of the forearm, depends on the integrity of the humerus and ulna and their proper articulation. If the ulna is broken high up the forearm is deprived of its support on the inner side and it sags inward, thus approximating the bones, obliterating the interosseous space, and diminishing the carrying angle. In treatment care should be taken that the forearm be not allowed to incline toward the inner side.