THE FOREARM. 



325 



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 



External condyle 



Lower fragment 

 of ulna 



Upper fragment 

 of ulna 



Tendon of triceps 



Anconeus covered by expansion 

 of the tendon of the triceps 



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



upper and lower fragments have a tendency to incline toward the radius and so 

 obliterate the interosseous space and interfere with rotation. 



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 pro- 

 nation. The ulna is to be pushed away from the radius by pressure made between 

 them with the thumb and fingers 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. 



