3o6 HUMAN ANATOMY. 



Lateral dislocations of the separate bones are infrequent for the same reason ; 

 of both bones because of the great relative width of the joint, its irregular undulating 

 transverse outline, the prominences of the border of the trochlea and of the capi- 

 tellum, the strength of the lateral ligaments, and the presence of the flexor and 

 extensor muscular masses arising from the condyles. 



Inward dislocation is the rarest on account of the greater projection of the inner 

 border of the trochlea. 



When either bone is dislocated separately , it is most apt to be the radius, and 

 in the forward direction on account of the slightness of its humeral connection, its 

 mobility, its direct relation with the hand and wrist, and the effect of muscular action 

 (biceps) upon its upper extremity. The orbicular ligament offers the chief, if not 

 the only resistance to this forward pull of the biceps. Therefore, if this is torn, 

 recurrence of the luxation is common, unless the arm is kept in the acutely flexed 

 position. When the ulna is dislocated alone, it is almost always backward for 

 reasons already mentioned. 



In the common backward dislocation of both bones, the tip of the coronoid may 

 rest upon the posterior surface of the trochlea, or may ascend to the level of the 

 olecranon fossa, which, however, it is prevented from actually entering by the pres- 

 ence of the soft parts and by the tension of the structures on the front of the joint. 

 The most easily recognized symptom of this displacement is the change in the rela- 

 tion of the tips of the condyles and the olecranon, the latter occupying a much higher 

 position in extension, or lying much more posteriorly in flexion (page 287, Fig. 

 301). In making this measurement it is important to be sure that the line uniting 

 the tips of the condyles, and in full extension in the normal arm, crossing the olec- 

 ranon about one-sixteenth of an inch below its tip, is a straight line at right angles 

 to the long axis of the humerus. Any upward or downward curve given to this line 

 destroys its diagnostic significance. 



The large majority of cases of dislocation of the elbow occur in young males, 

 usually below the age of twenty. Kronlein has called attention to the fact that at 

 this age fractures of the clavicle are also common and luxation of the shouldfer is 

 rarely met with, while after twenty both clavicular fracture and elbow dislocation 

 are comparatively rare and shoulder dislocation is common. He concludes that in 

 childhood fracture of the clavicle is the equivalent of dislocation .of the shoulder by 

 direct violence, and dislocation of the elbow is the equivalent of the shoulder dis- 

 location from indirect violence. 



The anatomical explanation may be that the disproportion between the head of 

 the humerus and the glenoid cavity (page 278) is less marked in childhood, the 

 articular surfaces are therefore not so easily separated, and force applied to the point 

 of the shoulder is more apt to reach and be expended upon the clavicle. 



As to the elbow, the shallowness in children of the fossae which receive the 

 processes and a corresponding want of prominence in the latter, together with the 

 ease with which the elbow-joint in childhood may be hyperextended (which is not 

 the case in adult life), are possible explanations of the frequency of this dislocation 

 in young persons. 



Congenital dislocations occur. In some instances they haye been associated 

 with deficiency of the capitellum, and haye then been accompanied by such elonga- 

 tion of the radial neck as to place the head of that bone on a level with the tip of the 

 olecranon. 



This affords an illustration of the general law, which may be mentioned here, 

 that the rate of growth of epiphyses is inversely as the pressure upon them. Other 

 examples are to be seen in the overgrowth of the cranial bones in hydrocephalus, 

 when their edges are separated by the pressure of the ventricular fluid ; in the pro- 

 jection of the vomer and intermaxillary bones beyond the level of the alveolar arch 

 in some cases of cleft palate ; in the bony outgrowths that fill up the glenoid cavity 

 or the acetabulum in unreduced luxations of the humerus or femur ; and in many 

 other similar conditions. 



Disease of the elbow-joint is most often tuberculous, but may be of any variety. 

 In spite of the constant exposure of the joint to traumatism, it is not attacked by 

 disease with exceptional frequency. This is probably partly due to the firm, inter- 



