PRACTICAL CONSIDERATIONS : THE ELBOW-JOINT. 305 



capsule ajr^ least tense. It is probably assisted by a want of perfect adaptation 

 between the articular surfaces. These experiments on the dead body do not quite 

 fulfil the conditions of the living, because we have no evidence that then the muscles 

 can produce quite the same movement ; moreover, Cathcart has shown that in anky- 

 losis of the shoulder-joint this motion is greatly impaired, thus proving that in life a 

 small amount of motion at that joint is an essential part of free twisting of the hand. 

 Experiments by Hultkrantz on the living subject tend to show that the slight motion 

 of the ulna is in the opposite direction to that described. There is probably much 

 individual variation. ^ 



PRACTICAL CONSIDERATIONS. 



The Elbow-Joint. — This joint is dependent for its strength more upon the 

 shape of the bones that enter into it than upon the ligaments or muscles. As the 

 elbow ceased to be useful for support, but became of the utmost importance for 

 prehension, the radius became movable instead of fixed, and the strength of the joint 

 came to depend in much larger proportion upon the ulna. 



Force applied in the line of the long axis of the limb, as in hanging by the 

 hands (the weight being transferred from the wrist and the radius to the ulna and 

 the elbow, largely by means of the triangular and orbicular ligaments, with very 

 slight help from the oblique ligament), is resisted in the order of effectiveness (a) 

 by the hook of the olecranon over the trochlea ; (d) by the lateral ligaments ; (<r) 

 by the biceps, triceps, and brachialis anticus, aided by the flexors, extensors, prona- 

 tors, and supinators. The lower part of the lesser sigmoid cavity of the ulna under- 

 hangs the inner edge of the radial head, and aids in preventing the radius from being 

 drawn away from the ulna. 



Force applied in the same line, but in the opposite direction, as in falls upon 

 the hands (the thrust being transferred from the radius to the ulna by means of 

 the oblique fibres of the interosseous membrane), is resisted almost exclusively by 

 the coronoid process, aided perhaps by the surface of contact between the radial 

 head and the capitellum, which is diminished in full extension. 



As the dislocation usually occurs with the forearm hyperextended, the lateral 

 ligaments, particularly the inner one, are often stretched and torn ; the brachialis 

 anticus is drawn tightly over the humerus and is sometimes ruptured. The coronoid 

 process is not infrequently broken. 



Aiitero-posterior dislocations are the most frequent, because of {a) the lesser 

 antero-posterior diameter of the joint as compared with the lateral diameter ; (3) the 

 varying efficiency of the hold of the ulnar processes — the coronoid and olecranon — 

 on the humerus in different positions of the elbow ; (r) the weakness of the anterior 

 and posterior ligaments, and the absence of effective nmscular support. 



Backward dislocation of both bones is far more frequent than forward, be- 

 cause : (i) The capsular ligament is weakest posteriorly. (2) The coronoid, which 

 resists backward displacement, is smaller, less curved, and received in a shallower 

 fossa than the olecranon, which prevents luxation forward. (3) It is in its relation of 

 least effectiveness when the joint is in full extension. (4) Falls upon the hand with 

 the forearm extended greatly outnumber all other causes of dislocation of the elbow. 

 (5) In full extension the already slight surface of contact between the radius and 

 humerus is diminished and the posterior articular edge of the radial head projects 

 behind the capitellum. (6) The ulna and radius are apt to be dislocated together 

 rather than separately because of the strong ligaments which hold them to each 

 other — the triangular ligaments below, the interosseous membrane, and the orbicular 

 and oblique ligaments above — and because of the absence of any such intimate 

 connection of either bone with the humerus. 



It is this ligamentous connection with the ulna which enables the radius, in spite 

 of the shallowness of the articular cup upon its head, to resist the powerful forward 

 pull of the biceps. 



^ Heiberg : Ueber die Drehung der Hand, 1884, contains an exhaustive bibliography. 

 Heiberg : Journal of Anatomy and Physiology, vol. xix., 1S85. Cathcart: ibid. Dwight : ibid. 

 Hultkrantz : Das Ellenbogen Gelenk and seine Mechanik, Jena, 1897, contains the later 

 bibliography. 



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