512 



ABNORMAL CONDITIONS OF THE HAND. 



Fig. 226. 



Luxation of the first phalanx of the thumb an the back 

 of the metacarpal bone. 



Anatomical characters of this accident. Op- 

 portunities of ascertaining by dissection the 

 actual condition of the parts when luxation 

 backwards of the first phalanx of the thumb 

 has recently happened, of course do not occur, 

 but from the dissection of old unreduced in- 

 juries of this kind,* and from experiments on 

 the dead subject, we are led to infer that the 

 immediate effects of the injury are, extensive 

 laceration of the anterior part of the synovial 

 membrane, and of one or both the lateral liga- 

 ments, while the posterior portion of the cap- 

 sule remains entire ; the base of the first pha- 

 lanx is dragged to a considerable extent upon 

 the dorsum of the metacarpal bone, elevating 

 with it the tendons of the extensor primi and 

 secundi internodii pollicis ; the tendon of the 

 flexor pollicis longus is carried inwards and 

 under the head of the metacarpal bone. As the 

 extensor ossis metarcarpi and opponens pollicis 

 are not attached to the first phalanx, they are 

 little affected by the luxation, but the con- 

 dition of the three remaining muscles which 

 are inserted into the base of the first phalanx 

 requires consideration. These short muscles 

 are the abductor pollicis, the flexor pollicis 

 brevis, and the adductor pollicis. 



When the dislocation backwards of the first 

 phalanx of the thumb has occurred, the large 

 head of the metacarpal bone is at the same 

 time thrown inwards towards the palm, and 

 having forced its way between the two origins 

 of the flexor pollicis brevis, the shaft of this 

 bone, which is comparatively much narrower 

 than the head, becomes tightly embraced by 

 the two fleshy columns of the muscle. This 

 is a state of things which should be taken into 

 account when the obstacles to the reduction of 

 this dislocation are considered, nor should it 

 be forgotten that the direction and relative 

 position of the points of attachment of all the 

 muscles concerned must be altogether changed 

 when the complete luxation has occurred; 

 their origins and insertions are more than na- 

 turally approximated, and the line of direction 

 of their action is thrown much behind the 



* See London Medical Gazette foi Oct. 14, 1837, 

 J. A. Lawrie, Glasgow. 



longitudinal axis of the metacarpal bone ; the 

 tendons of the extensor primi and secundi 

 internodii, and of the flexor pollicis longus are 

 of course carried by the dislocated bone behind 

 their usual line of action; hence the action of 

 all these muscles, after the luxation has oc- 

 curred, becomes materially altered, their con- 

 traction will no longer be resisted by the lateral 

 and capsnlar ligaments, and the bone will 

 be drawn upwards and backwards by them, 

 a considerable distance on the dorsum of 

 the metacarpal bone (Jig. 226). The flexors 

 have their direction so altered and so thrown 

 behind the longitudiual axis of the metacarpal 

 bone of the thumb, that they now no longer 

 act as flexors of the first phalanx to approxi- 

 mate it to the palm ; on the contrary, they now 

 have become extensors of the dislocated pha- 

 lanx, and tend much by their contraction to 

 increase the deformity.* 



This dislocation is difficult to reduce, par- 

 ticularly if the nature of the accident have not 

 been speedily recognized. Various causes have 

 been assigned for the opposition to the return 

 of the bone ; some think with the late Mr. Hey 

 of Leeds, that a transverse section of the head 

 of the metacarpal bone presents in its outline 

 somewhat of a cuneiform figure; and that, 

 in consequence of the narrowest part of the 

 wedge being thus placed anteriorly, it can 

 easily under the influence of accident glide 

 towards the palm by passing between the 

 lateral ligaments which remain unbroken, and 

 resist all return of the bone backwards to its 

 original situation. Others imagine that the 

 interposition of the anterior ligament and 

 sesamoid bone attached to it between the arti- 

 cular surfaces constitutes the principal ob- 

 stacles to the reduction of this luxation. Again 

 it has been asserted that the tendon of the 

 flexor longus pollicis has been twisted spirally 

 under the metacarpal bone, while some with 

 more appearance of truth have supposed that 

 the muscles are the principal sources of re- 

 sistance. The learned author of the First Lines 

 of Surgery has expressed his opinion that the 

 return of the dislocated phalanx to its place is 

 opposed by a combination of causes, viz. 

 the cuneiform shape of the bone and the re- 

 sistance of the lateral ligaments, as suggested 

 by Hey, the force of the muscles, and, lastly, 

 he adds, because the surface for the applica- 

 tion of the extending means is very limited. 

 To most of these observations we have reason 

 to object, particularly to the last, because we 

 believe that all the force which it is justifiable 

 to use may be easily applied ; and we should 



* In the experiments made by my colleague Mr. 

 Mayne and myself on the dead subject, when we 

 forcibly dislocated the first phalanx backwards, we 

 found the anterior part of the synovial membrane 

 and the external lateral ligament torn across ; the 

 first phalanx was placed as inyzj/. 226. We found the 

 head of the metacarpal bone driven between the two 

 heads of the flexor pollicis brevis in such a way, 

 that the external head of the muscle was placed 

 upon the outside of the shaft of the bone in com- 

 pany with the abductor pollicis, while the internal 

 was situated at the inside of it, along with the ad- 

 ductor and long flexor tendon. 



