Luxation of the phalanges the fingers.— The first phalanx of the thumb as well as the first phalanx of any of the fingers may be luxated backwards; the luxation forwards of the phalanx is very rare and perhaps impossible, except in the index finger and the thumb.
The mutual support which the first pha langes of the fingers afford each other laterally, and the strength of the lateral ligaments render the luxation outwards or inwards very difficult.
Luxations of the first phalanx of the thumb from the metacarpal bone. The first phalanx of the thumb may be luxated forwards to the palmar surface of the metacarpal bone, but this form of luxation is very rare, while the luxation of the same phalanx on the dorsuni of the metacarpal bone is the most common and important displacement of any to which the bones of the hand are liable. We shall therefore consider this accident in detail.
In some persons the first phalanx of the thumb can at will he dislocated backwards, solely by the contraction of the muscles. The displacement produced by accident, however, is much more extensive than this, which may be termed the voluntary luxation. When the first phalanx of the thumb is in a state of extreme extension, accident may dislocate it on the dorsum of the metacarpal bone. The signs of the injury are so evident that mistake appears impossible; the first phalanx is thrown back as if pulled by its two extensors, and forms nearly a right angle with its metacarpal bone (fig. 226); the head of the latter forms a remarkable tumour at the anterior part or palmar aspect of the articulation, while a prominence behind points out the situation of the base of the first phalanx : the last or distal phalanx is (in recent cases) flexed, and it soon becomes difficult or impossible to extend it, or to flex the first phalanx.
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 longitudinal axis of the metacarpal bone; the tendons of the extensor priori and secundi internodii, and of the flexor pollicis bonus 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 capsular ligaments, and the bone will be drawn upwards and backwards by them, a considerable distance on the dorsum of the metacarpal bone (fig. 226). The flexors have their direction so altered and so thrown behind the longitudinal 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 ever keep in mind a case given on the authority of Mr. Hey, who informs us that the celebiated Mr. Bloomfield reported to his class of pupils at St. Georges Hospital, London, that he knew a surgeon increase the force of extension to such a degree in attempting reduction of this dislocation, that he tore off the thumb at the second joint.