Though falls on the point of the shoulder are oftentimes productive of luxation of the clavicle, they are not the exclusive causes of that accident. Desault has seen the sternal extremity forced from its cavity by the knee being pushed violently against the middle of the back, while the shoulders were drawn at the same time backwards.
Case I. A porter dislocated his clavicle in the following manner. He was carrying a very heavy burden, suspended from his shoulders by cords that passed under each arm-pit. Being desirous of resting himself by the way, he placed on a block the burden he carried, which slipping backward, drew his shoulders in the same direction, and at the instant of his attempting to retain it and prevent it from falling, produced a luxation of the clavicle.
7. It follows from what has been said respecting the different states of the articulation, during its various motions (4), that the clavicle is not equally liable to be luxated in every direction. Inclining naturally backward, but a very slight degree of motion in that direction is necessary, to effect a luxation forward. To produce a luxation backward or inward, it is necessary, on the other hand, that the humeral extremity of the bone should make a sweep at least three times the extent of that required in the preceding case. Besides, motions in this direction are accompanied with pain, particularly if they be made by force. Luxation downward is prevented, on the one hand, by the cartilage of the first rib, which presents to the bone an insurmountable barrier. On the other hand, to produce this kind of luxation, it would be necessary for the external extremity of the clavicle to be forced upwards, an occurrence very seldom occasioned by falls. Luxation upward, or over the superior edge of the sternum, must be the effect of a stroke, which, by depressing the point of the shoulder, and forcing it at the same time forward, presses the sternal extremity against the internal and superior part of the capsule, which, being thus lacerated, suffers a luxation to take place. But such a derangement of the articulating surfaces is very rarely produced by falls. Whence it follows, that of the different kinds of luxation of the clavicle, that in a downward direction is altogether impracticable. Those backward and upward, though possible, occur but rarely in practice; while that in a forward direction, on the contrary, is not an unfrequent accident. This tends to confirm the observations of practitioners, and particularly of Desault, whose immense collection on the subject furnishes examples of the last kind of luxation only.
8. In these luxations, there is for the most part, a rupture of the capsular ligament, and an escape of the bone through the opening. But sometimes the ligament is only preternaturally distended, and then the luxation is incomplete.
§ III.
OF THE SIGNS.
9. But whatever may be the causes or kind of the luxation, its diagnosis is always easy. If it be forward, the direction of the stroke which the shoulder has received, furnishes, at first, some ground of suspicion. The accident is certainly known by the appearance of a hard and unnatural protuberance in front of the sternum, and behind the extremity of the sterno-cleido-mastoideus muscle, by the existence of a sensible depression or hollow at the joint, and by the situation of the shoulder, which is pushed further backward, and is less projecting and more approximated to the trunk, than in its natural state. Add to these, a difficulty in performing motions in a forward direction, which, when somewhat forced, reduce, in proportion as they are accomplished, the size of the protuberance formed by the displaced end. The head is always inclined towards the side where the luxation exists; an attitude which relieves the painful drawing or tension produced in the sterno-mastoideus muscle, by the humeral extremity pushing it forward.
10. A protuberance over the superior edge of the sternum, a difficulty in raising the shoulder, the pain which results from attempting such a motion, the diminution of the protuberance which it occasions, the absence of the sternal extremity from its natural cavity, the approximation of the shoulder to the thorax, and its depression and diminished projection, compared to its usual state, afford evidence of a luxation upwards.
11. A luxation inward or backward, would be characterized by a projection of the shoulder exteriorly, by a difficulty in performing motions in a backward direction, by the alarming effects, which, as Petit remarks, the compression of the trachea would doubtless produce, and by a depression or hollow at the joint, more perceptible here than in the two preceding cases.
12. These appearances will be more or less striking, accordingly as the membranes, lacerated or only distended, offer a greater or less resistance.