A Treatise on Fractures, Luxations, and Other Affections of the Bones

by P.-J. Desault

18. It appears from what has been said, that the humerus is subject to four different kinds of displacement. 1st, Downward: 2dly, outward, in both of which directions the luxation is always primitive; 3dly, inwards, where it is sometimes primitive, and sometimes consecutive; 4thly, upwards, where it can never be otherwise than consecutive.

The second and fourth are very rare occurrences, and bear so small a proportion to the others, that the latter alone ought to command the attention of the practitioner.

§ III.

OF THE CAUSES AND THE MECHANISM.

19. The causes and the mechanism of luxations of the humerus, vary according as the displacement is primitive or consecutive.

The action of external bodies, directed against the arm, but more particularly falls, where this part strikes forcibly against a resisting body, give rise in general to primitive luxations, and, according as it is differently situated at the time of the fall, the humerus determines, by its position, the different kinds.

20. If it be separated from the trunk, without being carried either before or behind it, if the elbow be elevated, and the fall be on the side, the weight of the body, being almost entirely supported by this bone, pushes its upper extremity downward, distends the inferior part of the capsule, lacerates it, and produces a luxation downward, in which it may even be favoured by the action of the pectoralis major, the latissimus dorsi, and the teres major, as has been judiciously observed by Fabre. In such a case, these muscles, involuntarily contracting to support the trunk, act like the power or force in a lever of the second kind, the resistance to which is formed by the head of the bone, which they draw downward, while the lower extremity of the humerus, resting on the ground, constitutes the fulcrum. Some authors even regard, as an immediate cause of luxation, the powerful contraction of the deltoid muscle, which depresses the head of the bone, and forces it through the lower side of the capsule, a mode of displacement, the existence of which observation incontestibly establishes. The case of a scrivener, so often cited, is well known, who in lifting a book of records luxated his humerus in a downward direction.

21. The mechanism of a primitive luxation inwards, differs a little from the preceding. The elbow, being at once separated from the trunk, and carried backward, the person falls: the weight of the body rests on the humerus; the capsule is lacerated in its fore-part; and a displacement in the same direction supervenes.

22. In a luxation outwards, the elbow is carried forward, towards the opposite shoulder; the capsule being stretched outwardly, gives way in that part, provided the humerus be acted on by a sufficient power. But what can this power be? In a fall, the arm being pushed against the trunk, and stopped by it, cannot carry its motion to a sufficient extent to produce a laceration of the capsule. Hence a luxation outwards must be extremely rare. Indeed no instance of it is to be found in books of surgery. Desault, in particular, never witnessed it. Besides, when in a fall, the arm, separated from the body, is carried backward or forward, the weight of the body acts obliquely on it, and it is but partially subjected to the action of the latissimus dorsi, the pectoralis major, and the teres major. So that no kind of luxation ought from these considerations, to be very frequent, except that in a downward direction, where the influence of both causes is direct. Yet luxation inwards is common enough, and in many instances Desault has observed this primitive mode of displacement, though many modern authors doubt the fact, believing, with Hippocrates, that, primitively, all luxations are downwards.

23. It may so happen that in a primitive luxation, the capsule is only greatly stretched, in which case, the articulating surfaces are but partially displaced; but this membrane more frequently suffers a rupture, through which the head of the bone escapes. To this phenomenon writers, in general, have paid too little attention, notwithstanding the opening of dead bodies has oftentimes demonstrated its existence to practitioners, particularly to Desault, who has given two examples of it modelled in wax; one, of a luxation inward, and the other downward, both found in subjects that died in the Hotel-Dieu. Bell relates some analogous facts, and another English surgeon has also had occasion to meet with them.