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.
24. Oftentimes, in compound fractures, one of the fragments passes
through the integuments. In the dislocation of the humerus something
similar to this occurs. The capsule is sufficiently lacerated to allow
the head to escape; but the opening, being then too narrow, forms
around the neck of the bone a noose or kind of collar, which prevents
it from re-entering the place which it originally occupied. Thus, in
the fractures of which I have just spoken, the aperture in the skin
does not, at times, admit of the reduction of the fragment, without a
previous dilatation.
In this case, an attempt is made to reduce the luxation: the capsule is
pressed in folds against the glenoid cavity, and, interposing itself
between it and the head of the humerus, renders fruitless the efforts
of the surgeon who would replace the bone. Desault was the first who
observed this practical fact, two instances of which are recorded in
his journal, and which has frequently since occurred in the Hotel-Dieu.
In such a case, the head is in general extremely moveable, because,
being entirely without the capsule, there is nothing to impede its
motion.
25. When, to a primitive luxation a consecutive one succeeds, several
causes may concur in its production. If a second fall happen, the arm,
being separated from the body, the head of the humerus having nothing
to retain and secure it, obeys, with great facility, the power tending
to displace it in that direction, and suffers a fresh removal from the
bed which it accidentally occupies.
CASE I. A man fell in descending a ladder, and luxated his humerus in
a downward direction. Desault being immediately called, discovered the
nature of the disease, but deferred the reduction till evening. In the
interval, the patient went to get into a chaise: his foot slipped, and
he fell a second time. The pains became more severe than at the time
of the first accident; and Desault, on his return, instead of finding,
as in the morning, the head of the humerus under the hollow of the
arm-pit, discovered it to be behind the pectoralis major.
26. Muscular action is one permanent cause of a new displacement.
Suppose the humerus luxated downward, the pectoralis major, and the
deltoid muscles draw its superior extremity upward and inward, which,
offering to their action but a feeble resistance, changes its position
and that in a two-fold direction.
27. The different motions of the arm may also, according to their
direction, produce the same effect. Thus we have often witnessed a
luxation inwards succeeding to a luxation downwards, in consequence of
unskilful attempts to reduce it.
§ IV.