4. The sternal articulation of the clavicle experiences different
changes, according to the different movements of this bone. If
these movements be in a backward direction, the articulating
surface is turned forward, straining the anterior part of the
capsule, the corresponding ligament, and the extremity of the
sterno-cleido-mastoideus muscle. If, on the other hand, they be in a
forward direction, the posterior ligament, and the adjacent portion
of the capsule are overstretched. In motions directed upwards, the
costo-clavicular ligament, and the external and inferior part of
the capsule, and in those directed downwards, the inter-clavicular
ligament, and the internal portion of the capsule, experience a similar
degree of tension.
5. Hence it follows, 1st. That the natural movements of the shoulder
may be regarded as predisposing causes of luxation, because at the part
where tension is excessive, the ligaments are disposed to give way,
and suffer the sternal extremity to escape: 2dly. That the efficient
causes will be, all external forces acting on the clavicle in such a
way as to increase its motions beyond their natural degree, and beyond
the resistance which the ligaments are capable of making. Thus a fall
on the point of the shoulder, forcing it suddenly backward and inward,
produces a luxation forward. But, in general, as the strength of the
articular ligaments is superior to the resistance of the clavicle
itself, a fracture takes place more frequently than a luxation, in the
proportion of nearly six to one.
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.