4. Writers have admitted in general four kinds of displacement;
backward, forward, outward, and inward. But all these are not alike
frequent, as is proved by experience, and demonstrated by the relative
situation of the parts.
5. In a backward direction the olecranon and the radius may pass up
behind the humerus, as the coronoid apophysis offers but little
resistance in consequence of its slight curvature. On the other hand,
the kind of hook formed by the olecranon, prevents it and the radius
from passing before the lower articulating extremity of the humerus,
and therefore, without a fracture of the olecranon, a luxation in that
direction is impracticable: at the sides, the two lateral ligaments,
but more particularly the reciprocal joining or interlocking of the
uneven articulating surfaces, present almost insurmountable obstacles
to luxations laterally. Whence it follows, that luxation backward is
much more frequent than the others; compared to lateral luxations, it
is, at least, in the proportion of ten to one: with luxations forward,
no comparison can be made; neither Petit nor Desault having ever met
with such.
6. An external force produces the whole of these luxations, but
according to each, this force must vary. In a fall sideways, suppose
the hand be applied to the ground, with the arm extended, to save the
body. It is evident that the resistance of the ground will tend to make
the bones of the fore-arm pass upwards over the humerus, while the
weight of the body pushing that bone downward and forward, will make it
glide over the coronoid apophysis. Thus, the capsule, being distended
before by the humerus, and behind by the bones of the fore-arm, will
give way in one or the other place, or in both, as Desault observed
in a man, who fell on his side, as he was carrying a heavy load: the
weight of the body increased by the load, had such an effect, that the
bones overlapped each other nearly two inches.
7. It appears from this, that a state of extension is the position
most favourable to a displacement backwards; a doctrine by no means
conformable to that of most practitioners, who consider a state of
flexion as necessary to the accident. But, then, in what direction
should the fall take place, in order that the olecranon may pass
upwards? Applied as it is against the side of the cavity that receives
it when the arm is extended, would not this apophysis prevent such
passage? Whatever may be the mode of displacement, the olecranon, in
passing upward and backward, may incline a little to the one or the
other side.
8. I have already said, that without a fracture of the olecranon, no
luxation forward can occur (5). But what cause can act with sufficient
power on the parts to produce both accidents at the same time? It
would be necessary that a fall which had produced a fracture should
be succeeded by another fall; but in such a case, the fore-arm would
be half-bent, and it is in a state of extension alone (7) that the
luxation can take place.
9. Lateral luxations, that is, luxations at the sides have been divided
into complete, when the two articular ranges of the arm and fore-arm,
have lost their connexion entirely, and incomplete, when only one bone
or one part of a bone has been separated from its natural connexion
with the humerus. But what cause can act with sufficient force to
produce the first kind of luxation, namely, that which is complete? In
such an accident so great would be the extent of the wreck and ruin of
the part, that without doubt amputation would be the only resource.
10. The second kind of lateral displacement is the result of a stroke
which forces violently the extremity of the fore-arm outward or
inward. A footman, says Petit, in falling from a carriage, had his arm
entangled between the spokes of the wheel, and suffered in consequence
a luxation outwards. Another produced one inwardly, by being thrown
from his horse, and falling with his arm under him, on rough ground.
Strokes of this kind may, as that author remarks, vary in a singular
manner. But, in general, in all of them, the fore-arm must represent a
lever of the first kind, where the power acts on the end next the hand;
the resistance being in the joint, and the fulcrum in the middle.
§ III.