7. Most of the phenomena which accompany fractures of the fore-arm
(5), are evidently the result of a displacement of the fragments; a
displacement, not, in general, very perceptible in the longitudinal
direction of the bones, because the muscular action, tending to produce
it in that direction, is not very powerful. When it does occur in this
way, it is most frequently the immediate effect of the stroke that
produced the fracture.
8. But it is different with respect to a displacement in the cross
direction of the bone. Here the cause of the separation of the broken
ends, may be the same with the cause of their fracture, as happens in
the passage of a carriage wheel over the limb, or in the falling of
some body against it; and then, 1st, the fragments are separated from
before backward, or contrarywise, and hence, a protuberance on the one
side of the limb, and a depression on the other; 2dly, or else they
are pressed against each other laterally from without inwards. From
this latter cause arises that inequality which the limb exhibits at the
place of the fracture; the slight depression which it manifests on its
sides; and the protrusion or bulging out of its anterior and posterior
surfaces, by means of the mass of muscles which are pushed in these
directions, by the approximation of the fragments to each other.
9. A proper reduction removes the first kind of displacement, namely,
that which occurs in the cross direction of the bone backward or
forward (8); and unless an external force be applied anew, it does
not again return. On the contrary, how exact soever the reduction may
be, in the second kind, namely, that which takes place laterally from
without inwards, the fragments are soon found to have approached each
other again. Above, the pronator teres presses the superior fragment
of the radius against that of the ulna; below, the two fractured
extremities are pressed against each other, by the contractions of the
pronator quadratus. From this double cause arises, unless something
prevent it, the contact of the four fractured ends, which have been
sometimes found united together by a common callus, as is proven by
several cases sent to Desault, and by the cases of different patients
admitted into the Hotel-Dieu, after having undergone an improper
treatment. In such cases, the movements of pronation and supination
being entirely destroyed, are but imperfectly supplied, as Duverney
remarks, by those of a rotation of the arm.
But if the four broken ends should not even be joined together by
a common callus, still the space between the bones being evidently
diminished, impedes muscular action and the motions of the limb
depending thereon.
§ V.