9. The causes which produce the displacement of the lower end of
the radius, are the same with those that give rise to other similar
affections. 1st, The convulsive action of the pronator and supinator
muscles, is doubtless a rare cause of the accident, since Desault never
met with an instance of it. 2dly, The action of external bodies, which,
by forcibly and suddenly producing the motions of pronation, rupture
the posterior portion of the capsule, or, by those of supination,
lacerate its anterior portion.
10. Hence two kinds of displacement, the one forward, the other
backward. The first is somewhat frequent; the second is much less so.
The latter was never seen by Desault but once, and that was in the
corpse of a man who had had both his arms luxated, but respecting the
circumstances of which he could receive no information. The other
kind occurred frequently in his practice, of which five examples
have been already published. The difference no doubt arises from
this circumstance, that the greater part of our powerful motions are
performed only in the direction of pronation. This appears to be proven
by the following circumstances.
11. If, in several dead bodies, we lay bare the bones of the fore-arm,
still united by their ligaments, and push the extremity of the radius
forcibly backwards, that is, in the direction of supination, a
laceration will as readily occur in the anterior part of the capsule,
as it will in its posterior part, when, by forcibly pushing the same
extremity forward, the motion of pronation is performed. Hence the
difference does not arise from the structure, but from the direction of
the motions impressed on the limb.
§ IV.