1st, Most authors who have written on the luxations of the fore-arm,
have omitted considering separately those confined to the radius
alone. Some detached observations may be found here and there, on the
luxations of the upper extremity of this bone, which Duverney alone
has treated at some length. Those of its lower extremity, though more
frequent, and more easily produced, appear to have almost entirely
escaped the attention of the French practitioners, who have transmitted
nothing to us on that point, owing, no doubt, to their having had no
knowledge of it from experience. But since, at the present day, a
sufficient number of facts are collected on the subject, some account
of these displacements cannot be a matter of indifference to the art,
and it may be traced with as much precision as the accounts of other
similar accidents.
§ I.
OF THE DIFFERENCES IN POINT OF STRUCTURE BETWEEN THE TWO ARTICULATIONS
OF THE RADIUS WITH THE ULNA.
2. The radius, the moveable agent in pronation and supination, rolls
on the ulna its fixed basis or abutment, by means of two small
articulating surfaces, the one at its upper end, slightly convex,
broad within, and narrow without, corresponding to the small sigmoid
cavity, in which it is lodged; and the other at its lower end, concave,
semicircular, and fitted to the convex edge of the ulna, which it
receives. Hence two kinds of articulation different from each other,
with respect to their motions, the connexion of their surfaces, and the
ligaments which strengthen them. Let us specify these differences; they
will serve to shed light on those that exist between the displacements
of the two extremities of the radius.
3. At its upper end, the radius, in performing pronation and
supination, moves only on its own axis; at its lower end, it rolls
round the axis of the ulna: therefore, being farther removed from
their centre, its motions must have both a greater range and greater
force, in the latter case than in the former. The head of the radius,
turning on itself within the annular ligament, cannot distend it in
any direction or part. The cellular membrane attached to this ligament
is alone slightly stretched, but being loose and elastic, it yields
without resistance. At its lower end, on the contrary, the radius,
turning from without inwards during pronation, keeps the capsule
posteriorly in a state of tension, and draws it against the immoveable
head of the ulna, which tends to pass through it, if the motion be
forcible. The same phenomenon occurs in a contrary direction, during
supination; the radius is directed backward, and the ulna inward. Being
in this case distended before, and relaxed behind, the capsule is
disposed to laceration anteriorly.
4. In addition to this disposition, the ligaments of the two
articulations are disproportioned in their strength. Thin and weak at
the lower articulation, thick and firm at the upper one, they are in
this respect strikingly different. The head of the radius, resting
against the small but firmly fixed head of the humerus, finds there, in
most of its movements, an obstacle to displacement. On the contrary,
its lower end, drawing along with it in its movements, the bones of the
carpus which are connected with it, derives from them no solid support.
§ II.
DIFFERENCES IN THE LUXATIONS OF THE RADIUS; DIFFICULTIES OF THAT AT ITS
UPPER END.
5. It follows from what has been said (3 and 4), 1st, that the lower
articulation of the radius is not only exposed to the action of more
causes of displacement, but possesses fewer means of resisting those
causes, and that, from the threefold consideration of its motions, the
ligaments which connect its articulating surfaces, and their relation
to each other, it must be frequently subject to luxations: 2dly, that
for reasons the very reverse of these, its upper articulation must be
very seldom subject to luxation.
6. Indeed, what cause is there to produce luxation in this latter
joint. Is it from a forcible exertion of pronation or supination that
this accident can occur? Surely not: for, on the one hand, as the
lower articulation offers less resistance than the upper one, it is
evident that, in either state of motion, it being the weakest, will
be displaced first, and the motion being thus checked, can no longer
operate to the displacement of the other. On the other hand, however
forcible the motion may be, there will be in the upper articulation,
nothing but a rotation of the bone on its own axis (3). How, then,
without being carried forward, backward, &c. can the head be displaced?
Indeed, it would be necessary that all the fastenings or bonds of
attachment muscular and ligamentous, should be first broken. But these
are too strong, and the motion is too weak. Can the displacement be
produced by a blow impressed on the radius from below upwards? By no
means: because the head of the humerus making, in this case, a solid
resistance will not permit it to escape from the capsule (4). Can it
arise from a violent extension or flexion of the fore-arm? No. This
effort being altogether confined to the ulna, affects the radius in but
a very faint degree.
7. It appears from hence, that the accidental luxation of the upper end
of the radius, suddenly produced by external causes, must, if it ever
occur, be extremely rare. But it is not so with respect to luxations
which take place slowly in this joint, particularly in children, where,
in consequence of repeated efforts, the ligaments become relaxed.
But this kind of displacement, being almost always complicated with
a swelling of the joint, and sometimes not to be reduced by the
expedients of art, cannot be comprised in my present plan.
9. But experience would seem to have at times exposed the fallacy of
these considerations and reasonings, founded merely on the structure
of the parts. Duverney relates some instances of luxations of the
head of the radius, produced suddenly by external causes. Two other
practitioners are also of opinion that they have witnessed similar
displacements. But did they examine the subject with all that attention
which it required? A similar case was reported to the Academy of
Surgery, by one of its associates; but doubts were entertained with
regard to its reality: and, ultimately, there were so few facts in its
favour, and such strong presumptions against it, that Desault was
induced to deny the luxation altogether, till its reality should, by
new proofs, be more certainly established.
After all, if it should occur, the same signs which announce the
luxation, when the ligaments, in consequence of being gradually
relaxed or in some way distended, permit the head of the radius to
be insensibly displaced, would then appear as the sudden effect of
external violence.