SUMMARY
The operations called "muscle transpositions " have been very usefull for the correction of certain kinds of strabismus, both comitant and incomitant. The mechanical interpretation of the results of these surgeries, however, has been subject of discrepancies, which begin with the interpretation of some physiologic facts concerning the rectus muscles.
The old treatises stated that an ocular duction perpendicular to a rectus muscle plane of action causes a side slip of this one on the sclera, so that it assumes always the shortest path between its two insertions. Modern theories, however, have stated the opposite, i.e., that the rectus muscles pass through some structures like pulleys, situated around the
region of the eye equator, that constrain strongly the translation movements. This fact would warrant the stability of the muscle action in any direction of sight. Some papers that assume this fine of thinking state that those structures impede the lateral sliding of the muscle, including in surgical transpositions, as the Hummelsheim operation.
The author shows photographs of surgical acts that show that those structures are destroyed during that kind of operations; the positions of the muscles show that there is a full translation, at least from a point situated just behind the posterior pole of the eye.
Taking the chance, the author shows two lenght-tension curves, made during the operations. It can be seen that the transpositions type Carlson & Jampolsky for the correction of the abduction palsy produces some adduction passive limitation, differently that what occurs in the Knapp 's operation for the correction of the so called double paralysis of the elevators.
As a corollary of those demonstrations, the author contests the way that Knapp proposes the reinsertion of the transposed horizontal muscles toward the superior rectus insertion. He reinserts them in a direction almost perpendicular to the spiral of Tillaux, when they must be reinserted almost parallelly to it. Knapp's reinsertion way causes a reduction of the desired elevationg effect of the operation.
Key-words:
Muscle transpositions; mechanical principles