Graded recessions of the superior oblique muscle: biomechanical analysis of their effects upon its vertical, torsional and horizontal force components.
OBJECTIVE
To obtain objective information about the effects of different amounts and directions of superior oblique muscle (SO) recession, on its three force components: torsional (incyclotorsion), vertical (depression), and horizontal (abduction).


METHODS
A well known Biomechanical Model of Ocular Motility (Orbit 1.8 Gaze Mechanics Simulation), was used to simulate different amounts of recession of the superior oblique muscle, along three different and commonly used axes of recession: a) along its anatomical path (the hypothetical line uniting the trochlea and the scleral insertion of the SO); b) anteroposteriorly on the nasal side of the superior rectus muscle (recession with posterior transposition); and c) straightforward nasal transposition around the globe. Changes in its force fraction components (torsional, vertical and horizontal) were obtained from the "Mechanical State Viewer" of Orbit, for each amount of recession in three gaze positions: in and down (infra-adduction), downgaze, and out and down (infra-abduction). Several graphics were created with these data for comparison of results.


RESULTS
Any one of the axes of simulated recession of the SO proved to be effective in changing the main actions of the muscle in the gaze positions that were analyzed. The effects upon any one of the oblique muscle force components, were typical for any type of recession, as would be expected from its relationships to the three axes of rotation of the globe. Significant differences were noted among procedures when progressivity of effects, according to the amount of the simulated recession, was analyzed. Qualitative changes in one or more of the force components of the SO were also found with some procedures and amounts of recession, thus confirming some complications reported in the literature.


CONCLUSIONS
A separate analysis of the effects of surgery upon any one of the three force components of the SO can best be obtained with the help of biomechanical models of ocular motility as the one herein tested. Such an analysis may help dilucidate the most convenient point of recession of the SO, when seeking for a specific effect (torsional, vertical or horizontal) along with a minimization of the effects upon the rest of them.
Castanera De Molina
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Instituto de Oftalmologia Pediatrica, Barcelona, Spain. castenera@comb.es
D001696:Biomechanical Phenomena; D005133:Eye Movements; D006801:Humans; D008954:Models, Biological; D009801:Oculomotor Muscles / Q000503:physiopathology / Q000601:surgery*; D013285:Strabismus / Q000503:physiopathology / Q000601:surgery*; D015348:Vision, Binocular / Q000502:physiology