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PURPOSE: 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.

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PURPOSE: To determine the incidence of postoperative persistent or recurrent, true or pseudo, superior oblique muscle overaction (SOOA) following treatment of A-pattern strabismus by a selective surgical approach; and to seek its cause. SUBJECTS AND METHODS: Retrospective analysis of the records of 104 consecutive patients with A-pattern strabismus and SOOA treated, with 4 different superior oblique (SO) weakening procedures, selected according to the amount of preoperative anisotropia (in order, smallest to largest): posterior (partial) tenectomy, scleral disinsertion, tenectomy at the insertion and recession with posterior transposition. After exclusions for cause, there were 93 patients who received the same procedure in both eyes. The mean preoperative A anisotropia was 18.30delta+/-8.05delta (range 8delta to 50delta). The mean preoperative SOOA was +2.06 (range +1 to +4). RESULTS: The mean correction obtained by surgery was 16.20delta+/-8.7delta (88.5% of the preoperative deviation). The mean reduction of SOOA was -1.89 (-91. 7% of the preoperative SOOA, range -75.8% to - 103.1%). During postoperative followup, a pattern of persistence of downshoot in the infra-adducted position of gaze (true/pseudo SOOA) was found in 37. 6% of the cases (28% following tenectomy, 30.7% after recession, 40. 4% after posterior tenectomy and 62.5% after disinsertion). A complication, Superior Oblique Inclusion Syndrome was discovered in two cases. CONCLUSIONS: Persistence or recurrence of postoperative downshoot in the infra-adducted position is a common finding following any of the SO weakening procedures analyzed in this study. Its occurrence does not seem to be related either to the amount of preoperative SOOA and/or A anisotropia, nor to the amount of A anisotropia correction obtained. Relatively uncontrolled procedures, such as disinsertion, showed the highest incidence of this sequel of surgery.

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PURPOSE: To evaluate the ability of a computer assisted model of ocular motility, to reproduce this syndrome using known ocular anatomic biomechanical muscular parameters, and to hypothesize pathogenetic mechanisms deduced from clinical and laboratory experiences. METHODS: The manipulation of anatomic, elastic, contractile and innervational parameters available in the computer software program Orbit 1.6, were used to model: a) anatomic ocular and orbital parameters found in the 12 months old infant; b) static (biomechanical) changes measured intraoperatively in the extraocular muscles (medial and lateral rectus muscles) of infantile esotropes; and c) dynamic components of the horizontal deviation, including crossed fixation and bilateral limitation of abduction. Bilateral medial rectus recession surgery was also simulated on the completed model by moving backwards the insertions of both medial rectus muscles. RESULTS: Small, fairly comitant, esotropic deviation were simulated using real data of the mechanical status of medial and lateral rectus muscles collected intraoperatively (length-tension measurements). The more typical large esotropic deviations with bilateral limitation of abduction of the congenital/infantile esotropia syndrome were only obtained when subtle modifications of the medial and lateral rectus muscles' thresholds to innervation were added to the anatomical-mechanical model. An orthotropic primary position binocular alignment with full rotations was obtained in this model following standard bilateral medial rectus recessions, showing surgical dose/response figures close to those usually found in the treatment of real cases of congenital/infantile esotropia. CONCLUSIONS: This biomechanical computer-assisted model may provide a very useful tool not only to test new or different surgical procedures and dosages but also to explore different pathogenetic mechanisms.

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Ocular alignment is usually more divergent in strabismus patients under general anesthesia than in the awake state. Although clinical and anesthetized alignments have been reported to be correlated through the equation A = 0.8P + 30, significant differences in the amount of divergence under anesthesia have been reported between esotropes and exotropes. We found differences in response to general anesthesia in a group of 83 congenital esotropes. The averaged anesthetized alignment pertained to the whole group of 6.2 prism diopters of esotropia, which correlated poorly with the preoperative deviation. Mean anesthetized alignments were similar for any amount of preoperative deviation. Recommendations of intraoperative adjustment of surgery based on the anesthetized position should, in our opinion, be reconsidered and recalculated when treating congenital esotropes. A prediction of surgical results following these recommendations suggested an increase of the undercorrection rate, probably linked to an incorrect identification of normal and anomalous cases to the Apt-Romano test.

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