Aim: To present the results of a new technique that the authors have developed to weaken the extraocular muscles. The biomechanics of this technique, which is termed ''Y-split recession,'' are given in the companion paper. Methods: A retrospective study, testing the effects of a new surgical technique on strabismus, nystagmus, and visual acuity. 228 patients (aged 6.8 (SD 6.0) years) with variable angle strabismus and nystagmus were treated by splitting and detaching two rectus muscles, and re-attaching the two halves at an angle of about 65˚to each other. Subjects were examined with the usual orthoptic tests immediately after the operation, and up to 96 months later. Results: The operation reduced the strabismus, eliminated or weakened the nystagmus in primary position, and improved binocular vision and the development of visual acuity. Only minor side effects were observed. Conclusions: The Y-split recession can be used to reduce the torque for extraocular rectus muscles, and provides an alternative to ''Cüppers Fadenoperation,'' recession, and similar procedures.

ABSTRACT.Purpose: To compare the results of two different surgical techniques: 'Cu¨ppers technique', in which the torque of oculomotor rectus muscles is reduced by suturing the muscle to the globe in the posterior half of the globe; and 'Y-split recessions', in which the muscle torque is reduced by Y-splitting the rectus muscles, and reattaching the two halves at an angle to each other. Methods: We carried out a retrospective analysis of the outcome of surgery on 100 patients with infantile esotropia. Results: Both techniques show a sufficient reduction of strabismus angle variability, and minimal and maximal strabismus angle. Conclusions: Both techniques achieve satisfactory results. In addition, the Y-split technique allows for accurate control of the muscle torque and requires no access to the posterior half of the eye, which can facilitate the surgical approach. For a reduction in muscle torque, the Y-split recession is a good alternative to the established Cu¨ppers technique.

The consequences of changes in the oculomotor system on the three-dimensional eye movements are difficult to grasp. Although changes to the rectus muscles can still be approximately understood with simplified geometric models, this approach no longer works with the oblique muscles. It is shown how SEE++, a biomechanical model of the oculomotor plant that was built on the ideas of Miller and Robinson (1984) can improve the understanding of the effects of changes to the oblique eye muscles. By displaying only selected muscles, and by illustrating the relative contribution of these muscles through color-coding the bulb surface, the functional properties of the oblique muscles can be presented in a much clearer way. Investigating the effects of a hyperactive inferior oblique muscle shows that this type of model can help to clarify the functional cause of a pathology, which can otherwise be unclear, even for common pathologies.

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