We thank Kekunnaya and associates for their interest in our recently published article, “Biomechanical Analysis of X-Pattern Exotropia.” We do agree with them that X-pattern exotropia may involve a large number of neural and biomechanical parameters that were not modulated systematically in our simulations and that multicenter studies would be necessary to obtain a larger number of patients. But exactly for those reasons we confined our investigation to 2 central questions: (1) What do the best available biomechanical simulations say about existing hypotheses on the causes of X-pattern exotropia? and (2) Is there a simple, biomechanically plausible surgery to treat these patients sufficiently?

The comments by Kekunnaya and associates question neither our approach to these 2 questions, nor the answers that we have given. Regarding patient selection, we already had to include patients who underwent surgery 20 years previously, sometimes at different hospitals. Many of the desirable detailed information (preoperative and postoperative strabismus angles for all gaze directions, surgical details, high-resolution magnetic resonance imaging results) simply were not available to us, and should be collected for future multicenter studies. We also stated that the 2 groups (with and without previous strabismus surgery) showed no significant difference, so we decided to pool them.

We also do not claim that surgery on the horizontal rectus muscles alone abolished the X-pattern: we clearly stated that it reduced the strabismus to a level that was acceptable for the patients, and did so in a way that was biomechanically plausible.

Our study cannot definitely explain X-pattern exotropia, and also does not aim to do so. For that, much more detailed information, especially high-resolution preoperative and postoperative magnetic resonance imaging scans, would be required, as well as a larger number of subjects. These parameters, combined with their effects in realistic 3-dimensional oculomotor models, can provide further insight into the causes of the pathologic features.

We believe that our study has shown that the oculomotor system is so complex that speculations based on intuition alone are not sufficient. But even when these data are available, it is still challenging to assess the importance of possible innervational patterns on the observed strabismus, because such data are very hard to obtain in humans.