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Reply: Residual astigmatism after toric intraocular lens implantation

Felipe, Adelina PhD; Artigas, José M. PhD

Journal of Cataract & Refractive Surgery: April 2012 - Volume 38 - Issue 4 - p 731-732
doi: 10.1016/j.jcrs.2012.01.017
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According to recent clinical studies, current toric IOLs almost never rotate after their implantation. Nevertheless, we are in agreement with Drs. Berdahl and Hardten that the alignment of the axis is critical. Errors in positioning the IOL can occur and their effects are of interest, mainly for the future of multifocal toric IOL implantation.

Although we have not had the opportunity to use the toric results analyzer, it seems an interesting and useful tool to solve clinical problems that unavoidably appear with the use of multifocal toric IOLs. Because the recipient of a multifocal IOL has high expectations for complete refractive error correction, he/she will not be satisfied if there is appreciable residual astigmatism.

The example presented by Drs. Berdahl and Hardten suggests a good outcome. After considering their arguments carefully, we have only one concern. It seems that they have used the theory that supposes both vectors, corneal and IOL cylinders, have the same value. According to this theory, 10-degree rotation of a toric IOL results in a 30% reduction in the IOL’s effectivity. In our paper, however, we commented that in practical situations, the corneal and toric IOL cylinders do not generally have the same value. In those cases, the residual astigmatism could be 40% to 50% of the corneal astigmatism, depending on each case, for an IOL rotation of 10 degrees. Also, the axis direction of the final astigmatism, the resultant vector, depends on the real value of the 2 vectors added. In our opinion, the approximate theory that considers the 2 cylinders as equals could be enough to solve the problem with an acceptable degree of accuracy, but the use of a more rigorous theory, assuming the real difference between the 2 vectors, would provide more precise results. Of course, the number of errors that can occur in the measuring instruments and also in the surgical procedure, ie, marking the axis position, may make the improvement obtained with the rigorous theory too small to appreciate. In any case, future data might elucidate this extreme.

In summary, both the calculations developed and the presentation in an open web site seem very useful but only practice can determine what degree of improvement this new tool means in the accuracy of astigmatism correction.

© 2012 by Lippincott Williams & Wilkins, Inc.