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Paul van Saarloos; Toric Calculation Errors and Optical Misalignment creating Astigmatism. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2212.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the effect of optical misalignment of an IOL, from the optical center of the cornea, on the amount of astigmatism induced, and assess how that may affect predicting Toric lens power.
A model eye was created with an IOL in the lens position that can be decentred with a micrometre adjustment. The induced aberrations, including astigmatism and coma were measured with an iTrace (Tracey Technologies) device, for different decentrations and different IOL powers.
The amount of astigmatism increased to the square of the decentration distance. The amount of astigmatism induced was in proportion to the power of the IOL. The axis was in the direction of the decentration. The maximum induced astigmatism was over 5 Diopters, however, decentrations below 750 microns were below 1 Diopter, except for high IOL powers combined with high decentrations. The amount of Coma also increased with increasing decentration.
This model eye shows that the optical misalignment between the cornea and IOL is likely to induce astigmatism and coma. Angle Alpha is a potential predictor of IOL misalignment, and is within 10 degrees of horizontal in 95% of 5000 cataract patients in a paper presented at the Tracey Technology Users Meeting at AAO 2017. This study shows misalignment of the IOL in the horizontal direction will induce a different amount of astigmatism for correction of with-the-rule and against-the-rule astigmatism, due to the different IOL power in that meridian. This matches the prediction of the Baylor nomogram for calculating Toric power in IOLs. Optical misalignment is a good alternative theory, compared to astigmatism in the back surface of the cornea, for the errors in calculating the power of Toric IOLs to correct total eye astigmatism.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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