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Natascha Bayer, Nino Hirnschall, Lukas Traxler, Andreas Drauschke, Rainer A Leitgeb, Sverker Norrby, Oliver Findl; Analysing the impact of a misaligned toric intraocular lens on wave front aberrations. Invest. Ophthalmol. Vis. Sci. 2017;58(8):1121. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Misalignments of intraocular lenses (IOLs) after cataract surgery influence the image quality negatively. Parameters to judge the effect of misalignments on visual performance are wave front aberrations. The effect of a decentred and tilted toric IOL on wave front aberrations was tested using numerical raytracing software.
An astigmatic model eye was implemented using the optical ray tracing program Optic Studio (Zemax). As basis for modelling the Liou & Brennan schematic eye was chosen, having a 5° temporal tilt of the visual axis and a 0.5 mm nasal decentration of the pupil. In order to simulate a pseudophakic eye, the IOL was introduced into the model, so that the first principal plane of the natural lens of the Liou & Brennan eye coincides with the first principal plane of the IOL. This refers to an anterior chamber depth of 5.32 mm. The IOL was further characterized by a toric anterior surface and an aspheric posterior surface. In order to simulate a certain degree of astigmatism the anterior surface of the cornea was adapted to a biconic surface with 49.89D @180° and 46.89D @90°. The thickness of 0.5 mm, the refractive index of 1.376 and the conic constant -0.18 were adopted from the data published by Liou & Brennan. The posterior corneal surface was modelled as conic surface having a radius of curvature of 6.4 mm and a conic constant of -0.6. IOL decentration ranged from -0.5 mm to +0.5 mm along the x-axis, while tilt was performed around the y-axis amounting -5° to +5°. Zernike coefficients up to the 4th order were analysed in dependence on tilt and decentration.
An increase of decentration leads to a reduced performance of the toric IOL. This effect maximizes if both tilt and decentration are prominent. The coefficients, which are influenced the most by an IOL misalignment, are defocus (Z4), astigmatism (Z6), and coma (Z8). Spherical Aberration (Z11) shows more sensitivity to IOL decentration than tilt. Minimal influence is also investigated concerning secondary astigmatism (Z12).
Numerical simulations allow estimating the effect of lens misalignments on wave front aberrations and other optical quality parameters. Similar to monofocal IOLs, position tolerance for toric IOLs also has clinical relevance when judging the postoperative vision of the patient.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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