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T. Padrick, E. Sarver, M. Hall; Astigmatic Effect of Spherical IOL Tilt and Decentration. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1158.
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To evaluate the theoretical effect of a spherical IOL's decentration and tilt on astigmatic correction at the corneal plane, which may occur at the time of cataract surgery and effect intraoperative wavefront measurements.
We generated an astigmatic eye model in Zemax Optical Design program (November 10, 2008) to determine the astigmatic correction at the corneal plane as the spherical IOL was decentered and/or tilted. The anterior and posterior corneal surfaces were modeled as aspheric surfaces with 7.8 and 6.5 mm radii, respectively, and a conic constant K = -0.25. The thickness of the cornea was 0.55 mm and the corneal index of refraction was 1.376. The IOL was modeled as a 20D biconvex spherical lens located 4.6 mm behind the anterior cornea and had an index of refraction of 1.42. The aqueous index of refraction was 1.336, the pupil diameter was 6.0 mm, and the axial length was 23.6 mm. We allowed the IOL decentration to range up to 1.0 mm from the optical axis and allowed the IOL to tilt up to 10 degrees about the X axis. A toric paraxial lens was placed at the anterior corneal vertex and the Zemax built-in optimization features were used to determine the astigmatic correction as the IOL was decentered and tilted. For fixed values of decentration of (x,y=0,0), (x,y=1,0), and (x,y=0,1), the IOL tilt about x was varied from 0 to 10 degrees. At each of these misalignment values, the change in astigmatic refraction at the corneal plane was calculated. Second-order polynomials were fit to the sets of data corresponding to the decentrations.
The resulting change in astigmatic refractions for the three sets of decentrations were very well described by the second-order polynomials. These polynomials for spherical equivalent (SEQ) and cylinder (CYL) along with the R^2 values are shown in Table 1. For no decentration, the maximum SEQ and CYL are -0.45 and -0.46 D, respectively. For decentration x,y=1,0, the maximum SEQ and CYL are -0.81 and -0.14D, respectively. For decentration x,y=0,1, the maximum SEQ and CYL are -1.48 and -1.52, respectively. The changes in astigmatic refraction at the corneal plane are clinically significant for lens tilts up to 10 degrees for all values of decentration.Table 1. Second-order fit for SEQ and CYL. x = tilt angle in degrees.
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