Purchase this article with an account.
E. J. Sarver, T. D. Padrick, M. T. Hall; Astigmatic Effect of Toric IOL Tilt and Decentration. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5613.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To evaluate the theoretical effect of a toric IOL's decentration and tilt on astigmatic correction at the corneal plane.
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 toric IOL was decentered and/or tilted. The anterior cornea was modeled as a biconic surface with K = -0.25 in the principal axes and power 46.6D @ 180 and 48.1D @ 90. The posterior cornea was modeled as a conic surface with radius 6.5 mm and conic constant K = -0.25. The toric IOL was modeled with equal toric surfaces with power 20D + 2D x 90. The corneal thickness was 0.55 mm, the anterior IOL surface was located at 4.6 mm behind the anterior corneal surface, and the axial length of the eye was 23.6 mm. The indices of refraction for the cornea, aqueous, and IOL were 1.376, 1.336, and 1.42, respectively. For all calculations, the pupil diameter was 6.0 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. In all cases, the minimum R2 was 0.9996. For no decentration, the maximum SEQ and CYL are -0.47 and -0.44 D, respectively. For decentration x,y=1,0, the maximum SEQ and CYL are -0.96 and 0.06D, respectively. For decentration x,y=0,1, the maximum SEQ and CYL are -1.55 and -1.46, 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.
Tilt of a 2D toric IOL up 10 degrees will lead to clinically significant changes in astigmatic refraction in the corneal plane. The effects are increased if decentration of the IOL is also present. For a given decentration, the provided second-order equations as a function of tilt, predict the change in astigmatic refraction with high precision.
This PDF is available to Subscribers Only