Tomography-Based Customized IOL Calculation Model
Medical Optics at the Institute of Medical Physics, University of Erlangen-Nuremberg, Erlangen, Germany. Current eye research
(Impact Factor: 1.64).
06/2011; 36(6):579-89. DOI: 10.3109/02713683.2011.566978
To provide a mathematical calculation scheme for customized intraocular lens (IOL) design based on high resolution anterior segment optical coherence tomography (AS-OCT) of anterior eye segment and axial length data.
We use the corneal and anterior segment data from the high resolution AS-OCT and the axial length data from the IOLMaster to create a pseudophakic eye model. An inverse calculation algorithm for the IOL back surface optimization is introduced. We employ free form surface representation (bi-cubic spline) for the corneal and IOL surface. The merit of this strategy is demonstrated by comparing with a standard spherical model and quadratic function. Four models are calculated: (1) quadratic cornea + quadratic IOL; (2) spline cornea + quadratic IOL; (3) spline cornea + spline IOL; and (4) spherical cornea + spherical IOL. The IOL optimization process for the pseudophakic eye is performed by numerical ray-tracing method within a 6-mm zone. The spot diagram on the fovea (forward ray-tracing) and wavefront at the spectacle plane (backward ray-tracing) are compared for different models respectively.
The models with quadratic (1) or spline (3) surface representation showed superior image performance than the spherical model 4. The residual wavefront errors (peak to valley) of models 1, 2, and 3 are below one micron scale. Model 4 showed max wavefront error of about 15 µm peak to valley. However, the combination of quadratic best fit IOL with the free form cornea (model 2) showed one magnitude smaller wavefront error than the spherical representation of both surfaces (model 3). This results from higher order terms in cornea height profile.
A four-surface eye model using a numerical ray-tracing method is proposed for customized IOL calculation. High resolution OCT data can be used as a sufficient base for a customized IOL characterization.
Available from: Timo Eppig
- "Therefore, we adopted the Liou-Brennan model eye used in our previous studies    including the decentered pupil. There are various model eyes available for optical simulation which can be customized to individual biometric properties . A thin diaphragm (aperture stop) was placed in a distance of 200 microns behind the corneal surface (virtual flap generation for KAMRA implantation). "
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. To evaluate the effect of the KAMRA corneal inlay on the retinal image brightness in the peripheral visual field.
. A KAMRA inlay was “implanted” into a theoretical eye model in a corneal depth of 200 microns. Corneal radius was varied to a steep, normal, and flat (7.37, 7.77, and 8.17 mm) version keeping the proportion of anterior to posterior radius constant. Pupil size was varied from 2.0 to 5.0 mm. Image brightness was determined for field angles from −70° to 70° with and without KAMRA and proportion of light attenuation was recorded.
. In our parameter space, the attenuation in brightness ranges in between 0 and 60%. The attenuation in brightness is not affected by corneal shape. For large field angles where the incident ray bundle is passing through the peripheral cornea, brightness is not affected. For combinations of small pupil sizes (2.0 and 2.5 mm) and field angles of 20–40°, up to 60% of light may be blocked with the KAMRA.
. For combinations of pupil sizes and field angles, the attenuation of image brightness reaches levels up to 60%. Our theoretical findings have to be clinically validated with detailed investigation of this vignetting effect.
Available from: Achim Langenbucher
- "They are referred to as " higher-order samples. " The third group holds two customized lenses derived from biometric patient data   . A general quadric surface is described by the following equation: í µí± (í µí±¥, í µí±¦, í µí± §) = í µí°´í µí±¥ 2 + í µí°µí µí±¦ 2 + í µí° ¶í µí± § 2 + 2í µí°·í µí±¥í µí±¦ + 2í µí°¸í µí±¦í µí± § + 2í µí°¹í µí±¥í µí± § + 2í µí°ºí µí±¥ + 2í µí°»í µí±¦ + 2í µí°¼í µí± § + í µí°¾ = 0. (2) "
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In order to establish inspection routines for individual intraocular lenses (IOLs), their surfaces have to be measured separately. Currently available measurement devices lack this functionality. The purpose of this study is to evaluate a new topography measurement device based on wavefront analysis for measuring individual regular and freeform IOL surfaces, the "WaveMaster Reflex UV" (Trioptics, Wedel, Germany).
Measurements were performed on IOLs with increasingly complex surface geometries: spherical surfaces, surfaces modelled by higher-order Zernike terms, and freeform surfaces from biometrical patient data. Two independent parameters were measured: the sample's radius of curvature (ROC) and its residual (difference of sample topography and its best-fit sphere). We used a quantitative analysis method by calculating the residuals' root-mean-square (RMS) and peak-to-Valley (P2V) values.
The sample's best-fit ROC differences increased with the sample's complexity. The sample's differences of RMS values were 80 nm for spherical surfaces, 97 nm for higher-order samples, and 21 nm for freeform surfaces. Graphical representations of both measurement and design topographies were recorded and compared.
The measurements of spherical surfaces expectedly resulted in better values than those of freeform surfaces. Overall, the wavefront analysing method proves to be an effective method for evaluating individual IOL surfaces.
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ABSTRACT: The purpose of the present study was to demonstrate a method of how to calculate intraocular lenses with a customized asphericity and how to apply this strategy to clinical examples in cases where biometric data of the cornea (front and back surface topography) as well as distances in the eye are known.
(1) we demonstrated an algebraic method for tracing a bundle of rays through a schematic eye containing surfaces which can be represented by 2nd order surfaces (quadric surfaces), and (2) we introduced a strategy for customization of the lens' back surface for compensating the optical path length differences of the rays from object to image in terms of a wave front correction while predefining the lens front surface.
The presented method was applied to three working examples: example 1 referred to a centered optical system with a spherical cornea (front and back surfaces) and a predefined spherical lens front surface, example 2 referred to a centered optical system with aspherical surfaces for the corneal front and back surfaces and a predefined spherical lens front surface, and example 3 referrred to a non-centered system with a decentered aspherical cornea (front and back surface), and a predefined spherical lens front surface. The parameterized ray intersection points with the lens back surface were optimized in terms of equalizing the ray path lengths and a quadric surface was fitted to these ray intersection points to characterize the customized lens. The fitting error, ray spot diagram, and the optical path length of the rays are provided.
This simple calculation strategy may be the first step in developing individual aspherical lenses, which have the potential to fully compensate spherical aberrations based on individual measures of the eye.
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