Computerized Calculation Scheme for Bitoric Eikonic Intraocular Lenses

Abstract

Abstract: : Purpose: To describe an iteration strategy for tracing an axial pencil of rays through the 'optical system eye' containing astigmatic refractive surfaces with their axes at random to calculate a thick bitoric lens implant which eliminates image distortion. The capabilities of this computing scheme are demonstrated in a clinical example due to high corneal astigmatism. Methods: We initialized the system with a spherical front surface of the thick lens implant and corrected the eye for an arbitrary spherocylindrical target refraction with a toric back surface. From two characteristic object points at distance the retinal image of a circular object (incident slope angle of 10 prism diopters) was derived and an ellipse was fitted to this image. From the orientation of the image a cylinder lens was added to the front surface increasing the magnification of the semi-minor meridian of the ellipse and reducing image distortion. Then, a new correcting toric back surface was calculated to this new front surface. This process was iterated until the semi-major-to-semi-minor meridian ratio (MMR) was minimized to eliminate image distortion. Results: In our example corneal power was 42.5+3.5 D /15° and the thick lens implant (refractive index 1.42, central thickness 1.5 mm) was predicted to be positioned at 4.6 mm behind the corneal vertex. Intending emmetropia / a target refraction of -0.75-0.5 D/90° we started from a 10.0 D spherical front surface of the lens (back surface power 6.53+5.25 D/105° / 7.33+5.86 D/103.2°) and optimized to a bitoric eikonic lens with a front surface power of 10.0+14.75 D/105° / 10.0+20.25 D/100° and a back surface power of -5.26+11.81 D/105° / 10.01+17.37 D/8.9° with a retinal image size (radius) of 1.67 / 1.65 mm (MMR <1.01). Conclusions: We presented a mathematical straight-forward computer-based strategy for calculation of thick bitoric eikonic lens implants using an iteration algorithm. This may be of clinical relevance for elimination of image distortion (monocular) and aniseikonia (binocular) after implantation of toric intraocular lenses for correction of corneal astigmatism.

Keywords: astigmatism • cataract • computational modeling