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A. Langenbucher, N. Szentmáry, T. M. Eppig; Effect of Axis Misalignment in Toric Intraocular Lenses - Paraxial vs. Numerical Raytracing. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5614.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the effect of axis misalignment of toric intraocular lenses (TIOL) on the image performance and the refraction error at spectacle plane with numerical raytracing (NR) and to compare with the respective results of a simple paraxial raytracing (PR) scheme using 4x4 matrices.
The Liou-Brennan schematic model eye (4.5 mm pupil) was modified by making the anterior corneal surface astigmatic with an cylinder of 5 and 10 diopters (D). The respective keratometric radii were 7.387/8.192 mm for the 5 D astigmatism and 7.041/8.663 mm for the 10 D astigmatism (refractive index = 1.376). The crystalline lens was extracted and replaced by an appropriate posterior chamber TIOL with a backtoric lens (10 D spherical anterior surface, located at 4.9 mm from anterior corneal apex), which was rotated (misaligned in axis) from 0° to 10°. The TIOL power was calculated using PR or NR. Backward raytracing from the retinal plane to the spectacle plane yielded the wavefront error with NR or the error refraction using both, NR and PR.
Best optical performance at the focal plane was achieved with the 5D/10D corneal astigmatism with a 15.82+7.98/11.70+15.82 D (NR) or 16.73+7.00/13.12+14.00 D (PR). Strehl ratio decreased from 0.0933/0.0551 for 0° to 0.0074/0.0030 for 10° misalignment. Backward raytracing yielded a wavefront error (peak to valley) of 0.84/1.02 µm for 0° and 5.90/11.73 for 10°. Refraction error with NR was 0.01-0.00/0.05-0.07 D for 0° to 0.98-1.94/2.02-3.99 D for 10° misalignment, and with PR it was 0.0/0.0 for 0° to 0.87-1.74 D/1.74-3.47 D for 10° misalignment.
For low corneal astigmatism the optical performance can be estimated with good approximation with the paraxial model even in case of misalignment of the cylinder axis. For high corneal astigmatism, especially in combination with misalignment of the implantation axis, numerical raytracing is obligatory to estimate the optical performance after TIOL implantation.
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