Toric intraocular lenses for compensation of corneal astigmatism

Background: Besides arcuate or straight transverse keratotomies, toric intraocular lenses are of increasing popularity to enhance the visual function in cases of lens opacification for the correction of corneal astigmatism and the following ocular surgery such as penetrating keratoplasty. The purpose of this study was to present a generalized scheme for the calculation of toric intraocular lenses and to demonstrate its potential on a clinical example in a step-by-step approach. Methods: After providing some helpful approximations for the clinicians, i.e., for a conversion of radii differences to corneal astigmatism, the calculation scheme using vergence transformation in a paraxial space and the dualism of the standard and component notation for the description of spherocylindrical vergences and spherocylindrical refractive surfaces is described. Results: In a clinical example, a toric intraocular lens is calculated step-by-step using the above-mentioned calculation scheme. The methodology is designed in a matrix structure for the direct implementation in a computer language. In a second step, a toric lens similar to the calculated lens in the manufacturing grid is selected and inserted with a small angle of rotation from the ideal implantation axis to demonstrate the effect on postoperative refraction. Conclusions: The calculation scheme allows the determination of toric intraocular lenses with an astigmatic cornea and enables us to achieve a spherocylindrical target refraction with cylinder axis at random. Furthermore, the postoperative refraction at the spectacle or corneal plane can be simulated by inserting any toric intraocular lens oriented in any axis. The concept can be easily generalized to 'thick' toric intraocular lenses if the geometrical data and the refraction index of the material are known.