Analysis of the effect of intraocular lens tilt and decentration on induced astigmatism by ray tracing on the model eye

Purpose. Determination of the influence of the intraocular lens (IOL) inclination and decentration on the magnitude of induced astigmatism in a theoretical study using software on an eye model. Material and methods. An eye model with the specified parameters was performed to carry out the calculation. Modulation of the change in the wavefront was carried out using the Zemax program (LLC, USA). In the model eye, the probability of induction of astigmatism was analyzed depending on the change in the position of the aspherical IOL Acrysof IQ 16.0, 22.0 and 24.0 diopters and the toric model Acrysof IQ Toric SN6AT3, SN6AT4 and SN6AT5. The angle of inclination of the IOL relative to the horizontal plane was examined from 0° to 10° with a step of 1°, decentration from 0.1 to 1.5 mm with a step of 0.1 mm. At each IOL position, aberration data of the 2nd order Z (2, 2) were recorded. The obtained results were used to calculate the critical tilt angle and decentration. Results. The magnitude of induced astigmatism increased with the rise of the degree of inclination and decentration, corresponding to an increase in the optical power of the IOL. Accordingly, critically significant were the values of 3.0° inclination for the aspherical IOL with an optical power of 22.0 and 22.0 diopters and 3.5° for 16.0 diopters, decentration – 0.7 mm and 1.0 mm. The inclination of the TIOL relative to the horizontal plane revealed the induction of the magnitude of inverse astigmatism, according to an increase in the inclination angle of 0.3 diopters at a slope of 3–4°. Conclusions. The tilt and decentration of the aspherical and TIOL patterns induce astigmatism depending on the increase in IOL power. The inclination of the TIOL in relation to the horizontal plane set at 90 ° promotes the induction of astigmatism. Key words: astigmatism, toric intraocular lenses, tilt and decentration of the intraocular lens, ray tracing

Effect of Intraocular Lens Tilt and Decentration on Visual Acuity, Dysphotopsia and Wavefront Aberrations

Tilt and decentration of intraocular lenses (IOL) may occur secondary to a complicated cataract surgery or following an uneventful phacoemulsification. Although up to 2–3° tilt and a 0.2–0.3 mm decentration are common and clinically unnoticed for any design of IOL, larger extent of tilt and decentration has a negative impact on the optical performance and subsequently, the patients’ satisfaction. This negative impact does not affect various types of IOLs equally. In this paper we review the methods of measuring IOL tilt and decentration and focus on the effect of IOL tilt and decentration on visual function, in particular visual acuity, dysphotopsia, and wavefront aberrations. Our review found that the methods to measure the IOL displacement have significantly evolved and the available studies have employed different methods in their measurement, while comparability of these methods is questionable. There has been no universal reference point and axis to measure the IOL displacement between different studies. A remarkably high variety and brands of IOLs are used in various studies and occasionally, opposite results are noticed when two different brands of a same design were compared against another IOL design in two studies. We conclude that <5° of inferotemporal tilt is common in both crystalline lenses and IOLs with a correlation between pre- and postoperative lens tilt. IOL tilt has been noticed more frequently with scleral fixated compared with in-the-bag IOLs. IOL decentration has a greater impact than tilt on reduction of visual acuity. There was no correlation between IOL tilt and decentration and dysphotopsia. The advantages of aspheric IOLs are lost when decentration is >0.5 mm. The effect of IOL displacement on visual function is more pronounced in aberration correcting IOLs compared to spherical and standard non-aberration correcting aspherical IOLs and in multifocal versus monofocal IOLs. Internal coma has been frequently associated with IOL tilt and decentration, and this increases with pupil size. There is no correlation between spherical aberration and IOL tilt or decentration. Although IOL tilt produces significant impact on visual outcome in toric IOLs, these lenses are more sensitive to rotation compared to tilt.