A new method for predicting the behaviour of multifocal IOLs has been developed. This methodology could be very useful for ophthalmologists, enabling them to know the potential effect of the implantation of an IOL in any type of eye prior to implantation.
In recent years, the correction of presbyopia has become one of the most relevant global challenges within the field of ophthalmology, with the increase in use of electronic devices having lead to an greater need to be able to focus at different distances all the time (and not only at near and far).
In an attempt to meet this demand, new designs of IOLs have been developed. Currently, there is a wide variety of surgical solutions for presbyopic patients, with the ability to provide good vision at different distances. One example of this is the multifocal IOL (MF IOL) that was developed with the aim of providing good vision at near and far distances – and even intermediate distances with some designs.
Bifocal IOLs improved far and near/intermediate vision by means of their optical addition, while trifocal IOLs represented a step forward by increasing the number of optical foci.1
Unfortunately, light scattering and an increased level of high-order aberrations (HOAs) are factors related to patient dissatisfaction following cataract surgery with MF IOL implantation.2 A key way of avoiding these effects is the control of HOAs. HOAs may increase the depth of focus of the eye, providing good functional intermediate and near vision, but may also significantly deteriorate quality of vision.3,4
For this reason, wavefront analysis has been widely used to detect the effects of lower- and HOAs and their contribution to optical quality, in both in-vitro and in-vivo settings, when implanting MF IOLs.5
In this context, it would be desirable to predict the performance of each type of MF IOL prior to implantation. It would also be useful to determine if analysis of the wavefront of the IOLs could aid in this prediction.
Our group has recently developed a methodology for predicting the optical performance of IOLs when implanted in a real eye. These studies-currently in a pre-validation phase-could prove useful, in the future, for gauging the potential effect of implanting a specific type of IOL in a specific patient.6
In the first step, our group designed an optical bench to measure the aberrometic profile of different types of IOLs. The system consisted of a diode collimated laser beam of 532 nm, a beam expander, a wet cell in which the IOL was submerged, a collimating lens and a Shack–Hartmann wavefront sensor.
The wet cell consisted of a chamber with transparent optical windows on its top and bottom encasing a lens solution (0.9% normal saline). The temperature of the cuvette with saline solution was 35ºC.
The IOL was placed on the bottom optical window of the wet cell and an XYZ translational stage was attached to the wet cell to align the IOL with the optical axis of the wavefront sensor. We obtained the spot field from the wavesensor (see Figure 1) and the aberrometric pattern from the Zernike polynomials.
Next, our group attempted to predict the effect of the IOL implantation on presbyopic patients. This was achieved by studying the through-focus modulation transfer function (MTF) for a spatial frequency (see Figure 2).
For instance, the curves obtained for a spatial frequency of 50 cycles/mm approximates the visual function assessment with an optotype used for measuring a decimal visual acuity of 0.50 (20/40 Snellen). The theoretical through-focus MTF of each specific IOL can be evaluated in eyes with different anatomical peculiarities prior to implantation.
To perform the simulations, the wavefront profile of each IOL was characterised using a Hartmann-Shack wavefront sensor with the IOL submerged into the cuvette. Once the wavefront aberration profile of the IOL was characterised, the phase transformation introduced by each IOL was calculated. After that, the topographic data of a specific eye was used for the simulations.
Finally, the IOL was introduced as a phase element in the eye model and the through-focus MTF for each IOL was simulated by ray tracing.
In the first study (‘In vitro aberrometric assessment of a multifocal intraocular lens and two extended depth of focus IOLs’),6 the aberrometric pattern of a refractive IOL (Mini Well, Sifi Medtech) and two extended depth-of-focus IOLs (Tecnis Symfony ZXR00, Johnson & Johnson and Lentis Mplus X LS-313 MF30, Oculentis) were evaluated. Three different pupil sizes were used for the comparison: 3, 4 and 4.7 mm.
In this study, we concluded that the refractive IOL induced negative primary and positive secondary spherical aberrations for all pupil sizes. The Tecnis Symfony ZXR00 aberrations seemed to be pupil dependent, not inducing negative primary spherical aberration for 3 mm but with a negative increasing value for higher pupil sizes.
Whereas HOAs for the refractive and Tecnis Symfony IOL were not clinically significant for all pupil sizes, the Lentis Mplus IOL showed, in all cases, higher levels of HOAs, becoming clinically relevant for pupils larger than 4 mm.
From this study, the following step was to develop a method for simulating the optical performance of an IOL before its implantation. For this purpose, the wavefront profile of each IOL characterised using the Hartmann-Shack wavefront in the first study was used.
According to this profile, the phase transformation introduced by each IOL was obtained. An eye model was built using ray-tracing software, offering the possibility of using real corneal topographic data for the simulations. Finally, the IOL was introduced as a phase element in the eye model and the through-focus MTF for each IOL was simulated by the ray tracing software.
In a preclinical experiment published in the Journal of Refractive Surgery (‘Simulation of the effect of different presbyopia-correcting intraocular lenses with eyes with previous laser refractive surgery’),7 the simulation of the through-focus modulation transfer function (MTF) was used for three different presbyopia-correcting IOLs in one eye with previous myopic laser in situ keratomileusis (LASIK) and another with hyperopic LASIK.
Our results suggest that the design of the MF IOL could condition the results in eyes with previous myopic or hyperopic LASIK. In fact, our study has found notable differences between the MF IOLs in providing acceptable near, intermediate and far foci, although these findings will need to be confirmed in future clinical studies involving more patients and IOLs.
In conclusion, a new method for predicting the behaviour of MF IOLs has been developed. This methodology could be very useful for ophthalmologists, enabling them to predict the potential effect of the implantation of an IOL in any type of eye prior to implantation. Thus, the surgeon would be able to more accurately plan and perform surgeries, thereby increasing the success rate of cataract surgery with presbyopia-correcting IOLs.
Vincente J. Camps, PhD
Dr Camps works at the Grupo de Ãptica y PercepciÃ³n Visual (GOPV), Department of Optics, Pharmacology and Anatomy, at the University of Alicante, Spain.
David P. PiÃ±ero, PhD
Dr PiÃ±ero works at the Grupo de Ãptica y PercepciÃ³n Visual (GOPV), Department of Optics, Pharmacology and Anatomy, at the University of Alicante, Spain.
1. Gatinel D, Pagnoulle C, Houbrechts Y, Gobin L. Design and qualification of a diffractive trifocal optical profile for intraocular lenses. Journal of cataract and refractive surgery. 2011;37:2,060-2,067.
2. de Vries NE, et al. Dissatisfaction after implantation of multifocal intraocular lenses. Journal of cataract and refractive surgery. 2011;37:859-865.
3.Bakaraju RC, Ehrmann K, Papas EB, Ho A. Depth-of-Focus and its Association with the Spherical Aberration Sign. A Ray-Tracing Analysis. Journal of optometry. 2010;3:51-59.
4. Legras R, Benard Y, Lopez-Gil N. Effect of coma and spherical aberration on depth-of-focus measured using adaptive optics and computationally blurred images. Journal of cataract and refractive surgery. 2012;38:458-469.
5. Alio JL, Plaza-Puche AB, PiÃ±ero DP. Rotationally asymmetric multifocal IOL implantation with and without capsular tension ring: refractive and visual outcomes and intraocular optical performance. J Refract Surg. 2012;28:253-258.
6. Camps VJ, et al. In vitro aberrometric assessment of a multifocal intraocular lens and two extended depth of focus IOLs. Journal of Ophthalomology. 2017.
7. Camps VJ, Miret JJ, Tolosa A, PiÃ±ero DP. Simulation of the effect of different presbyopia-correcting intraocular lenses with eyes with previous laser refractive surgery. Journal of Refractive Surgery. Accepted.