Visual Simulation: application to monofocal intraocular lens analysis

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1 ARTICLE Visual Simulation: application to monofocal intraocular lens analysis Alberto Domínguez Vicent, OD; Cari Pérez-Vives, MSc; Lurdes Belda-Salmerón, MSc; César Albarrán-Diego, MSc; Santiago García-Lázaro, PhD PURPOSE: Compare two monofocal intraocular lenses (IOLs) with different designs. METHOD: An adaptive optics visual simulator (crx1, Imagine Eyes, France) was used to simulate the aberration pattern of two IOLs, a spherical monofocal IOL (AcrySof SN60AT IOL, Alcon Laboratories, USA) and an aspheric monofocal IOL (AcrySof IQ SN60WF IOL Alcon Laboratories, USA). The Visual Acuity (VA) was measured at high- mediumand low- contrast when each IOL was centered, decentered and tilted different degrees. Depth-of-focus of both lenses was evaluated when each IOL was centered. The Modulation Transfer Function (MTF) and the Point Spread Function (PSF) were computed for all conditions. All measures were taken for 5 mm pupil diameter. RESULTS: There were no statistical significant differences in VA between both IOLs for centered and decentered 0.2 mm positions (P > 0.05). For these positions, both lenses showed good VA (about 20/20) at high contrast. But at 50% and 10% of contrast VA values were worse (up to 20/40). At 0.4 mm of decentering and both tilts, the SN60AT IOL has better VA values than SN60WF IOL showing statistically significant differences (P<0.05). The values of depth-of-focus were 0.25 and 0.75 dioptres (D) for aspheric and spherical IOLs, respectively. MTFs and PSFs decrease when both IOLs were decentered and tilted. The spherical IOL shows worse PSFs than aspheric IOL in centered position. CONCLUSION: It seems that spherical design is more robust and provides more depth of focus than the aspheric model. Although, aspheric model induces better optical quality in centered position. J Emmetropia 2011; 2: INTRODUCTION It is necessary to introduce some optical concepts before discuss about the different applications of adaptive optics (AO) in human vision. Wave aberration 1 is defined as the difference between the perfect (spherical) and the actual wavefront (WF) for each point over the pupil. Figure 1 shows this concept schematically. AO may be defined as an optical system which adapts to compensate aberrations which are induced by the medium between the object and the image. Submitted: 7/27/2011 Revised: 8/27/2011 Accepted: 8/29/2011 From the Optometry Research Group (GIO), Department of Optics. University of Valencia. Spain. Acknowledgements and Disclosure: This research was supported in part by Ministerio de Ciencia e Innovación Research Grant #SAF E#. The authors have no proprietary interest in any of the products mentioned in this article. Address: Alberto Domínguez Vicent. Department of Optics University of Valencia. C/ Dr Moliner, Burjassot. Spain. aldovi@alumni.uv.es Figure 1. Scheme of wavefront definition, showing the reference spherical wavefront (black dot line), the aberrated wavefront (red line) and the wave aberration SECOIR Sociedad Española de Cirugía Ocular Implanto-Refractiva ISSN:

2 144 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS AO is applied to the human eye because it has optical aberrations that reduce the retinal image quality. The main objective to applied AO principle to the human eye is to improve retinal image quality and retinal image visualization (retinal imaging), being the first to achieve better visual acuity (VA) and the second one for observational purposes. Improving retinal image quality To correct ocular aberrations considering AO has been recently applied to 2 contact lens (CL) and intraocular lenses (IOLs) designs or corneal refractive surgery ablation algorithms. The main idea that these solutions use to correct the whole eye aberrations is to make a CL, IOL or an anterior surface of the cornea which induces an anti-aberration of the whole eye. Consequently a perfect image on the retina, which is only affected by diffraction, may be obtained. Figure 2 shows a scheme of this concept, applied specifically to a CL 3. The others devices follow the same correction principle explained above. On the other hand, these devices must be static, centered and customized for each patient to obtain the best retinal image quality. If they are not centered, static or customized, they will induce new aberrations and the retinal image quality will deteriorates. This is the main disadvantage for ophthalmic devices. Soft CLs may be considered as the first choice for optical aberration correction because they have less movement than rigid gas-permeable CLs (RGPCLs). Scleral RGPCLs are more stable than the RGPCLs and may be also considered for this purpose. Then, soft CLs and Scleral RGPCLs must be considered as the first choice for optical aberration correction. The advantage of customized CL, in comparison with refractive surgery, is its reversibility because the CL can be removed if the visual performance is not good. One disadvantage is that CL can rotate, translate and flex with each blink, and the retinal image quality cannot be good. Figure 2. Principle of aberration correction using a contact lens. The ocular aberrations, as well as the aberration pattern induced by the contact lens are shown above. In relation to refractive surgery 1, the main objective is to customize the anterior surface of the cornea to compensate ocular aberrations with laser Excimer ablation. To obtain this, surgeons use WF-guided ablation. This technique consists on obtaining the ocular aberrations and to ablate the anterior surface of the cornea using the WF data. The advantage of refractive surgery is that customized treatment induces less high order aberrations (HOAs) than conventional treatments 4,5. The main disadvantage of this optical aberration correction is that the effect cannot be removed if patient s visual performance is not good. Additionally other factors tend to limit the success of WF-guided treatments and will need to be overcome to optimize clinical results (also considered for CLs and IOLs). These factors are: Age-related changes in HOAs, pupil size accommodation, ocular surface, retinal limits, neuroplasticity, postoperative issues: wound healing and biomechanical effects. Finally, we know that society is aging and the number of cataract surgeries is increasing. Currently, surgeons have different monofocal IOL 1 designs available in the market to be considered for replacing the crystalline lens, as an aspheric and a spherical IOL. The former design is the spherical IOL, but this one induces a residual spherical aberration (SA) and, theoretically, the retinal image quality deteriorates. The aspheric design tries to compensate the corneal SA to improve the retinal image quality. Theoretically it can be concluded that an aspheric monofocal IOL design induces better VA than spherical monofocal IOL design if exists balance between corneal and IOL aberrations. However, clinical studies did not find VA differences between aspheric and spherical designs Only two studies found 11,12 that an aspheric design has better VA than the spherical one. Discrepancies between theoretical and clinical studies can be: clinical studies do not consider the IOL position into the eye, these studies do not check if exist a balance between corneal and IOL aberrations, and there are different philosophies of IOL manufacturers exist to correct full or part of the corneal SA 10. On the other hand, we should considered that the potential benefits of aspheric IOLs are limited by inaccurate or absent preoperative measurement of the ocular parameters necessary for IOL power calculation, inaccurate manufacturing and inability to locate the IOL in the correct plane. However, in centered position, the optical and visual performance of aspheric IOLs is, even in the worst cases, equal to or better than that with spherical IOLs. But independently of the image quality that the IOL induces, there is an external problem which depends of the surgeon. This is an error placing the IOL in the capsular bag because surgeons can tilt or decenter the IOL. Consequently, surgeons should know what IOL design is most robust when it is placed inside the eye.

3 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS 145 Considering the differences reported above on this topic, we have done a pilot study to analyse two monofocal IOL design. One with an aspheric design, which partially compensates corneal SA, and the other one with a spherical design which does not compensate corneal SA. We used the visual simulator to induce the aberration pattern of each IOL + cornea, previously obtained, to measure the VA at different contrast under different conditions, when the lenses were centered, decentered and tilted. Depth-of-focus for the centered condition of both lenses was also measured. PATIENTS AND METHODS Subjects Seven individuals, aged between 21 and 30 years and experienced in psychophysical experiments were included in this study. Spherical refractive errors ranged between 2.00 and dioptres (D) with astigmatism < 0.50 D. They had clear intraocular media and no known ocular pathology. The tenets of the Declaration of Helsinki were followed. Informed consent was obtained from each participant after verbal and written explanation of the nature and possible consequences of the study. Adaptive Optics Visual Simulator A visual simulator 1,13-16 is a device which uses the principle of AO. This device can measure, simulate, correct and manipulate any aberration pattern. It has three basic components: WF sensor, control computer and WF corrector-simulator. On the following section we describe briefly these elements relating them with the components of the crx1 AO visual simulator (Imagine Eyes, Orsay, France) used during the experimental procedure. Figure 3 shows a scheme of this equipment. Wavefront sensor The WF sensor measures the optical aberration at the pupil plane of the eye. A recent review of Cerviño et al. 17 discussed about the advantages and disadvantages of different clinical WF sensors available in the market. Clinical aberrometers may be based on the Tscherning s aberroscope, the cross-cylinder technique, the spatially resolved refractometer, the laser ray tracing technique and the Shack-Hartman (S-H) sensor. We are going to focus on the S-H WF sensor because it is the WF sensor used in the visual simulator. The basic components of S-H WF sensor 1,18 are the lens let array, a monochromatic light source and a charge-coupled device (CCD) detector 1. The lens let array consists of two-dimensional array of 1024 lenses, Figure 3. Visual simulator scheme of the crx1 system. all with the same diameter and the same focal length. The typical diameter range of these lenses is about 100 to 600 µm. The typical focal length range is from 30 nm to a few millimetres. The light source that is used to measure aberrations uses a wavelength of 850 nm. The CCD detector is a device which is placed in the focal plane of the lens let array. Its function is to record the spot array pattern to calculate the WF. This WF sensor has some advantages and disadvantages 1. The advantages consider that the measurement is objective, automatic and quick; it can operate in real time and it has high spatial resolution. Disadvantages of this WF are that it has small dynamic range (it is limited by spacing and the focal length of the lens let) and that the spot pattern can be blurred. This defocus can cause an error in estimating the centroid of the spot. The measure procedure of the aberrations for the whole eye consists on sending into the eye a light from a laser. This is focused by the optics of the eye to a point on the retina. This light is reflected on the retina and emerges through the pupil as an aberrated WF. This aberration pattern travels from the eye through a lens let array, which is conjugate with the pupil of the eye. As the lens let array is in a pupil conjugate plane, the WF shape at the lens let plane is identical to the shape at the pupil of the eye. Finally, the lens let array forms an array of spots image on a CCD detector. This explanation is summarized on figure 4. This figure shows a schematic diagram of the AO system during the WF measurement. The red line shows the way that the light uses to do the measure. On the other hand, the same acquisition provides a calculation of the WF refraction which is compatible with optical corrections. The control computer This component 1 converts the raw output of the WF sensor into voltage commands that are sent to the WF corrector. Control systems are broadly classified in

4 146 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS Figure 4. Schematic diagram of the adaptive optics system during wavefront measurement. two group, open-loop systems and closed-loop systems. The closed-loop system is used in this visual simulator where the WF sensor measures the wave aberration after it is corrected by the WF corrector. Figure 5 shows a block diagram of closed-loop AO system for vision science. This control computer is faster and more accurate than open-loop control system. It increases the system accuracy and reduces the sensitivity of the ratio of the output to input due to variations in system characteristics. The disadvantages of closed-loop control are its increased complexity and cost. The wavefront corrector The WF corrector 1 compensates the aberration pattern generating a surface shape that is ideally conjugate to the aberration profile. In the visual simulator, the WF corrector has another application. This is inducing the aberration pattern that we need. This device is placed in the conjugated plane for both the pupil eye and the WF sensor. The WF correctors which are used in AO vision system to correct the ocular aberrations may be macroscopic discrete actuator deformable mirrors, bimorph mirrors and liquid crystal spatial light modulators. The present visual simulator uses the macroscopic discrete actuator deformable mirror 19. It consists of a continuous mirror surface whose profile is controlled Figure 6. Schematic diagram of the adaptive optic system during wavefront static correction/induction. by an underlying mini-magnet array. These mini-magnets electromagnetically push and pull on the surface, transforming it into a desired shape. Specially this WF corrector has 52 independent mini-magnets and can correct aberrations up to 5 th order. Figures 6 and 7 show a brief scheme about the working of this equipment. Figure 6 shows a diagram of the visual simulator during a correction of any aberration pattern and figure 7 shows a diagram of the visual simulator during a simulation of any aberration pattern. Other components of the visual simulator crx1 The crx1 has other components 19 such as Badal system, a micro-display and an artificial control pupil size. The Badal system controls the vergence of the stimulus. This device allows us to compensate the subject s refractive error or study the change of the ocular aberrations during accommodation (see figure 8 for an example). The micro-display has pixels and it works with white and black light. Consequently we only can show images which have these colours. Finally, the artificial control pupil size controls the aperture of the system. This device is optically conju- Figure 5. Closed-loop system for correcting wave aberration. Figure 7. Schematic diagram of the adaptive optic system during visual simulation.

5 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS 147 Table.1. Zernikes values of the intraocular lens centered plus cornea Zernike Coefficients Values (µm) SN60AT * SN60WF +CORNEA +CORNEA Figure 8. Variation of the wavefront pattern during accommodation. Left image shows the wavefront without accommodation and right image shows the wavefront with 5 dioptres of accommodation (5 mm pupil). gated with the eye-pupil plane and its aperture can be artificially adjusted selecting the diameter of an internal aperture. Then we can control the subject s pupil diameter if this is longer than the pupil diameter that we need for any experiment. Figure 9. Maltese cross. z (3, 3) z (3, 1) z (3, 1) z (3, 3) z (4, 4) z (4, 2) z (4, 0) z (4, 2) z (4, 4) * Spherical intraocular lens; Aspheric intraocular lens. Experimental Procedures Approximately 30 min before the experimental measurements, three drops of cyclopentolate hydrochloride 0.5% were instilled to paralyze the subject s accommodation and achieve pupil dilation up to 5 mm. While patient was dilating the mirror of the visual simulator was reshaped to flat shape using the commercially available program (HASO, Imagine Eyes, France). During all measurement procedure, we put 5 mm on the artificial control pupil size to ensure that all measures had the same pupil diameter. Once this process has been carried out, we used the commercially available program (Irx3, Imagine Eyes, France) program to measure subject s ocular aberrations while he was fixing to the Maltese cross (fig. 9). After that, we used the commercially available program (SVAO, Imagine Eyes, France) program to compensate the subject s aberration pattern and induce the aberration pattern which we needed. Before VA measurement we compensated the defocus that aberration pattern had induced using the Badal system. After that we measured the depth-of-focus when the IOL was centered. When these procedures were carried out, we measured the VA for 5 IOL positions (centered, decentered 0.2 mm and 0.4 mm and tilted 2 and 4 ). For each position we measured the VA for three contrast values: 100%, 50 % and 10%. All of these procedures must be done with all lenses positions. In addition, modulation transfer functions (MTFs) and point spread functions (PSFs) for both IOL + cornea were computed for all conditions (centered, decentered 0.2 mm and 0.4 mm and tilted 2 and 4 ). Corneal and Intraocular lenses Zernike s values The IOLs which we used were the monofocal AcrySof IQ SN60WF IOL (Alcon Laboratories, USA) which has an aspheric design; and the monofocal AcrySof SN60AT IOL, (Alcon Laboratories, USA) which has a spherical design. Figure 10 shows a frontal and a cross-section view of both lenses. For visual simulation we considered the Zernike s values of the IOLs 20 and the cornea which were obtained previously. Corneal Zernike s values used were obtained by Wang et al. 21 for a normal population. Table 1 shows the whole eye Zernike s values considering both corneal and IOL aberrations (defocus and astigmatism were cancelled). These values were obtained for a 5 mm pupil diameter. Visual Acuity Measurements To measure VA we have used the Freiburg Acuity Test (FrACT) 22, where we used the Landolt C at different contrasts. Subjects were instructed to recognize the orientation of the aperture of the C (forced-choice test). To do that, the subject had a numeric keypad and he had to press the number which corresponds with the orientation of the aperture.

6 148 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS Statistical analysis Data analysis was performed using SPSS for Windows software (version 14.0, SPSS, Inc.). Normality was checked using the Kolmogorov- Smirnov test, and outcomes were compared using t tests. Differences were considered statistically significant when the P value was less than RESULTS Tables 2 to 6 and figure 11 show the experimental results. Each table includes the VA values for the same IOL position at different contrasts (100% 50% and 10%). The contents of each table consider logmar VA values, its standard deviation (sd), the VA interval and the p-value. Figure 11 shows the depth-of-focus for both lenses. Table 2 includes the VA values obtained when both IOLs were centered. Here we can see that SN60WF IOL and SN60AT IOL provide good VA values at high contrast (about 20/20). However, at 50% and 10% of contrast VA values were worse (up to 20/40). When both lenses were decentered 0.2 mm (table 3), VA values were also good at 100% of contrast. Similarly to the centered situation, at 50% and 10% of contrast the VA values were reduced. At 0.4 mm of decentration (table 4) the VA values for the SN60WF IOL were considerably reduced (about 2 lines of VA) in relation Figure 11. Comparative of the depth-of-focus curve of the intraocular lenses used in this study. Y-label shows the visual acuity values in decimal scale. X-label shows the vergence of the stimulus in dioptres. Error bars are omitted for clarity. to the previous situation for all contrasts. In contrast, the SN60AT IOL showed better outcomes for all contrasts, which were significantly better in relation to the aspheric design (P<0.05). Table 5 shows the VA values when both IOLs were tilted 2. For the SN60AT IOL the VA values were good at 100% and 50% of contrasts. At 10% of contrast the VA value was highly deteriorated (about 20/50). For the SN60WF IOL at all contrasts the VA values were significantly worse than the spherical model (P<0.05). At 4 of tilt (table 6) both lenses provide good VA at 100%, being reduced at 50% and 10% of contrast. The spherical model showed statistically significant better VA in relation to the aspheric model (P<0.05). Table 2. Visual acuity values expressed in logmar scale at three different contrasts when the lenses are centered VA* 100 % VA 50 % VA 10 % SN60WF SN60AT SN60WF SN60AT SN60WF SN60AT Mean±sd 0.050± ± ± ± ± ±0.110 Range [ ] [ ] [ ] [ ] [ ] [ ] P value * Visual acuity; Aspheric intraocular lens; Spherical intraocular lens; Standard deviation. Table 3. Visual acuity values expressed in logmar scale at three different contrasts when the lenses are decentered 0.2 mm VA* 100 % VA 50 % VA 10 % SN60WF SN60AT SN60WF SN60AT SN60WF SN60AT mean±sd 0.030± ± ± ± ± ±0.100 Range [ ] [ ] [ ] [ ] [ ] [ ] P value * Visual acuity; Aspheric intraocular lens; Spherical intraocular lens; Standard deviation.

7 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS 149 Figure 11 shows the depth-of-focus of both IOLs. The SN60AT IOL showed the best VA value at 0.00 D of vergence. For ±0.50 D of vergence we can appreciate that this lens maintains good and comparable VA than the value found at 0.00 D of vergence. At ±1.00 D of vergence, the VA values decrease significantly (about 4 lines of VA). In relation to the aspheric model, the SN60WF IOL showed its best VA also at 0.00 D. When defocus was introduced the lens shows a similar pattern which was found for the spherical model. Differences statistically significant were not found between both IOLs in any of the defocus degrees studied (P>0.05). Figures show the MTFs and the PSFs for both IOLs. A descriptive analysis was done. In relation Figure 12. Variation of SN60AT intraocular lens (IOL) modulation transfer function (MTF) for all IOL positions studied. X label shows the frequencies in cycles per degree. Y label shows the MTF radial. Table 4. Visual acuity values expressed in logmar scale at three different contrasts when the lenses are decentered 0.4 mm VA* 100 % VA 50 % VA 10 % SN60WF SN60AT SN60WF SN60AT SN60WF SN60AT mean±sd 0.120± ± ± ± ± ±0.120 Range [ ] [ ] [ ] [ ] [ ] [ ] P value * Visual acuity; Aspheric intraocular lens; Spherical intraocular lens; Standard deviation; Differences statistically significant (P<0.05). Table 5. Visual acuity values expressed in logmar scale at three different contrasts when the lenses are tilted 2 VA* 100 % VA 50 % VA 10 % SN60WF SN60AT SN60WF SN60AT SN60WF SN60AT mean±sd 0.050± ± ± ± ± ±0.080 Range [ ] [ ] [ ] [ ] [ ] [ ] P value * Visual acuity; Aspheric intraocular lens; Spherical intraocular lens; Standard deviation; Differences statistically significant (P<0.05). Table 6. Visual acuity values expressed in logmar scale at three different contrasts when the lenses are tilted 4 VA* 100 % VA 50 % VA 10 % SN60WF SN60AT SN60WF SN60AT SN60WF SN60AT mean±sd 0.070± ± ± ± ± ±0.080 Range [ ] [ ] [ ] [ ] [ ] [ ] P value * Visual acuity; Aspheric intraocular lens; Spherical intraocular lens; Standard deviation; Differences statistically significant (P<0.05).

8 150 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS Figure 13. Variation of SN60WF intraocular lens (IOL) modulation transfer function (MTF) for all IOL positions studied. X label shows the frequencies in cycles per degree. Y label shows the MTF radial. Figure 14. Variation of the point spread function of the two lenses studied under 5 conditions studied. Left image (ideal) only affected by diffraction. to the spherical model, SN60AT IOL, the MTF is reduced in relation to the diffraction-limited and decreases when decentration and tilt were provoked. A similar pattern was found for the aspheric model, the SN60WF IOL. In relation to spherical design, SN60AT IOL, the PSFs are worse than diffraction-limited for all positions, centered, decentered and tilted. However, for SN60WF IOL, aspheric design, the PSF is similar to diffraction-limited in centered position. Though, for both decenters and tilts the PSFs are worse than centered position. DISCUSSION In relation to VA, we may observe that for centered and decentered 0.2 mm situations, both IOLs provide similar VA values. For other situations of the IOLs (high decenter and both tilts), the SN60AT IOL (spherical design) provides better VA than the SN60WF IOL (aspheric design). Considering these outcomes, it seems that the spherical design is more robust than the aspheric model for decentering and tilting. These values agree with some theoretical and clinical papers which compare both IOLs designs. On the theoretical paper, Dietze and Cox 23, with model-eye simulation, found that spherical IOLs perform more robust than aspheric IOL when it was displaced. This result agrees with the VA values obtained here. It can be observed from tables 2-6 that the VA values for the spherical design are similar at the same contrast and different IOL position. Figure 14 correlates with this too. Here, the PSFs of the spherical IOL are more constant than the PSFs of the aspheric IOLs at different situations. In a clinical paper, Rocha et al. 9,24 found that the aspheric design in a centered position induces lower HOAs than spherical IOL for a 5 mm pupil. Figures 12, 13 and 14 show an example of this outcome. We can appreciate, in centered position, that the PSF of the aspheric design is better than the PSF of the spherical one. Moreover, in terms of ocular aberrations, the aspheric design induces lower HOAs than spherical IOLs for 5 mm of pupil 9,24 because these lenses try to compensate ocular SA. Then, it can be concluded that, in centered position, aspheric IOL will induce better VA than spherical IOL. However, it must be taking into account that after performing theoretical simulations, Wang and Koch 25 concluded that there are two main reasons for customizing the asphericity of the IOL. First, there is a wide range of corneal SA within population 26,27. Second, other corneal HOAs interact with SA to increase or decrease optical performance. In fact, some residual defocus or astigmatism combined with SA may produce different outcomes depending on the balance between lower-order aberrations and HOAs. Although IOL asphericity can be customized according to corneal SA and desired postoperative ocular aberration, the following must be taken into account: the pupil in older patients is relatively small (senile miosis), HOAs are dominated by asymmetrical aberrations and thus cannot be compensated for by symmetrical IOL designs, and some tilting or displacement of the IOL and changes in corneal optical aberrations can occur after surgery (increasing asymmetrical aberrations). All these factors may contribute to unexpected visual performance outcomes after aspheric IOL implantation. There are two procedures to achieve good clinical results with an aspherical IOL. On the one hand, the asphericity of the lens must be customized because it produces better results compared with results in patient population which aspheric IOLs were not customized 28. On the other hand, the centering of the IOL into capsular bag must be very accurate because if it is not achieved, the IOL will induce more HOAs than spherical design. In terms of depth-of-focus, spherical IOL provides more depth-of-focus than aspheric IOLs because the tolerance to defocus is highest in the

9 VISUAL SIMULATION IN MONOFOCAL IOLs ANALYSIS 151 spherical design. On figure 11, it can be appreciate that aspheric IOL has about 0.25 D and the spherical has about 0.75 D of depth-of-focus (we have considered depth-of-focus the range of VA values that are equal to or higher than 1.00). Marcos et al. 29 found that the tolerance to defocus was significantly higher with spherical IOLs than with aspheric IOLs. This was explained by Rocha et al. 24 because they pointed out that residual SA can improve depth-offocus and that the tolerance to defocus seems to be higher in eyes with a spherical IOL than in eyes with an aspheric IOL. However, it must be kept in mind that this refers to a monofocal IOL that was not designed to provide a large depth of focus, as are pseudoaccommodating IOL. In summary, the most clinical advantage of the visual simulator is that patients can check, with objective evidences and with a non-invasive procedure, their visual performance before any clinical procedure is carried out (i.e refractive surgery or adapt a visual corrector device). Clinically, visual simulator can be used by ophthalmologists and optometrists. With the visual simulator, ophthalmologists can show to their patients their visual performance before the refractive procedure is carried out. On the other hand, optometrists can show to their patients their visual performance if they adapt some visual corrector device. In both cases, they use the same simulation procedure. Firstly, they must measure and correct patients ocular aberrations. After that, they must induce patients ocular aberrations that their eye will have after the clinical procedure. On the other hand, researchers may use the visual simulator for other activities. They use this device to study which ocular surgery, ocular compensator device or compensator design induces better visual performance after their adaption. For example in the pilot study, we studied the visual performance of two IOLs designs when they are centered, decentered or tilted. Another use that researchers do of this device is to study the effect of ocular aberrations in the visual performance. All this research activities have the same aim: to design more physiologic ocular compensation devices to obtain better visual performance. REFERENCES 1. Poter J, Queener H, Lin J, Thorn K, Awwal A. Adaptive Optics for vision science. Canada:Wiley Interscience Pérez-Vices C, Belda Salmerón L, García-Lázaro S, Madrid Costa D, Ferrer-Blasco T. Adaptive optics, wavefront and visual simulation. J Emmetropia 2011; 2: López-Gil N, Castejón-Mochón J, Fernández-Sánchez V. Limitations of the ocular wavefront correction with contact lenses. 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