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Assessment of the image quality of extended depth-of-focus intraocular lens models in polychromatic light

Lee, Yumi MD; Łabuz, Grzegorz PhD; Son, Hyeck-Soo MD; Yildirim, Timur M. MD; Khoramnia, Ramin MD, FEBO; Auffarth, Gerd U. MD, PhD

Author Information
Journal of Cataract & Refractive Surgery: January 2020 - Volume 46 - Issue 1 - p 108-115
doi: 10.1097/j.jcrs.0000000000000037
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In cataract surgery, removal of the crystalline lens and implantation of a monofocal intraocular lens (IOL) leaves the eye unable to see clearly objects located at various distances. Multifocal IOLs overcome this limitation and enable patients with pseudophakia to achieve good near and far vision.1 These IOLs usually provide good vision at 2 distinct foci (eg, far and near), but their optical quality decreases in between these separate points.2 In addition, multifocal optic designs can generate undesirable photic phenomena such as halos, glare, starbursts, or decreased contrast sensitivity, all of which may limit patient satisfaction.1,3 Further development in the field of IOLs led to the introduction of extended depth-of-focus (EDOF) designs.4–9 In contrast to traditional multifocal IOLs, the EDOF IOLs aim to preserve good optical quality continuously across a visual range.10 This has been associated with a lower incidence rate of photic phenomena such as halo and glare.10 The literature also shows a high rate of spectacle independence and postoperative satisfaction among patients with EDOF IOLs.8–10

The optics of EDOF lens design has been assessed in in vitro studies.4,11 A criticism has been made of those studies12,13 that measurements were performed using an aberration-free model cornea, whereas clinical studies demonstrate that the human cornea produces positive spherical aberration.14 Furthermore, in those studies, optical measurements were performed in monochromatic (green) light, which may be considered a poor indicator of IOL performance in real-life conditions. Instead, the use of polychromatic light has been proposed,12,13 as everyday tasks are typically performed in light composed of multiple wavelengths. In the polychromatic light, however, chromatic aberration can limit the imaging quality of an optical system, such as IOLs.15 Pseudophakic eyes exhibit a different level of chromatic aberration, which besides the intrinsic dispersion of the ocular media also depends on the optical properties of implanted IOLs.16 Therefore, an in vitro IOL assessment using polychromatic light and a corneal lens with a population level of spherical aberration would appear a better approximation of the clinical (in vivo) situation.

Our principle research aim was to evaluate the optical quality of 3 EDOF IOLs by measuring the modulation transfer function (MTF) in polychromatic light and with a model cornea with spherical aberration. In addition, the polychromatic and the monochromatic MTF were compared to demonstrate how uncorrected chromatic effects worsen each IOL's optical performance. Last, the IOL's tolerance to tilt and decentration was tested.

METHODS

Intraocular Lenses

The 3 EDOF IOLs studied were the Tecnis Symfony ZXR00 (J&J Vision), AT Lara 829 MP (Carl Zeiss Meditec AG), and Mini Well Ready (SIFI). The tolerance to defocus was compared with a monofocal AcrySof SA60AT (Alcon Laboratories, Inc.): a single-piece spherical IOL made of hydrophobic acrylic material with a 1.55 refractive index and an 37 Abbe number of 37. The in vitro analysis was performed at the David J Apple Center for Vision Research, Heidelberg University Eye Hospital, Heidelberg, Germany.

The Symfony is a single-piece lens with an anterior aspheric surface to correct 0.27 µm of corneal spherical aberration. Its posterior diffractive surface has an echelette feature to enhance the range of vision and to compensate chromatic aberration using the opposite chromatic aberration behavior of a refractive and diffractive optical component. It is made of hydrophobic acrylic material with a refractive index of 1.47 and an Abbe number of 55.

Similar to the Symfony, in using diffractive optics, the AT Lara takes advantage of its diffractive design to minimize chromatic aberration effects. It is a single-piece hydrophilic acrylic (25% water content) IOL with a hydrophobic surface. The lens has an aspheric refractive base, which is aberration neutral, and has a refractive index of 1.46 and an Abbe number of 56.5.

The Mini Well is a single-piece IOL manufactured from hydrophilic acrylate (25% water content) with a hydrophobic surface. This lens has 3 circular zones: a central zone with positive spherical aberration, a surrounding zone with negative spherical aberration, and an outer monofocal zone with no induced spherical aberration. According to the manufacturer, the central zone and the middle zone have a diameter of 1.9 mm and 3.0 mm, respectively. The Mini Well's refractive index is 1.46, and the Abbe number is 46.9.

To test the repeatability of the optical quality measurements, each EDOF lens group was comprised of 3 +21 diopter (D) lenses and two +20 D lenses. The monofocal IOL was included solely as a benchmark for the defocus test, thus only one sample with a refractive power of +21 D was used.

Image Quality Assessment

The optical quality of all lenses was evaluated using an optical metrology station (OptiSpheric IOL PRO 2; Trioptics GmbH), which follows the guidelines of the International Standard Organization (ISO 11979-2). The IOLs were measured in a balanced salt solution with a 1.336 refractive index at room temperature. The optical apparatus included an aberrated model cornea that simulated the mean spherical aberration level (0.28 µm) found in the human cornea.14 A white LED light source and two optical filters were used. To study the IOL performance in monochromatic light, we used a bandpass filter (10 nm full width at half maximum) with the center wavelength of 546 nm. The polychromatic condition was simulated with a Commission internationale de l'éclairage photopic-response filter. Image quality metrics were primarily determined for a mean photopic pupil size of 3.0 mm.17 Apertures of 2.0 mm and 4.0 mm sizes were also used to test IOL pupil dependence. The imaging ability of the lens was evaluated objectively by means of the MTF. The optical metrology device measures the MTF with a 2% accuracy and has proven to provide excellent repeatability.15 MTF measurements were taken at best far and intermediate points and at defocus ranging from −0.5 D to 2.5 D with a 0.25 D increment. The mean MTF (up to 30 cyc/deg), the through-focus MTF at 15 cyc/deg, and the visual Strehl ratio (VSR)18 were assessed. The VSR was calculated over 30 cyc/deg in the frequency domain and weighted by the neural contrast sensitivity function.19 In addition, a polychromatic point spread function (PSF), which was an image of a 0.1 mm point source, was recorded through the 3.0 mm aperture for different defocus values. The PSF data were used to visualize the optical performance of IOLs through simulated images of Early Treatment Diabetic Retinopathy Study optotypes. To this end, we used a truncated visual acuity (VA) chart image that covered a range between 0.5 and −0.3 logarithm of the minimum angle of resolution. This image was convoluted with the polychromatic PSF using custom-written software (MATLAB; MathWorks).

IOL tolerance to misalignment was tested by inducing (separately) up to 0.75 mm of decentration and 5° of tilt and calculating the loss of the MTF value at 15 cyc/deg.

RESULTS

Monochromatic vs Polychromatic MTF

Figure 1 shows the comparison between the monochromatic and polychromatic MTF within each lens model. The measurements were performed at the best primary (far) and secondary (intermediate) focus for the 3.0-mm pupil. The position of the secondary focus was 1.90 ± 0.01 D for the AT Lara, 1.76 ± 0.02 D for the Symfony, and 2.20 ± 0.11 D for the Mini Well. The far MTF performance was worse in polychromatic than monochromatic light for all IOLs, as the mean MTF loss was 15% for the AT Lara, 17% for the Symfony, and 26% for the Mini Well. At the secondary focus, the image quality was slightly better for the AT Lara (by 6%) and minimally worse for the Symfony (by 5%) and the Mini Well (by 7%) in polychromatic light. All but one EDOF model demonstrated excellent repeatability regarding MTF measurements, as 5 IOL samples per model were measured. In the Mini Well group, however, the MTF curves differed, particularly between the samples of +20 D and +21 D power.

Figure 1
Figure 1:
The MTF of the studied IOLs measured in polychromatic (black lines) and monochromatic (546 nm; green lines) light. The MTF was determined at the best primary (left panels) and secondary (right panels) focus of the IOLs. Five samples of each model were measured, with 3 lenses having +21 D (dashed lines) and 2 having +20 D (dotted lines). The solid lines refer to the mean MTF curve derived from 5 samples (IOL = intraocular lens; MTF = modulation transfer function).

Polychromatic Image Quality Comparison

Given good repeatability of the MTF in the +21 D IOLs (also in the Mini Well group), only one sample was used in further analysis and comparison of the image quality of the individual models. The polychromatic MTF of the 3 EDOF IOLs and the monofocal control are compared in Figure 2. At far, the SA60AT demonstrated higher MTF levels than those of the EDOF models. The Symfony and the AT Lara presented comparable image quality at the far focus. The Mini Well's MTF was, however, worse than that of the two diffractive lenses at higher spatial frequencies but better up to 6 cyc/deg. At the best secondary focus, the Symfony outperformed the two other EDOF models, of which the AT Lara appeared to provide minimally better optical quality than the Mini Well. Monofocal lens performance at a defocus of 2 D (the average secondary positions of the 3 EDOF IOLs) was inferior to that of the other designs.

Figure 2
Figure 2:
The polychromatic MTF of the AT Lara (green lines), the Symfony (red lines), and the Mini Well (blue lines) IOL measured at the best primary (left panel) and secondary (right panel) focus. For comparison, MTFs of a monofocal IOL (black lines) assessed at zero (left panel) and 2 D (right panel) defocus are shown (IOL, intraocular lens; MTF, modulation transfer function).

Figure 3 presents the VSR over a −0.50 D to 2.50 D defocus range for the 3.0 mm aperture. All 3 EDOF models yielded a lower VSR value than that of the monofocal lens up to approximately 1.0 D. Above this range, the optical quality of the EDOF IOLs provided enhanced optical performance compared with the monofocal one. The Symfony and AT Lara VSR demonstrated 2 clear peaks at 1.75 D and 2 D, respectively, and at zero defocus, with the valley occurring at about 1 D. Although the Mini Well produced lower primary and secondary peaks than the diffractive lenses, its optical performance was nearly constant for an extended range with a small improvement at far point.

Figure 3
Figure 3:
Defocus tolerance of the AT Lara (green line), the Symfony (red line), the Mini Well (blue line), and a monofocal (black line) IOL. The polychromatic modulation transfer function (MTF) was measured for a 3.0 mm aperture with a defocus resolution of 0.25 D starting from the best far focus. The visual Strehl ratio was calculated over 30 cyc/deg (IOL, intraocular lens).

The visual acuity chart simulations that are presented in Figure 4 confirm the MTF results. The monofocal lens provided an excellent image at zero and satisfactory at ± 0.50 D, but it sharply decreased beyond this level. Of the 3 EDOF lenses, at no defocus, the VA prediction of the AT Lara was noticeably better than the Symfony (less intense optotype shadowing) and particularly better than the Mini Well, which appeared blurred. At 1 D, however, the refractive EDOF lens demonstrated a higher image quality than the diffractive ones, which was also better than the AT Lara but worse than the Symfony at 1.5 D. The two diffractive designs proved to be better than the Mini Well at 2 D, of which the Symfony was less affected by shadowing. The simulated resolving power decreases for all the EDOF lenses at 2.5 D, particularly for the Symfony.

Figure 4
Figure 4:
Visual acuity chart simulation at a range of defocus. The chart image, which represents the optotype range from 0.5 to −0.3 logarithm of the minimum angle of resolution, was convoluted with a polychromatic point spread function of the Symfony, Mini Well, AT Lara, and monofocal (SA60AT) lenses.

The through-focus MTF at 15 cyc/deg was measured for 2.0 mm, 3.0 mm, and 4.0 mm apertures to test how the optical performance changes with the pupil size (Figure 5). The Symfony exhibited only a small effect when changing the aperture, as the MTF values remained high for all conditions. However, the lens appeared to provide slightly better optical quality at the secondary than primary focus for 2.0 mm and 3.0 mm, which was reversed for the 4.0 mm pupil. The Mini Well demonstrated a clear pupil dependency, as the lens was intermediate dominant at 2.0 mm. The refractive lens produced the EDOF effect for the 3.0 mm aperture. However, it became more far dominant at 4.0 mm with an increased primary and decreased secondary peak. Similar to the Symfony, the AT Lara also provided a slightly higher MTF peak at intermediate than zero defocus, but this relationship was reversed for 3.0 mm. Although for the 4.0 mm aperture, the optical quality of the lens was lower, the MTF value decreased proportionally at the far and intermediate point.

Figure 5
Figure 5:
The through-focus modulation transfer function measured at a single frequency of 15 cyc/deg in polychromatic light. The pupil size impact on the image quality was assessed at an aperture of 2.0 mm (dotted lines), 3.0 mm (solid lines), and 4.0 mm (dashed lines).

The simulation of IOL tilt up to 5 degrees did not have a major impact on IOL performance; the MTF at 15 cyc/deg decreased by 0.07 for the AT Lara, 0.04 for the Symfony, 0.02 for the monofocal lens, and did not change for the Mini Well. Decentration by 0.75 mm also had a marginal effect on the performance of the AT Lara (ΔMTF = 0.02), the monofocal IOL (ΔMTF = 0.01), and the Mini Well (ΔMTF = 0.01), but the effect was larger for the Symfony (ΔMTF = 0.09).

DISCUSSION

We found that the polychromatic light does indeed affect the optical quality of the IOLs. The EDOF IOLs proved to extend the range of vision compared with the monofocal lens; however, apparent differences between the 3 EDOF models exist.

The comparison between the monochromatic and polychromatic MTF revealed a loss of optical quality in the latter condition. This chromatic effect results from the IOL's internal dispersion of light and the chromatic aberration of the model eye, which is about 1.0 D between 480 nm and 644 nm. The results indicate that the far MTF of the AT Lara and the Symfony decreased less than that of the Mini Well. The material properties may be one factor, as the Mini Well has a lower Abbe number, thus higher dispersion, than that of the diffractive IOLs. Another explanation for better polychromatic image quality of the diffractive IOLs might be that the two models feature the chromatic aberration correction. At the secondary focus, the chromatic effects were lower in all the EDOF lenses. For the AT Lara, we found a small MTF improvement in the polychromatic light. This can be understood as the ability of the lens to correct the chromatic shift and to bring more wavelengths (in contrast to only one in the monochromatic condition) into focus, which has a constructive effect on the optical quality. The Symfony's chromatic effects were lower at intermediate than far point, but the lens demonstrated a minimal deterioration in the image quality in this in vitro setting. In both diffractive IOLs, the chromatic aberration correction was more effective at the secondary than the primary focus. This can be explained by the diffraction grating design of the EDOF IOLs that use the first and second diffractive orders to diverge the light to far and intermediate point, respectively.6 In this design, the chromatic aberration correction is expected to double at the second order (intermediate) as compared to the first order (far).6 Interestingly, the Mini Well showed a comparable MTF loss to the Symfony despite its purely refractive design. This may result from an extended intermediate focus of the Mini Well, which did not show a distinct secondary peak so that it may have compensated the chromatic shift at this position.

Although all the studied IOLs share the EDOF principle, a number of key differences in their optical behavior emerge from the study results. At the far focus, the AT Lara had a slightly higher mean MTF than the Symfony with both providing better optical performance than the Mini Well. At the best intermediate focus position, which differed between the IOLs, the Symfony's MTF level was higher than that of the other EDOF lenses and that was followed by the AT Lara and the Mini Well. These differences in the objective optical quality can also be seen in the simulation of the VA chart. Although the measured differences are not likely to affect patients' VA, they may have an impact on the overall quality of vision by creating a ghost (out-of-focus) image of different size and intensity that looks like letter shadowing20 as seen in Figure 4. Although the readability of the 0.0 logarithm of the minimum angle of resolution line is preserved for all the EDOF IOL at 2 D, the Symfony produces less such shadowing effects than the AT Lara and the Mini Well, which may explain the higher VSR in Figure 3. In addition, the defocus MTF and the VA simulation indicate that the image quality of the diffractive IOLs changes considerably with defocus with two optical points matching the position of their main foci. By contrast, the Mini Well's optical quality is less affected by the defocus change, but none of the optotype images is as good as that of the AT Lara at no defocus or the Symfony at 2 D. Thus, one may conclude that enlarging the depth-of-focus comprises a tradeoff between, on the one hand how far the visual range can be extended and on the other, the optical quality achieved at each point.

The effect of pupil size varied for the studied EDOF IOLs. The diffractive lenses demonstrated minimally better intermediate than far performance at small apertures; however, it was reversed when the aperture increased. In contrast to the Symfony, the AT Lara's MTF (at 15 cyc/deg) was low through the 4.0 mm pupil. This may result from the difference in the spherical aberration corrections of both IOLs. The Symfony produces 0.27 µm of negative spherical aberration to counteract a positive spherical aberration of 0.28 µm found in the human cornea.14 This is similar to the level of spherical aberration in our in vitro model. Given a good match between the spherical aberration of the model cornea and the Symfony, its MTF remains high despite increased pupil size. If a model cornea free of spherical aberration had been used, the Symfony's MTF results at a larger pupil would have deteriorated significantly, as shown in a study by Domínguez-Vicent et al.4 This was the case with the AT Lara, which does not correct corneal spherical aberration, and thus, the MTF level for the 4.0 mm pupil was greatly affected. Figure 5 indicates strong pupil dependency of the Mini Well. At 2.0 mm, the refractive lens demonstrated one extended through-focus MTF peak centered at the intermediate distance (about 1.75 D). For such a small aperture, the first optical zone is dominant. The central area contains positive spherical aberration, which alone would make the eye hyperopic.21 For that reason, the first zone has a focus offset that adds refractive power to counter this undesired hyperopic shift.21 It appears that for the 2.0 mm pupil, induced spherical aberration and the focus offset are not matched, making the eye slightly myopic. This may result in improving intermediate and the expense of distance vision in patients with small pupils. At 3.0 mm, the IOL MTF increased at far but decreased at intermediate point as the two first zones take part in the image creation providing balanced optical performance for the far–intermediate range. A large pupil (4.0 mm) improved the far but worsened intermediate focus, which could have been expected given the outer monofocal zone of the Mini Well. Similarly to the AT Lara, spherical aberration effects may also affect the performance of the Mini Well as a consequence of the aberration neutral design of the IOL's peripheral zone and the positive spherical aberration of the corneal lens. Domínguez-Vicent et al.4 measured the Mini Well using an aberration-free model cornea, which showed MTF (at 15 cyc/deg) = 0.55 with no defocus and a 4.5 mm pupil. In this study, we found a discrete MTF value of 0.19 at 4.0 mm, which illustrates the extent of spherical aberration impact on the image quality. Although Wang et al.14 have reported the average level of corneal spherical aberration in the normal population as 0.28 µm, that study has also demonstrated a large variability of this parameter ranging from 0.055 to 0.544 µm.14 Thus, IOL manufacturers and clinicians may expect that aspheric lenses will not function optimally in all patients with larger pupils.

Clinical studies on IOL tilt and decentration have shown highly variable results. The mean decentration value ranges from 0.19 to 0.7 mm, but values greater than 1.0 mm have also been reported.22–24 A mean value of 7.8 degrees tilt was shown in one report,22 but in others, it was 3.1 degrees and 2.9 degrees.23,24 In this study, we demonstrated that limited tilt does not have an impact on the MTF of all but one IOL, which in principle is in line with results found in the literature.5,25 Although the AT Lara was slightly affected by tilt, it proved more robust against decentration. Only a small effect of decentration on MTF was found in the Mini Well IOL, which was comparable to that of the monofocal lens. The MTF (at 15 cyc/deg) loss was 0.01, which is lower than a value reported by Bellucci and Curatolo,5 who demonstrated a 0.06 drop under 0.5 mm of decentration in the green light condition. The Symfony's image quality appears to be most sensitive to decentration, which could have been expected, given its high level of spherical aberration correction as shown by Fujikado and Saika.26 In the study by Yoo et al.7 a 15 cyc/deg MTF of the Symfony was reduced by approximately 0.1 at 0.75 mm of decentration, which is close to the 0.09 found in this study.

The repeated measurements of each lens demonstrated a nearly perfect alignment of the MTFs (Figure 1) of different (+20 D and +21 D) samples in the Symfony and the AT Lara groups. This good repeatability was observed at the primary and secondary focus for both lighting conditions (Figure 1). By contrast, the Mini Well showed slight misalignment of the MTF curves measured from the samples of different power. The power difference between the two sets of samples seems to be negligible, as the theoretical cutoff frequency of both differs only by about 1 cyc/deg. Thus, we cannot find a possible explanation for this finding, which should be addressed by the manufacturer.

In conclusion, chromatic and spherical aberration have an essential impact on the in vitro image quality of IOLs. The correction of both aberrations improves IOL MTF, which in turn may improve subjective visual experience.27 The studied EDOF IOLs demonstrated clear potential for enlarging the visual range of patients with pseudophakia. The diffractive IOLs showed a comparable optical behavior, with the main differences being the intermediate optical performance and the management of spherical aberration. The Mini Well's tolerance to defocus proved more robust but yielded lower image quality. In contrast to the diffractive IOLs, the refractive zonal design of the Mini Well exhibits a high level of pupil dependency, which needs to be taken into account in preoperative decision making.

WHAT WAS KNOWN

  • Extended depth-of-focus (EDOF) intraocular lens (IOL) models create a continuous range of vision for patients with pseudophakia, but the provided visual quality may differ, given the variety of designs and working principles.

WHAT THIS PAPER ADDS

  • Although all studied EDOF IOLs proved more tolerant to defocus than a standard monofocal IOL, the key differences lie in their management of chromatic and spherical aberration effects, pupil dependency, and the optical quality at discrete visual points, which may be taken into account to optimize postoperative visual outcomes.

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