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O R I G I N A L A R T I C L E

Since the first monocular sequential implantation of two intraocular lenses (IOLs) in 1993,1 polyp-seudophakia has gained in popularity. However,

it was not without complications, especially in the early years when two posterior chamber IOLs were implanted in the capsular bag.2-6 This approach was used with success to correct refractive error,1,7 but it

was quickly recognized that it also increases the risk of hyperopic shift and interlenticular opacification.2-4

It was suggested by David Apple and others that these postoperative complications could be overcome by implanting the anterior of the two IOLs in the cili-ary sulcus. Only the primary lens would be located posteriorly in the bag.2,4 However, because capsular

ABSTRACT

PURPOSE: To compare the optical performance of a two-intraocular lens (IOL) system with that of a single capsular bag trifocal IOL.

METHODS: The two-IOL configuration of a monofocal RayOne Aspheric (Rayner Intraocular Lenses, Ltd) and a Sulcoflex Trifocal (Rayner Intraocular Lenses, Ltd) lens was compared in vitro with a single-lens option (RayOne Trifocal; Rayner In-traocular Lenses, Ltd). Two samples of each IOL model were studied with an optical metrology device. The optical quality was assessed using the area under the modulation transfer function (MTF). The impact of the supplementary lens mis-alignment on the MTF was tested. The light loss was also measured using a power meter.

RESULTS: The two-IOL system produced three well-defined focal peaks comparable to those of the single lens. The MTF area of the single- and two-IOL configuration was, respectively, 22.5 and 20.7 at far, 16.4 and 15.4 at intermediate, and 14.9 for each con-figuration at near. A moderate decentration (up to 0.6 mm) had a minimal effect at intermediate and near on the supplementary lens MTF and no impact at far. A 5° tilt did not alter the MTF. The supplementary lens caused a 1.3% decrease in the optical power.

CONCLUSIONS: The optical quality of the two-IOL system matched that of the single trifocal lens. A low-power supple-mentary IOL demonstrated high tolerance to misalignment and minimal light attenuation. The reversibility of the two-IOL approach may prove advantageous clinically.

[J Refract Surg. 2020;36(9):570-577.]

From David J. Apple Center for Vision Research, Department of Ophthalmology, University Hospital Heidelberg, Heidelberg, Germany (GŁ, GUA, H-SS, TMY, RK); and Medical Faculty Mannheim of the University of Heidelberg, Mannheim, Germany (MCK).

© 2020 Łabuz, Auffarth, Knorz, et al; licensee SLACK Incorporated. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (https://creativecommons.org/licenses/by-nc/4.0). This license allows users to copy and distribute, to remix, transform, and build upon the article non-commercially, provided the author is attributed and the new work is non-commercial.

Submitted: April 3, 2020; Accepted: July 7, 2020

Supported by an unrestricted research grant from Rayner.

Dr. Auffarth reports grants from Klaus Tschira Stiftung, Heidelberg, Germany. Drs. Auffarth and Khoramnia report grants from Acufocus, Carl Zeiss Meditec, PhysIOL, and Powervision; grants and non-financial support from Kowa and Ophtec; and grants, lecture fees, and non-financial support from Alcon, Hoya, Johnson & Johnson, Oculentis, Polytech, Rayner, and SIFI. The remaining authors have no financial or proprietary interest in the materials presented herein.

The authors thank Donald J. Munro for his review of the manuscript.

Correspondence: Gerd U. Auffarth, MD, PhD, Universitäts-Augenklinik Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany. Email: Gerd.Auffarth@med.uni-heidelberg.de

doi:10.3928/1081597X-20200715-01

Trifocality Achieved Through Polypseudophakia: Optical Quality and Light Loss Compared With a Single Trifocal Intraocular LensGrzegorz Łabuz, PhD; Gerd U. Auffarth, MD, PhD; Michael C. Knorz, MD, PhD; Hyeck-Soo Son, MD; Timur M. Yildirim, MD; Ramin Khoramnia, MD, PhD

• Vol. 36, No. 9, 2020 571

bag IOLs were not intended for sulcus fixation, this caused other problems, such as pigmentary dispersion syndrome or pupillary block.5,6 In 2010, Kahraman and Amon8 described the first supplementary IOL spe-cifically designed to be implanted in the sulcus and made of hydrophilic acrylic, demonstrating excellent clinical results. Since then, sulcus implantation of a supplementary IOL is widely accepted as a safe and predictable procedure.8-15

During the past decade, sulcus-fixated lenses have been refined to offer astigmatism correction and bifo-cality.9-11,13,16 Recently, however, a new trifocal ver-sion of a supplementary lens has been introduced.15 The concept of a trifocal sulcus lens is particularly at-tractive because the trifocality is more readily revers-ible than when using a single capsular lens.15 Revers-ibility may be of critical importance in cases where there is patient dissatisfaction after surgery or patients develop diseases later in their life, when multifocal-ity could be a hindrance (eg, macular degeneration or glaucoma). Although reversibility could be a definite advantage, one may question whether the supplemen-tary trifocal lenses offer a standard of imaging quality comparable to those implanted in the capsular bag.

In this study, we compared the optical perfor-mance of a two-IOL system with a single trifocal IOL. We simulated clinically relevant conditions by using polychromatic light and an aberrated corneal model. We measured the effect on the image quality of tilt and decentration of a supplementary IOL. Clinicians have frequently raised this as a concern because mis-alignments of varying extents have been reported in polypseudophakic eyes.8,9,12 Also, we assessed theoretically and experimentally the light loss in the two-IOL configuration and whether it has a potential clinical impact.

MATERIALS AND METHODSIOLs

We performed benchtop measurements of a cap-sular bag trifocal IOL (+20.00 diopter [D], RayOne Trifocal, RA0603F) and a supplementary zero-power lens (Sulcoflex Trifocal, IOL703F) with a monofocal IOL for capsular bag implantation (+20.00 D, RayOne Aspheric, RAO600C) to assess the impact of polypseu-dophakia on image quality metrics. Two samples of each IOL model were used. All of the study IOLs were from Rayner Intraocular Lenses, Ltd, and had the same hydrophilic acrylic material with a refractive index of 1.46 and an Abbe number of 56. The IOLs have an aspheric, aberration-neutral design.

The two trifocal models (RayOne and Sulcoflex) share the same non-apodized diffractive design. The

IOLs contain 16 diffractive steps confined to a 4.5-mm diameter zone, which leaves the optic periphery used solely for distance vision. At 3 mm, the energy split between the three foci favors distance with 52% of light, and the remaining part is allocated to the inter-mediate (22%) and near (26%) focus. The add power for the intermediate and near range is 1.75 and 3.50 D, respectively. Despite these similarities, the supple-mentary lens has a different geometry, suited to sulcus implantation, compared to the capsular bag lenses. The key differences are summarized in Table 1.

OptIcaL MetrOLOgy The optical comparison between the single- and

two-lens systems was made using an OptiSpheric IOL PRO2 (Trioptics GmbH), which was described in detail in earlier publications17,18 (Figure A, available in the on-line version of this article). All measurements were per-formed under simulated in situ conditions using a bal-anced salt solution (with a refractive index of 1.336). To mimic polypseudophakia (ie, combined Sulcoflex Trifo-cal and RayOne Aspheric), the supplementary IOL was positioned at the pupil plane with a 2-mm separation (to-ward the retina) between the two lenses. In the eye, the lenses are closer together, with a distance between them of approximately 0.5 mm.13 Although the lens position in vivo significantly affects the eye’s refractive error,19 in our in vitro set-up, the adjustable camera distance com-pensates for this effect. We also expect that this configu-ration has a minimal effect on the IOL’s image quality, which may result from a slight (approximately 4%) in-crease of the exit pupil size as indicated by the analysis of the set-up modeled in optical design software (Optic-Studio 19.4; Radiant Zemax LLC).

The optical assessment was divided into two parts, each performed at a 3-mm pupil. First, the refractive power (including add powers) measurements were ob-

TABLE 1Key Characteristics Differentiating

the Sulcus-Fixated IOL From the Two Capsular Bag IOLs

Characteristic Sulcoflex TrifocalRayOne Aspheric/

RayOne TrifocalOptic/overall diameter 6.5/14.0 mm 6/12.5 mmOptic shape Convex anterior

and concave posterior

Biconvex

Optic/haptic edge Round SquareHaptic angulation Posterior 10° Uniplanar 0°IOL = intraocular lens All lenses are manufactured by Rayner Intraocular Lenses, Ltd.

572

tained in monochromatic (green) light with the magni-fication method described in the ISO 11979-2 standard. These measurements were done without a model cor-nea. Second, image quality was tested by measuring the IOLs’ modulation transfer function (MTF). For this part, we used a model cornea having 0.28 µm of spheri-cal aberration, and polychromatic (white) light with its spectrum modified to correspond to the photopic sen-sitivity of the human eye. Sagittal and tangential MTFs were averaged, and we calculated the area under the MTF (MTFa), as described by Vega et al.20 The through-focus MTF was assessed at 50 lp/mm with a defocus range of +1.00 to -5.00 D. Furthermore, to visualize and compare the optical performance of each IOL, we took photographs of the U.S. Air Force resolution test charts, also performed at a 3-mm aperture.

The MTF metrics were used to assess how misalign-ment of only the supplementary lens (keeping the pri-mary lens centered) can impact the entire image quality in polypseudophakia. First, we induced a 5° tilt with a custom-designed insert. Second, we forced the decentra-tion of the sulcus-fixated IOL and measured the optical quality. Although the OptiSpheric features a motorized stage to simulate decentration effects, this would decenter the two-IOL system, so we could not use it. Consequently, to induce decentration, we intentionally placed the Sulco-flex lens off-center in the model eye while maintaining a proper alignment of the primary lens. The extent of decen-tration was later derived from the analysis of photographs taken during the course of the test.

LIght transMIssIOnImplantation of a supplementary lens introduces into

the eye new and additional surfaces that reflect a small part of the incoming light. We used Fresnel equations to quantify the amount of reflected (R) and transmitted (1-R) light at the interface between media having dif-ferent refractive indices (eg, the interface between the aqueous humor and an IOL).21 Given the nearly normal incidence of the light at the first IOL surface, we could

neglect that the reflection coefficient changes with light polarization.22 Thus, a simplified formula was used:

where nA = 1.336 is the refractive index of the aqueous humor and nIOL = 1.46 is that of the Sulcoflex Trifocal lens.

Laboratory measurements of the light attenuation followed the theoretical assessment. To this end, the optical power was compared between a single lens (RayOne Trifocal) and the two-IOL configuration (Sul-coflex Trifocal and RayOne Aspheric). We used an illu-mination system of the OptiSpheric, which projected a collimated uniform beam (without a test object) onto a model eye without a model cornea. The light loss was assessed using an optical power meter (PM100D; Thorlabs) with a photodiode power sensor (S121C; Thorlabs), which was placed behind a flat window of the model eye. We used a 3-mm aperture to narrow a cone of light and to ensure that all light falls onto the photodiode. Three measurements were taken for both single- and two-IOL configurations with a monochro-matic blue (480 nm), green (546 nm), and red (644 nm) light. Results were averaged, and the light loss was calculated using the following formula:

RESULTStwO-agaInst-One cOMparIsOn

Table 2 presents the mean nominal power measure-ments of the single- and two-IOL systems. Both condi-tions yield comparable dioptric power results.

Figure 1 shows the average MTF curves. The image quality of the two-IOL approach matched that of the single RayOne trifocal IOL at intermediate and near, but it was minimally lower at far. The MTFa of the single- and two-IOL systems was 22.5 versus 20.7 at infinity, 16.4 versus 15.4 at intermediate, respectively, and 14.9 for both configurations at near.

The results of the through-focus MTF scan are shown in Figure 2. Measurements taken in the single- and two-IOL models revealed a clear separation of through-focus MTF peaks corresponding to the designed far, in-termediate, and near focus. The two approaches dem-onstrated equivalent optical performance at those three foci. However, a small difference was observed at zero defocus. The through-focus analysis confirmed a larger allocation of energy to far than to the other distances,

TABLE 2Nominal and Add Dioptric Power of a

Single- and Two-Lens System

Distance RayOne Trifocal

Sulcoflex Trifocal With RayOne

AsphericFar (D) 20.36 ± 0.03 20.28 ± 0.17Intermediate (D) 1.79 ± 0.00 1.77 ± 0.03Near (D) 3.47 ± 0.01 3.38 ± 0.02D = diopters All lenses are manufactured by Rayner Intraocular Lenses, Ltd.

• Vol. 36, No. 9, 2020 573

but the MTF peak was slightly higher at intermediate than near.

U.S. Air Force resolution chart photographs were tak-en at the best foci and are presented in Figure 3. Those images confirm the comparable quality of the studied IOLs as differences between the single- and two-IOL configurations are hardly noticeable also at far-point.

MIsaLIgnMent Of the suppLeMentary LensFigure B (available in the online version of this ar-

ticle) shows the photographs of a supplementary lens decentered by 0.2, 0.4, and 0.6 mm, and with an ex-treme shift of 1.8 mm. The corresponding resolution target images are presented in Figure C (available in the online version of this article). The MTF at 50 lp/mm of the dual configuration with a perfectly centered supplementary lens was 0.22 at far, 0.15 at intermedi-ate, and 0.13 at near (Figure 2). The far MTF did not change with 0.6 mm of decentration. The shift of up to 0.4 mm did not affect the image quality at the ad-

Figure 1. The modulation transfer function (MTF) as a function of the spatial frequency. The comparison between a capsular bag trifocal lens (black) and polypseudophakia with a supplementary trifocal and a monofocal intraocular lens (IOL) (green). The dashed line shows the results of individual IOLs; the solid line shows the average value.

Figure 2. The through-focus modulation transfer function (MTF) of single- (black) and two-intraocular lens (IOL) (green) trifocal arrange-ments. The dashed line shows the results of individual IOLs; the solid line shows the average value. D = diopters

574

ditional foci. However, a small decrease of the MTF value was noted at the intermediate (MTF = 0.13) and near (MTF = 0.10) focus at 0.6 mm. Severe decentra-tion resulted in an improvement of the image quality at far (MTF = 0.28), but trifocality was virtually lost with the intermediate and near MTF values of 0.04 and 0.02, respectively (Figure C). The 5° tilt of the sup-plementary lens did not affect the optical performance of the two-IOL model, with no effect on the discrete MTF value.

LIght-LOss assessMentThe theoretical estimation of reflectance yielded a

value of 0.2% at one interface. Consequently, the R parameter increases to 0.4% for the single-IOL system and 0.8% for the two-IOL system.

The optical power obtained with the Sulcoflex Tri-focal and the monofocal RayOne Aspheric IOLs was compared with that of the capsular bag RayOne Trifocal IOL. The experimental measurements showed a 1.2% ± 0.2% loss in blue light and 1.3% ± 0.1% in green and red light due to the presence of the supplementary lens. Thus, the two-IOL configuration yields, on aver-age, minimally decreased light transmission by 1.3% ± 0.1%, compared to the single-lens arrangement.

DISCUSSIONWe showed that a polypseudophakic configuration

of the Sulcoflex Trifocal and the monofocal RayOne Aspheric (two IOLs) provides good MTF performance at the range of distances comparable to that of the (sin-gle) capsular bag RayOne Trifocal IOL.

The dioptric power measurements (Table 2) indi-cate an equivalent trifocal behavior of both the single- and two-IOL systems. The MTF analysis demonstrated that the polypseudophakic approach could be used to extend the range of vision without sacrificing the op-tical quality, because the Sulcoflex Trifocal IOL with the RayOne Aspheric IOL performed comparably to that of the RayOne Trifocal IOL. At the intermediate and near focus, the MTF curves obtained in both con-ditions demonstrated a clear overlap (Figures 1-2). The far-focus MTF of the single-optic lens was mini-mally better, though, which may stem from chromatic aberration effects combined with differences in the ef-fective lens position, or it is due to only one of these effects. A shift of the primary lens (RayOne Aspheric) in a dual configuration slightly increases the exit pu-pil size and thus may create more spherical aberration effects. Alternatively, a compensation of chromatic aberration by the RayOne Trifocal IOL might explain

Figure 3. U.S. Air Force target images recorded at the best far, intermediate, and near focus. IOL = intraocular lens

• Vol. 36, No. 9, 2020 575

the better far-focus MTF if the lens uses a non-zero diffractive order at the far focus.17,23 Given the zero power at far of the supplementary trifocal lens, any chromatic aberration correction at that focus cannot take place.17,23 To assess how this MTF difference may affect postoperative visual acuity, we applied a model by Vega et al20 that incorporates the MTFa as a param-eter. The visual acuity estimation yields a difference of less than one letter (< 0.02 logMAR), which can-not be detected in a standard test. Despite these dif-ferences, the U.S. Air Force chart images also appear identical (Figure 3), confirming that the image quality of the two-IOL system matches that of the single-optic trifocal lens. Although both trifocal IOLs from Rayner Intraocular Lenses, Ltd were designed to transfer more light to the near than intermediate focus (26% vs 22%), our optical measurements showed the opposite. The reason for that is that the imaging quality does not exclusively depend on the light distribution but can also be affected by factors such as monochromatic and chromatic aberrations.17,18 Both have the potential to increase the depth of focus,24,25 which in this case appears to slightly enhance the intermediate over the near focus.

The extent of supplementary lens misalignment has been assessed clinically. Kahraman and Amon8 implanted a monofocal sulcus-fixated lens in 12 eyes with only one case of decentration, and it was less than 0.5 mm. No IOL tilt or rotation was observed in that study.8 Prager et al12 analyzed retro-illumination photographs of polypseudophakic patients and found the average value of 0.22 to 0.23 mm with a maximum of 0.6 to 0.7 mm, depending on the reference object. Furthermore, the comparison between the position of sulcus-fixated and capsular bag IOLs revealed a sig-nificantly better centration of the former.12 Gerten et al9 reported a decentration level of 0.5 mm or less in 55 eyes that had received a multifocal supplementary lens. Decentation was higher in only one case (ap-proximately 0.8 mm).9 Despite misalignment, no com-plaints on visual performance were reported in those studies, nor was a secondary procedure required.8,9,12 In the current study, we also demonstrated that mod-erate tilt and decentration of the Sulcoflex Trifocal lens does not substantially affect the optical quality, which is in line with those clinical results. Only ex-treme misalignment, as simulated here, has the poten-tial to compromise the visual performance. However, such cases are rarely seen clinically, and are often re-lated to a traumatic incident occurring postoperative-ly.13,26 In our simulation of a 1.8-mm shift, the MTF was slightly improved at far. This rather unexpected result can be explained, noting that a smaller portion

of the light passes through the diffractive element be-cause more than half of its structure falls beyond the pupil area (Figure C). As a result, rays that miss the diffraction grating are not split, which favors the far focus by increasing its partition of light. Also, we used zero-power supplementary IOLs, so even if extremely decentered, this cannot affect the far-focus MTF. On the other hand, the image quality at secondary foci was dramatically degraded (Figure C). One would also expect photopic phenomena originating from the lens edge,26,27 so in a clinical situation, such a severe case would certainly require surgical intervention. Moder-ate misalignment of the supplementary lens, as report-ed clinically,8,9,12 does not, however, yield an impor-tant effect on the image quality in polypseudophakia.

The theoretical calculations revealed that inter-face reflections of an IOL result in a 0.4% light loss, but this value doubled for two IOLs. However, the R parameter strongly depends on the refractive index difference between two media.21,22 So, because the refractive index of the aqueous humor remains con-stant, the higher the refractive index of an IOL, the stronger the interface reflects the light. For instance, one AcrySof IOL (Alcon Laboratories, Inc) with a high refractive index of 1.55 would have a reflectance of 1.1%. In the dual configuration, with two Rayner IOLs both having a 1.46 refractive index, the reflectance was 0.8%. Hence, one would expect that the visual function of patients with polypseudophakia is not af-fected by interface reflections because the estimated R coefficient was lower than that of an Acrysof IOL, a commonly implanted, singular IOL with a high refrac-tive index. Schrecker et al22 studied internal reflec-tions in a two-IOL configuration using a ray-tracing model. They concluded that supplementary IOLs do not produce glare symptoms that could be relevant to a patient’s visual function compared with conven-tional single-lens implantation. Our results also con-firmed that there is no disadvantage to implanting a supplementary lens, at least not in terms of optical metrics such as the MTF.

The optical power assessment confirmed that the attenuation of light passing through supplementary IOLs is low. However, it is apparent that the light loss does not stem only from reflections because the mea-sured values were higher than 0.4% derived from the theoretical calculations. The reason for this discrep-ancy is that the Fresnel equations do not account for other factors, such as material absorption and light scattering,28 which in addition to the internal reflec-tions also have the potential to decrease the optical power we measured with the photodiode sensor. Note that the light loss by a diffractive element to higher

576

diffraction orders29 cannot be quantified using our methodology and such quantification was not part of our research aim. To estimate the impact of the mea-sured optical power loss on the visual function, we performed the conversion from radiometric to photo-metric units to calculate the Weber fraction. Weber’s law describes the relationship between the change in brightness and an initial brightness of a stimulus that can be perceived by the human eye. Cornsweet and Pinsker30 showed that in patients viewing a white-light stimulus, the Weber fraction is approximately 0.15. The calculation of the Weber fraction based on the optical power difference found in this study yields a value of 0.012 for the green light. Thus, the light loss due to the presence of a supplementary IOL is unlikely to affect the brightness perception in pa-tients with polypseudophakia.

Our research confirmed that polypseudophakia with the Sulcoflex Trifocal lens is optically equiva-lent to the single-lens RayOne Trifocal model. We demonstrated that although two-IOL implantation doubles interface reflections, the absolute values of the light loss are low, and we can assume this is clini-cally insignificant. Tilt and decentration of low-power supplementary IOLs have but minimal impact on the MTF in the two-IOL configuration. From these labo-ratory results, we would expect a similar effect in a polypseudophakic eye.

AUTHOR CONTRIBUTIONSStudy concept and design (GŁ, GUA, RK); data col-

lection (GŁ, RK); analysis and interpretation of data (GŁ, GUA, MCK, H-SS, TMY, RK); writing the man-uscript (GŁ); critical revision of the manuscript (GŁ, GUA, MCK, H-SS, TMY, RK); statistical expertise (GŁ, GUA, RK); administrative, technical, or material sup-port (GŁ, GUA, RK); supervision (GŁ, GUA, RK)

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Figure A. OptiSpheric IOL PRO2 (Trioptics GmbH).

Figure B. Photographs of a decentered trifocal supplementary lens by (A) 0.2, (B) 0.4, (C) 0.6, and (D) 1.8 mm.

Figure C. Comparison of the U.S. Air Force test chart images taken after the misalignment of a trifocal supplementary lens.