Restoring vision by replacing the opacified crystalline lens with an intraocular lens (IOL) remains the main goal of cataract surgery. However, modern patients are more demanding and have higher expectations regarding visual quality, comfort, and spectacle independence after IOL implantation. For that reason, cataract surgery is currently performed at increasingly earlier ages and has become a consolidated option within the portfolio of refractive procedures.1 This has prompted the constant evolution of IOL designs intending to achieve the best visual function possible, especially at intermediate and near distances, while maintaining perceived good image quality at a far distance. Designs currently available on the market are diffractive, refractive, or combined refractive/diffractive lenses with low, intermediate, or high addition power, each providing distinct vision at different distances. With regard to the IOLs' focal features, the lenses are commonly categorized as multifocal (ie, bifocal and trifocal) and extended range of vision, the latter being commonly referred to as extended depth of focus (EDOF) IOLs.2,3 Effective extension of the depth of focus from distance to intermediate and near distances has been achieved with either diffractive-based bifocal IOLs that combine low addition and chromatic aberration correction,4–6 or more recently by means of a refractive-based IOL with alternate zones of different focus power and spherical aberration.7 Examples of these EDOF IOLs are the Tecnis Symfony (Johnson & Johnson Vision, Inc) and Mini Well (SIFI), respectively.
The Tecnis Eyhance IOL model ICB00 (ICB-IOL) (Johnson & Johnson Vision, Inc) is a new monofocal refractive lens aimed at extending depth of focus in comparison to a standard monofocal IOL. The manufacturer's goal with this new design is to offer the patient better visual acuity at intermediate viewing distances that are required for many important daily tasks while maintaining the quality and amount of vision the patient gets for far vision. In addition, the ICB-IOL should not produce more photic nuisance than a conventional monofocal IOL does,8 although this feature has yet to be confirmed by clinical studies. The ICB-IOL incorporates a modified aspheric anterior surface that differs from that of its predecessor, the Tecnis one-piece model ZCB00 (ZCB-IOL), an IOL that has been widely implanted throughout the world and yields well-known outcomes.9–12
The aim of this study was to evaluate in vitro the optical performance of the new ICB-IOL compared to the standard monofocal ZCB-IOL.
Materials and Methods
Two monofocal IOLs produced by the same manufacturer (Johnson & Johnson Vision, Inc) were included in this study: Tecnis-1 model ZCB00 (ZCB-IOL) and Tecnis Eyhance, model ICB00 (ICB-IOL). Both lenses share the same platform, have a biconvex design, and are made of the same ultraviolet light–absorbing hydrophobic material with a refractive index of 1.47 (at 35°). In addition, for a 6-mm eye entrance pupil (5.3 mm at the IOL plane),13 they both produce negative 4th-order spherical aberration of −0.27 μm.8,14 The studied lenses had the same refractive power (20.00 diopters [D]).
The ZCB-IOL is a standard monofocal lens with an anterior aspheric surface and a posterior spherical surface. The optical and clinical performance of this lens have been extensively reported in previous articles.9,10,12,14–17
The new ICB-IOL features a modified higher order aspheric anterior surface intended to produce a continuous power increase from the periphery to the center of the lens. More concretely, whereas power in the ZCB-IOL increases from the periphery to the center of the lens, the power change in the ICB-IOL is continuous but faster, with most of the change occurring in the central part of the lens.8 The posterior surface of the lens is spherical.
Optical Quality and Halo Assessment
The optical performance of the IOLs was evaluated with a test bench that has been described in detail elsewhere,17,18 and mainly consists of three parts: the illumination system, the model eye, and the image acquisition system (Figure A, available in the online version of this article). Because the ICB and ZCB IOLs share the same hydrophobic acrylic material of the Tecnis one-piece family of IOLs, their chromatic properties and spectral performances should be similar. We also have considered green illumination (530 ± 20 nm) exclusively in our experimental tests. The green LED source illuminated either a four-slit test or a pinhole object for modulation transfer function (MTF) measurements17,19 and halo assessment,20–22 respectively.
Optical set-up used for in vitro assessment of the intraocular lenses (IOLs).
The model eye was formed by an artificial cornea and a wet cell with balanced salt solution where the IOLs were placed. A variable aperture diaphragm, placed in front of the artificial cornea, was used as the entrance pupil to control the size of the beam on the artificial cornea and hence the level of corneal spherical aberration of the wavefront that impinged on the tested IOL (Figure A). The entrance pupil size also determined the beam size on the IOL plane (referred to hereafter as IOL-pupil).13 The ratio of IOL-pupil to entrance pupil was experimentally calibrated to be 0.56. From now on, all pupil diameters are referred to as the IOL plane.18,23 The cornea was an achromatic doublet (Lambda-X) that induced +0.175 μm of 4th-order spherical aberration for a 5-mm IOL-pupil. The model eye with the IOL formed an image of the test object at its best focus that was projected through a 10× infinity corrected microscope onto an 8-bit charge-coupled device (CCD) camera. All optical elements in the set-up were mounted in high-precision mechanical holders with three axis (x, y, and z) micrometer precision adjustments.
The MTF of the IOLs placed in the model eye was measured at their best focus plane for distance vision. This focus plane was experimentally determined as the one that maximized the MTF for a 3-mm IOL-pupil and was set as the origin for defocus (ie, 0.00 D).
The through-focus MTF curves were obtained between −3.00 and +1.00 D in 0.10-D steps with three IOL-pupil sizes: 2, 3, and 4.5 mm, the last two simulating photopic and mesopic illumination conditions in the clinic. Additionally, the optical quality was also evaluated with the area under the MTF metric (MTFa) given its potential significance as a preclinical metric.14,17 The MTFa was obtained by integrating the corresponding MTF values from 0 to 50 cycles/mm as reported elsewhere.14 The MTF was computed from the images of the four-slit object and, more specifically, from the modulus of the Fourier transform of the line spread function of each slit (ie, four MTF curves).19 The mean and standard deviation of the MTF and MTFa were derived from these four measurements. The higher the MTFa value, the better the optical quality of the IOL.
For the halo assessment, we determined the halo energy as illustrated in Figure B (available in the online version of this article). The image provided by the CCD camera (linear scale of intensity) consisted of the sharp and intense image of the pinhole (referred to from now on as the core) surrounded by a faint halo (Figure BA). When the image was displayed in logarithmic scale of intensity (Figure BB),16,21,22,24 which is a closer representation of how the human eye would see the image, the halo became evident.
(A) Image of the pinhole object in linear scale of intensity obtained at the best focus position of the ZCB-IOL (Johnson & Johnson Vision, Inc). (B) Same image but in logarithmic scale. The log-transform energy in the core and halo regions was obtained with Equation 1 inside and outside the black dashed line, respectively.
In the image in logarithmic scale, we computed the log-transform energy of the region of interest as:16,18,25
stands for the total image, the core region (inside the dash black circle in Figure BB
), or the halo (outside the dash black circle in Figure BB
= total, core or halo), n
is a pixel contained in the R
region, and e(n)
is the pixel gray level. For each pupil size, the log-transform energy obtained with Equation 1 in the core and halo regions (Ecore
, respectively) was compared to Etotal
and expressed as percentages. This normalization is necessary for quantitative comparison of the pinhole images recorded with different pupil sizes and thus with different energy. Moreover, because the human eye responds to differences of energy, we have also computed the non-normalized differences between the core energy and the halo energy, which estimates the weight of the halo in the image: the larger the difference of energy, the lower the weight of the halo. The uncertainty in the computed values of the energy was basically due to the precision in the determination of the size of the core. Assuming an uncertainty of ±1 pixel in the diameter of this region of interest, the highest error corresponded to the lower IOL-pupil (2 mm) and was 5% for both IOLs.
Wavefront Aberrations Measurement
The higher order wavefront aberrations of the IOLs were measured from IOL-pupils ranging from 2 to 5 mm,26 modifying the optical configuration of the test bench. As shown in the layout of Figure C (available in the online version of this article), the artificial cornea was removed from the set-up and the microscope and CCD camera were replaced by an aberration-free collimating lens and a Shack-Hartmann wavefront sensor (HASO 76; Imagine Optics). This sensor has an array of 76 × 100 microlenses, thus providing an excellent spatial resolution during wavefront sampling, and a maximum aperture size of 8.7 × 11.4 mm. Wavefront fitting was made with a linear combination of 32 Zernike polynomials from 3rd to 6th order. Each wavefront was measured three times to obtain the mean value and standard deviation of the Zernike coefficients.
Optical set-up used for measuring the wavefront aberrations of the intraocular lenses (IOLs).
Finally, we also computed the wavefront aberrations of the achromatic lens used as an artificial cornea. They were obtained versus IOL-pupil size by ray-tracing simulation using a dedicated software (Zemax OpticStudio, Zemax Europe Ltd) and the lens parameters (curvature radius, thickness, and refraction index) provided by the manufacturer.
Figure D (available in the online version of this article) shows the influence of pupil size on the MTFs of both IOLs. For IOL-pupil sizes of 2 and 3 mm, the MTF curves of the ZCB-IOL were nearly diffraction limited, whereas those of the ICB-IOL were lower, indicating worse optical quality. For larger pupils, the MTF curves of both IOLs tended to decrease and get closer. Overall for both IOLs, it is worth remarking the close coincidence of their curves in the range of spatial frequencies of primary interest (0 to 50 cycles/mm).14,17
Modulation transfer function (MTF) curves for intraocular lens (IOL)-pupil sizes ranging from 2 to 5 mm at the best focus of the (red) ZCB-IOL and (blue) ICB-IOL (Johnson & Johnson Vision, Inc). The inserts show the MTF curves in the range of spatial frequencies (0 to 50 cycles/mm) used to compute the area under the modulation transfer function (MTFa) metric.
The MTFa metric versus IOL-pupil size is shown in Figure 1. For pupil sizes smaller than 3.5 mm, the ICB-IOL had a smaller MTFa than the ZCB-IOL. For larger pupils, the MTFa values of both IOLs were similar within the experimental uncertainty and tended to decrease, the latter showing the deleterious influence that the increase of the pupil size has on optical quality.
Area under the modulation transfer function (MTFa) values (average ± standard deviation) for intraocular lens (IOL)-pupil sizes ranging from 2 to 5 mm of the (red) ZCB-IOL and (blue) ICB-IOL (Johnson & Johnson Vision, Inc). These values were obtained on integration of the corresponding MTF curves between 0 and 50 cycles/mm.
The through-focus MTFa curves of the ZCB-IOL and ICB-IOL, obtained with IOL-pupils of 2, 3, and 4.5 mm, are shown in Figure 2. Given the monofocal design of both lenses, the curves showed just one peak of maximum MTFa that corresponds to the best focus of the lenses for distance vision.
Through-focus area under the modulation transfer function (MTFa) curves of the (red) ZCB-IOL and (blue) ICB-IOL (Johnson & Johnson Vision, Inc) obtained with intraocular lens (IOL)-pupil sizes of (A) 2, (B) 3, and (C) 4.5 mm. The arrows indicate the (A) myopic and (C) hyperopic shifts of the MTFa peaks. D = diopters
For both IOLs, the smaller the pupil, the wider the MTFa peak, proving that regardless of the IOL design, there was an effect of focus extension produced as a consequence of reducing the pupil size. With a 2-mm IOL-pupil, the maximum MTFa of the ZCB-IOL was higher than that of the ICB-IOL (45.69 ± 0.23 vs 40.61 ± 0.49) and, interestingly, there was a myopic shift of −0.40 D in the position of the MTFa peak of the ICB-IOL (Figure 2A). Even at this position, however, the MTFa value reached by the ICB-IOL was not higher than that of the ZCB-IOL. With a 3-mm IOL-pupil (Figure 2B), the maximum MTFa of the ZCB-IOL was still slightly higher (46.11 ± 0.58 vs 42.79 ± 0.77 in the case of the ICB-IOL), whereas such differences between both IOLs practically vanished with the 4.5-mm IOL-pupil (Figure 2C). With this pupil, the MTFa peak of both IOLs shifted slightly toward hyperopic defocus (+0.20 D).
The cornea of our model eye induced positive 4th-order spherical aberration (Figure 3) to mimic the natural aberration of the human cornea. The maximum spherical aberration was +0.175 μm for a 5-mm pupil at the IOL plane. Although this value is somehow less than the amount reported on average for the human cornea, +0.27 μm for a 6-mm entrance pupil (5.3 mm at the IOL plane),27 Wang et al28 found that 15.4% of their patients had corneas with spherical aberration values smaller than +0.2 μm.
4th-order spherical aberration versus intraocular lens (IOL)-pupil size obtained separately for the artificial cornea (black bars), and each IOL: ZCB-IOL (red bars) and ICB-IOL (blue bars) (Johnson & Johnson Vision, Inc).
The most significant higher order aberration found with both IOLs versus pupil size was negative 4th-order spherical aberration (Figure 3), logically intended to compensate for the positive corneal spherical aberration. With the 3-mm IOL-pupil, the wavefront aberration of the ICB-IOL also showed small contributions of positive 6th-order (0.028 ± 0.001) and negative 8th-order (−0.018 ± 0.001) spherical aberration. The rest of the higher order aberration terms were negligibly small for all pupil sizes.
For IOL-pupil sizes smaller than 3.5 mm, the ICB-IOL lens had more negative spherical aberration values than the ZCB-IOL. For instance, with a 2-mm IOL-pupil the spherical aberration of ICB-IOL is, in absolute value, 3.7 times larger than the spherical aberration of the ZCB-IOL (−0.056 ± 0.003 vs −0.015 ± 0.003 μm, respectively). For larger pupils, both lenses had similar spherical aberration values.
The images of the pinhole as a function of the pupil size, obtained with the model eye including either the ZCB-IOL or ICB-IOL, are shown in logarithmic scale in Figure E (available in the online version of this article) because it better approaches human perception. With both lenses, halos around the pinhole image could be hardly observed for IOL-pupil sizes up to 3 mm, but became apparent for larger IOL-pupil sizes of 4 and 5 mm.
Images of the pinhole object formed by the model eye including either ZCB-IOL or ICB-IOL (Johnson & Johnson Vision, Inc) at their best focus for increasing pupil sizes. The images are displayed in logarithmic scale of intensity.
The relative energy of the core and halo to total energy (all calculated with Equation 1), are shown versus IOL-pupil size in Figures FA–FB, respectively (available in the online version of this article). On the other hand, Figure FC shows the non-normalized energy difference between the core and halo regions.
Relative core (A) and halo energy (B) to total energy (all calculated with Equation 1) and (C) non-normalized energy difference between core and halo, versus IOL-pupil size, obtained with the ZCBIOL (red bars) and ICB-IOL (blue bars) (Johnson & Johnson Vision, Inc).
In the case of the ZCB-IOL and for IOL-pupils ranging from 2 to 3.5 mm, a constant, high fraction of the energy (approximately 60%) was correctly focused on the core, whereas with the ICB-IOL we found less energy correctly focused. Conversely, there was more energy spread to the halo with the ICB-IOL. Not surprisingly, the ICB-IOL showed smaller values of the energy difference in this pupil range than the ZCB-IOL (Figure FC), indicating images with more significant halos with ICB-IOL. This trend could already be noted in Figure E, where the halo for the 2-mm IOL-pupil with the ICB-IOL was clearly more visible and larger than that of the ZCBIOL, but was only slightly more visible and larger for the 3-mm IOL-pupil. The differences between the two IOLs tended to decrease for increasing pupils. As such, for mesopic and scotopic pupils (≥ 4 mm), both IOLs exhibited close results. The halo energy for both IOLs increased with pupil size (Figure FB), reaching similar maximum values for the 5-mm IOL-pupil: 71.9% ± 1.8% (ZCB-IOL) and 68.5% ± 1.7% (ICB-IOL). Closely related, the smaller value of energy difference occurred for this IOL-pupil (Figure FC), which accounts for the significant halos observed in Figure E with both IOLs.
A summary of the results versus IOL-pupil size is presented in Table 1.
MTFa, Relative Core Energy and Halo Energy, and Non-normalized Energy Difference Between the Core and Halo Obtained Versus IOL-Pupil Size With the ZCB00 and ICB00 IOLs
To our knowledge, this is the first study that evaluated in vitro the optical performance of the new monofocal Tecnis Eyhance model ICB00 (ICB-IOL), whose optical design aims to extend the depth of focus in comparison to a standard monofocal lens and to provide better intermediate vision while keeping similar distance vision and comparable incidence of photic phenomena (glare and halo). A meaningful comparison has been done using the Tecnis one-piece model ZCB00 (ZCB-IOL) as the standard monofocal lens, because both IOLs, manufactured by the same company, share basic features such as platform and material. Therefore, the fundamental difference between both designs is the modified aspheric anterior surface of the ICB-IOL that is referred to by the manufacturer as a continuous higher order aspheric surface. We have found that this modification in the optical design of the ICB-IOL has a measurable impact on the optical quality, the spherical aberration, and halo energy for relatively small IOL-pupil sizes (< 3.5 mm). In contrast, for larger IOL-pupils (≥ 3.5 mm) the results of the new ICB-IOL tend to be similar to those of the standard monofocal ZCB-IOL.
The negative values of spherical aberration versus pupil obtained with the ZCB-IOL are in good agreement with previous results reported with multifocal and EDOF Tecnis IOLs that share the same aspheric design.26,29,30 The measured experimental values of −0.28 ± 0.01 μm (ZCB-IOL) and −0.27 ± 0.01 μm (ICB-IOL) for a 5-mm IOL-pupil would fully compensate for the +0.27 μm value of the corneal spherical aberration of a representative average human cornea.27,28 Interestingly, we have found differences between the spherical aberration values of both IOLs for IOL-pupils smaller than 3.5 mm and it is in this range of small pupils where worse MTF curves and lower MTFa values are obtained for the new ICBIOL in comparison to the ZCB-IOL. Moreover, comparing the through-focus MTFa curves of the two IOLs, the largest differences occurred with the smallest IOL-pupil (2 mm). For this pupil in particular, the MTFa curves of both IOLs are considerably broader, proving that small pupils are an effective strategy to expand depth of focus in general,31 although they require good lighting. More important, in comparison to the standard ZCB-IOL, the MTFa curve of the new ICB-IOL showed a myopic shift of −0.40 D.
To explain this result, we recall that for small pupils the ICB-IOL induced more negative spherical aberration than the standard ZCB-IOL, and they were larger (in absolute value) than the spherical aberration of the cornea. The model eye with the ICB-IOL must have a remaining negative spherical aberration as a result of the insufficient compensation between the positive and negative spherical aberration values of the cornea and ICB-IOL, respectively. In a converging optical system with negative spherical aberration, the paraxial rays have more dioptric power than the peripheral rays. Thus, in eyes with a relatively large pupil (eg, mesopic illumination conditions) and negative spherical aberration, the emmetropia condition is achieved when the circle of least confusion lies on the retina, with the paraxial and peripheral rays focused in front of and behind the retina, respectively. Because the eye focusing with small pupils relies only on the paraxial rays, there would be a myopic shift of the best focus condition, as experimentally observed in the ICB-IOL (Figure 2A). This result confirms the power increase from the periphery to the center in the design of the new ICBIOL. More concretely and in comparison to the standard ZCB-IOL, there is an additional power close to +0.50 D in the central 2-mm region of the lens, which is based on the larger negative spherical aberration of the ICB-IOL with this IOL-pupil. Because the differences between the spherical aberration of the two IOLs decreased from 2 to 3 mm and were practically equal for IOL-pupils larger than 3.5 mm, one can conclude that the aspheric curvature of the ICB-IOL originates the +0.50 D additional power in the central 2- to 3-mm region and decreases toward the periphery of the lens. This is the basis for the intermediate performance and extension of the depth of focus with this new design, which could improve intermediate vision, especially in high light conditions and/or small pupils.
Several studies have additionally shown that higher order aberrations, particularly spherical aberration, help to increase the depth of focus.32,33 However, the addition of spherical aberration to increase the depth of focus has the potential drawback of lowering the visual acuity.34 In the clinic, Rocha et al35 and Marcos et al34 found greater depth of focus in patients with the implantation of spherical IOLs (ie, eyes with higher spherical aberration) than in patients with aspheric IOLs that reduced the total spherical aberration of the eye. However, other studies failed to find statistically significant differences in depth of focus between patients implanted with aspheric IOLs with negative spherical aberration, aspheric aberration-free IOLs, and spherical IOLs.36,37 Neither did Gong et al38 find differences in the depth of focus of eyes with different amounts of 4th-order corneal spherical aberration implanted with the same aberration-free IOL model. More recently, Bellucci et al39 reported, in comparison to an aspheric monofocal IOL, greater depth of focus with the Mini Well IOL, a lens based on alternating positive and negative spherical aberration in the central 3-mm optical zone. Camps et al26 measured large negative 4th-order (−0.13 ± 0.01 μm) and positive 6th-order (0.12 ± 0.01 μm) spherical aberration within this 3-mm zone of the Mini Well IOL. Because the experimental spherical aberration values of the ICB-IOL are much smaller, it can be ruled out that the depth of focus expansion with this new monofocal lens be based on a spherical aberration design.
With regard to the differences in optical quality between the ZCB-IOL and ICB-IOL accounted for by the MTFa metric and its implication in the clinic, Alarcon et al14 found a high correlation between the MTFa metric and clinical visual acuity of pseudophakic patients. The results led the authors to suggest that the MTFa metric could predict clinical average visual acuity, thus becoming a preclinical metric. More recently, Vega et al17 stated that the estimation of achievable visual acuity, as a non-linear function of variable MTFa, showed limiting behavior for IOLs with larger MTFa values (ie, lenses with higher imaging quality). As a consequence, beyond an MTFa threshold, visual acuity tended asymptotically to be the best value clinically achieved in the patients, and any further increase in the imaging quality of an IOL (ie, MTFa values above the threshold) did not translate into visual acuity improvement. With the 3-mm IOL-pupil, the best MTFa of both IOLs (ICB-IOL = 43.15 ± 0.43, ZCB-IOL = 46.10 ± 0.33) was far larger than the reported threshold for the MTFa (approximately 20),17 and one would not expect the best visual acuity with the new ICB-IOL model to be worse than that of the standard ZCB-IOL. Nevertheless, clinical studies are needed to either confirm or refute this prediction.
Concerning the halos, the ICB-IOL lens showed similar values of energy on the optical bench correctly focused on the core region (or conversely spoiled in the halo) for mesopic and scotopic pupils (≥ 4 mm) compared to the standard monofocal lens. Thus, the new design of the ICB-IOL is more likely to induce a similar level of photic phenomena, if any, as the ZCBIOL, although clinical studies are needed to confirm or refute this prediction.
Regarding the differences found in the spherical aberration within the small central area between the two IOLs, Taketani and Hara40 showed that spherical aberration was negatively correlated with dioptric power in the case of the Tecnis ZA9003 aspheric IOL (a lens made of the same material and with the same optical design as the Tecnis ZCB00 IOL of our study). Additional work is still necessary to check whether the differences of spherical aberration we have found between the ICB-IOL and ZCB-IOL with lenses of +20.00 D are maintained for other dioptric powers. This would be especially relevant in the case of low and high dioptric values because it could provide valuable information on the performance of the new ICB00 design in the case of highly myopic and hyperopic eyes, respectively.
Finally, our results have shown that the ICB-IOL is a modified monofocal lens with 0.50 D of additional power in the central 2-mm zone. With regard to the potential impact that this new design may have on near vision, one could argue that pupil miosis would occur when looking at near, a situation under which the ICB-IOL has shown the capability, in the optical bench, of producing a myopic shift related to the power increase on the central region of the lens. However, near tasks at 30 to 40 cm that demand good quality of vision require add powers of 3.00 to 2.50 D, which are significantly larger than the maximum additional power (approximately 0.50 D) that the lens is able to provide. Furthermore, despite the potential to extend the range of vision of the ICB-IOL, it is still a monofocal design. Preliminary clinical results (unpublished data, N. Garzón) indicate that the ICB-IOL defocus curve is slightly broader than that of the standard ZCB-IOL, but still has the typical “monofocal shape” with a single visual acuity peak for distance vision (0.00 D defocus). Thus, it would not be realistic to expect that the ICB-IOL could compete in near vision with multifocal IOLs that provide the pseudophakic patient with an additional visual acuity peak at near.
According to our findings, the design strategy of the new ICB-IOL to extend depth of focus is based on a continuous power increase toward the center of the lens as a result of the increased amount of negative spherical aberration that occurs in the central region of the lens (IOL-pupils smaller than 3.5 mm). The halos measured on the optical bench are comparable to those of a standard monofocal IOL.