Optical Behavior of an Enhanced Monofocal Intraocular Lens Compared with a Standard One

Abstract

The aim of this work was to compare an enhanced monofocal (RayOne EMV RAO200E, Rayner) and standard monofocal (RayOne RAO600C Aspheric, Rayner) intraocular lenses (IOLs) for three nominal powers (+10.00 D, +20.00 D and +30.00 D) as a function of the optical aperture diameter (pupil diameter) using a commercial Schlieren phase-shifting deflectometer NIMO TR1504 (Lambda-X, Belgium). From the wavefront maps measured by this instrument, the radial power profiles, the spherical aberration coefficients of the Zernike polynomial expansion (as a function of the optical aperture radius), and the root-mean-square (RMS) of the high-order aberrations (HOAs) were obtained and analyzed by comparing the two models. The results showed that the effective added power that could be obtained with the enhanced model depended directly on the pupil size and the power of the IOL implanted. The higher additions were achieved with the higher nominal IOL powers. The relationship between the pupil diameter, the corneal aberration of the patients and the power profile of these IOLs could have a crucial implication on the far distance and the final effective addition. However, it is important to note that these findings should be clinically validated through the implantation of these models in patients’ lenses.

1. Introduction

In recent decades, the visual needs of the general population have significantly changed due to the incorporation of electronic devices in daily tasks, which increases the intermediate- and near-vision demands. In parallel with these changes, new intraocular lens (IOL) designs have been constantly evolving to meet these needs, with models appearing on the market that offer good functional vision at different distances. Despite such advances, currently, monofocals are still the most commonly implanted IOLs [1]. The reasons for not selecting a multifocal IOL design include disadvantages such as loss of retinal image contrast; greater vision loss when there is a residual refractive error; or the appearance of dysphotopsias, such as halos or glare [2,3], particularly for diffractive designs.

To overcome these problems, IOL manufacturers have focused on what is currently referred to as “enhanced monofocal lenses”. This “enhanced monofocal lens” design is based on a monofocal refractive lens with an added modification, such as a certain amount of induced spherical aberration, targeting a mildly deeper depth of focus. In this way, it is possible to improve intermediate vision while avoiding the shortcomings of multifocal lenses [4,5]. Such an increased amount of intermediate vision that the patient perceives does not usually fulfill the requirements established for these models to be considered extended range and, hence, the preferred terminology is enhanced monofocal lens [6]. While multifocal or extended-range lenses usually feature diffractive optic designs to obtain different focal points, with enhanced monofocal lenses, a certain amount of induced spherical aberration, whether positive or negative, is often a characteristic of these designs [7].

Given that the spherical aberration of any optical system is, by definition, dependent on the height of incoming light rays with respect to the optic axis, and therefore, on the diameter of the entrance pupil of the system, it is important to measure the power profiles of these lenses to understand their impacts on vision. The main aim of this study was thus to compare enhanced monofocal and standard monofocal IOLs of identical platform and material in terms of their radial profiles, spherical aberration values and high-order aberrations (HOAs). We also examined whether the nominal powers of these IOLs influenced the added power that can be obtained with these designs by assessing +10.00 D, +20.00 D and +30.00 D lenses. These two objectives could be key to deciding IOL indications and the management of patients implanted with this type of IOL.

2. Materials and Methods

This study was conducted at the laboratory facilities of the Optics and Optometry Faculty of the Universidad Complutense de Madrid, Spain, under stable conditions of temperature and humidity.

2.1. Intraocular Lenses

The IOL models employed were RayOne RAO600C and RayOne EMV RAO200E manufactured by Rayner Intraocular Lenses, Ltd. (Worthing, UK). Lenses of +10.00 D, +20.00 D and +30.00 D of the two models were evaluated.

RayOne RAO600C is a standard aspherical monofocal lens, which is biconvex when its power is positive and biconcave when negative. Its anterior surface is aspherical to induce neutral spherical aberration. According to the information provided by the manufacturer, the lens has a blended edge region that gradually reduces longitudinal spherical aberration to maintain contrast in the peripheral viewing area. This design would help to increase the range of functional vision while minimizing the potential for dysphotopsia at all pupil sizes [8].

The IOL RayOne EMV RAO200E, which was launched by Rayner in 2021, is described as an enhanced monofocal in its portfolio. According to the information provided by Rayner, its design incorporates an anterior aspheric surface and a posterior surface that can be either aspheric or spherical, depending on the dioptric power. Ref. [9] Rayner mentions it is optimized for monovision, and thus, recommends the pursuit of emmetropia in the dominant eye and a residual myopia target in the non-dominant eye. It also states that a myopization of 1.00 D in the non-dominant eye offers an extended depth of focus of 2.25 D in binocular conditions [10].

The two lenses analyzed share the same platform and material. They are made of hydrophilic acrylic (26%) with a refraction index of 1.46 at 35 °C and an Abbe number of 56. The platform diameter is 12.50 mm with an optic zone of 6.00 mm. The design of their non-angulated haptics is closed-loop with anti-vaulting haptic technology and 360° of square edge [10].

2.2. Power Profile Mapping and Wavefront Analysis

The device NIMO TR1504 (LAMBDA-X, Nivelles, Belgium) used in this study measures the refractive effective power and complete wavefront aberrations of monofocal, toric and multifocal IOLs. The device is based on the phase-shifting Schlieren principle [11]. By combining this principle with the phase-shifting method generally used in interferometry [12,13,14], the NIMO system is able to measure light beam distortions and use these to calculate the power characteristics of optical lenses; it also conducts wavefront analysis, considering up to order 13 of the Zernike coefficients. The measurement light source shows a radiance peak at 546 nm, which is close to the spectral relative luminance efficacy peak of the human visual system, located at 555 nm under photopic conditions [15].

Once the fringes pattern has been captured, NIMO TR1504 computes data and allows for a detailed measurement of the power distribution within any selected optic zone; the instrument’s software also enables wavefront analysis via Zernike polynomial decomposition at different optical aperture diameters of the lens [16]. In this study, the maximum analyzed optical zone for each IOL was set to 4.5 mm, which allowed us to provide information in a range of real-life scenarios, including pupil sizes up to this value.

To compute the addition, the difference between the radial power measurements obtained and the nominal power provided by the manufacturer of the lens was calculated.

The parameters measured in this study were radial power profiles expressed in diopters; the root mean square (RMS) of the total high-order aberrations (HOAs) (from the third to thirteenth order); and the coefficient of the primary ${(Z}_4^0)$, secondary ${(Z}_6^0)$ and high-order (${(Z}_8^0)$, ${(Z}_{10}^0)$ and ${(Z}_{12}^0)$) spherical aberrations expressed in microns for different optical aperture diameters. The RMS was studied for 3 mm and 4.5 mm.

The measurements were carried out following the protocol published by Gomez- Pedrero et al. [17] for IOLs. In order to prevent surface distortion and loss of moisture during the measurements, intraocular lenses were placed in a quartz cuvette without aberrations and immersed in a saline solution, as specified in the NIMO TR1504 manual. Without a lens present, the chamber maintained uniform illumination. Conversely, when a lens was introduced into the instrument’s object plane, the light rays originating from the source underwent a shift or deflection. This, in turn, modulated the intensity of light reaching the pixels of the high-resolution CCD camera, creating what is commonly referred to as Schlieren fringes. The higher the power of the test lens, the more pronounced the deviation of light rays and the increased occurrence of these fringes. Employing phase-shifting techniques, a cartographic representation of both horizontal and vertical components of ray deflection was generated. The power maps corresponding to these were derived from the ray deflection map. Ten measurements were conducted for each lens in every assessed area to derive average values. These measurements were executed without any filters, treating the lens as a thin lens, thereby obviating the necessity to know the central thickness and curvature, as reported by Pedrero et al. [17].

2.3. Data Analysis

The NIMO TR1504 generated a CSV-formatted file for each measurement, which was then input into an R-script specifically designed to directly extract power and aberrometric profiles. Statistical analysis and graphical representation were executed using Rstudio with R version 4.2.2, alongside the ggplot2 graphic package.

3. Results

3.1. Power Profile Mapping

Figure 1 provides the average radial power maps for the three lens powers tested and compares the standard monofocal lens (pink) with the enhanced monofocal (EMV) lens (blue).

It may be seen in Figure 1 that the average radial power of the standard monofocal lens remained more stable across the whole optic zone diameter, especially for lower powers. For the enhanced lens, we found that the power varied along the lens diameter, where it was more positive toward the lens periphery. This behavior is characteristic of an optical system presenting positive spherical aberration.

Figure 2 shows the difference between the measured average radial power and nominal power according to the optic zone, i.e., the added power over the nominal power for each IOL’s optic zone.

The maximum effective power increases observed for the enhanced model for an optical zone of 3.00 mm lens were 0.69 D, 0.95 D and 1.62 D for the nominal powers +10.00 D, +20.00 D and +30.00 D, respectively. For an optical zone of 4.50 mm, the additions to the nominal powers were 0.70 D, 1.19 D and 1.78 D for the +10.00 D, +20.00 D and +30.00 D lenses, respectively. Notice that we computed the addition as the difference between the power measured at the border of the optical zone and the nominal power of the lens (see Figure 2).

3.2. Wavefront Analysis

Figure 3 shows the sum of the actual values (neither RMS nor PV) of the coefficients of the spherical aberration for the different optic zones according to the nominal power of each model, as represented by the sum of the coefficients of the radially symmetric Zernike polynomials of orders 4 to 12 ${(Z}_4^0+Z_6^0+Z_8^0+Z_{10}^0+Z_{12}^0)$ in microns.

Figure 4 shows the weight of each Zernike coefficient representative of spherical aberration from polynomial orders 4 to 12 for the three nominal powers of the enhanced monocular IOL in microns. These results indicate that both the primary and secondary spherical aberrations were high, while the spherical aberration of order 8 was low, and those of orders 10 and 12 were practically null. It is interesting to notice that the resulting spherical aberration was positive in the central part of the IOL, up to an optic zone of approximately 4.75 mm for the +10.00 D and +20.00 D IOLs and 4.50 mm for the +30.00 D IOL (see Figure 3), turning into negative values for larger zones, which was driven mainly by the secondary spherical aberration negative values induced for optic zones higher than 4 mm (see Figure 4).

Figure 5 shows the root-mean-square (RMS) values in microns of high-order aberrations (polynomials of orders 3 to 13) of two optic zones (3 mm and 4.5 mm) for the three powers of both monofocal lens designs.

4. Discussion

Optical aberrations are imperfections of an optical system that may degrade the image quality of that system. In the case of the eye and human optical system, this translates into reduced quality in the perception of the object being viewed. Ocular aberrations, which are measured at the exit pupil of the eye, arise from their different ocular structures, with pupil size being a main determining factor. Aging also causes a marked increase in the eye’s total aberrations, and, to a lesser extent, in those of the cornea. This means that in most younger subjects, total ocular aberrations are lower than corneal aberrations, while in older subjects, the reverse occurs [18,19]. Thus, while in young eyes, some corneal aberrations are offset by those of the natural lens, in aged eyes, this situation is reversed [19,20]. After the implantation of an IOL, the aberration balance of the pseudophakic eye changes with respect to the preoperative eye. This is due to (1) the corneal aberrations being changed by the surgical procedure and (2) the geometry, thickness and location of the IOL are different from that of the removed lens, and thus, the aberrations of the IOL are different from that of the natural lens, thus changing the balance of aberrations for the whole eye [21]. Regardless of age, an optical system’s aberrations are aperture-size-dependent such that in the eye, these increase as pupil size increases.

Most manufacturers use a technology based on spherical aberrations in their enhanced monofocal lens designs. Compared with most standard monofocal lenses, for which the approach is usually to try to reduce the total spherical aberration of the system to improve the vision quality [22,23], with these newer designs, the objective is the opposite. That is, the aim is to induce spherical aberration, whether positive or negative, to increase the depth of focus and achieve better intermediate vision than with conventional monofocals. To do so, it is necessary that at least one of the lens surfaces be a highly aspherical surface. It should be considered that positive spherical aberration induces an extra positive power in the lens periphery compared with its central zone. Conversely, negative spherical aberration leads to greater power in the central zone. Whatever induced spherical aberration occurs, either positive or negative, it should be of such a magnitude that it has no significant impact on the image quality so that intermediate vision is improved without sacrificing the quality of distance vision. Bakaraju et al. [24] described that for presbyopia correction strategies based on the deliberate induction of aberrations designed to increase the depth of focus, positive spherical aberration values have the same power as their negative equivalents.

As far as we know, this is the first study in which standard and enhanced monofocal lenses of the same material and with the same platform were compared in terms of radial and spherical aberrations and RMS HOAs for different powers.

Here, we found that the radial powers for the standard monofocal IOLs of powers +10.00 D and +20.00 D were stable from the lens center to the periphery, while for the +30.00 D lens, we observed a lower effective power than the nominal in the central zone (0.00 mm). For the three enhanced monofocal IOLs tested, the radial power was noted to increase toward the periphery, while in the central zone (0.00 mm), the power was lower than nominal values in all three cases by an amount ranging from 0.2 to 0.5 D. We observed that the higher the nominal lens power, the greater the increase in power produced at the periphery.

In deflectometry measurement systems, the power obtained is derived from the reflection. In these systems, a noisy signal is generated at the origin of the coordinates. However, as shown in reference [17], this central noise does not impair the reliability of the instrument. Therefore, we can assume that compared with the enhanced EMV lens, the monofocal one presents a much flatter profile close to the optical center of the lens.

The abrupt discontinuity in all the enhanced lens profiles in the peripheral zone, which was approximately +/− 2.00 to 2.25 mm from the center (at the edge of the optical zone between 4 and 4.5 mm), could have been caused by the blending between the optic and the peripheral zones and is of little clinical relevance [25].

Despite this reduced power observed in the central zone, we believe this would not affect patient vision in the case of such a small pupil due to the stenopeic or pinhole effect that produces an increased depth of field, thus offsetting any retinal image blur caused by that possible refraction defect. The increased peripheral power of the enhanced model would produce the effect of an addition directly related to pupil size and the lens’ nominal power, giving rise to more added power the greater the nominal power. Accordingly, the EMV monofocal lens is able to enhance intermediate vision by offering an overplus correction of 0.5 D to 1.00 D for pupil sizes of 2 mm and 0.8 D to 1.5 D for 4 mm pupils according to the lens’ nominal power. These results align with recent publications by Schmid et al. [26] and Alarcon et al. [27], who, in their optical bench studies, reported strong correlations between the performance of the RayOne-EMV lens, increased pupillary size, improved intermediate simulated VA, and reduced distance simulated VA and MTF.

Spherical aberration is, by definition, a variation in the effective power of an optical system from the center to the periphery that remains constant across any lens meridian. Thus, this wavefront presents radial symmetry, with no dependence on the angular direction. For this reason, it is defined by central polynomials of the Zernike pyramid, which has 0 azimuthal orders, and therefore, these polynomials do not show dependence on the angular or azimuthal direction. It is common to identify spherical aberration with the so-called primary spherical aberration, as represented by the Zernike polynomial $Z_4^0$. However, this is not the only polynomial that contributes to the spherical aberration of an optical system, as all central azimuthal orders above the fourth can do so, which is referred to as secondary spherical aberration. It is because of this that in this study, we considered all central polynomials of the Zernike pyramid so that we could better characterize the spherical aberration induced by the EMV lens examined. We detected significant contributions of the polynomials $Z_4^0, Z_6^0$ and ${ Z}_8^0$, although the SA of the eighth order had less influence and null contributions of $Z_{10}^0$ and $Z_{12}^0$ were found. As may be seen in Figure 3, the standard monofocal lens showed almost no aberration across the whole optic zone for the three nominal powers examined. However, for all three powers considered, the enhanced monofocal designs showed a similar aberration pattern such that their central 2 mm featured neutral spherical aberration and this gradually increased to 4 mm, reaching a value of around 0.05 microns, but with a significant drop for an optic size above 4 mm, where the SA changed to negative values (see Figure 3 and Figure 4). This variation in spherical aberration from the central 2 mm toward the periphery (up to 4 mm) is consistent with the increase in positive power observed in the radial power map.

Our analysis of RMS HOAs for the two IOL models revealed a stepwise behavior of the standard monofocal whereby, for both 3 mm and 4.5 mm pupils, the higher the lens power, the greater the HOA. The behavior of the enhanced monofocal was, however, flatter in that less variation was produced in aberrations measured for the three lens powers and for both pupil sizes. The increment in the HOA at the enhanced IOL periphery was a result of its aspherical design. However, in photopic conditions, this increment has no effect on patient vision for normal pupil diameters.

Rayner markets this RayOne EMV RAO200E IOL, stressing it is possible to achieve good binocular outcomes when implanted using a monovision approach, leaving the dominant eye emmetropic and the contralateral eye with a slight myopia of 1.00 D. The fact that in the lens center, radial power is lower than nominal and that aberration is neutral could lead to less defocus than clinically expected. This is because the effective power would induce this central defocus of some 0.5 D, and this would not be increased by the primary spherical aberration.

Limitations

At the time of completing this study, no publications in the literature presented clinical data obtained with the RayOne EMV RAO200E IOL, preventing a direct comparison or correlation between our optical bench data and clinical data from patients with this lens implanted.

5. Conclusions

The present study revealed clear differences between Rayner’s standard and enhanced monofocal models. These differences took the form of greater central–peripheral power variation shown by the enhanced lens, which was even greater the higher the intraocular lens’ nominal power and the larger the optic zone considered. It should, nevertheless, be considered that these differences were observed in vitro and once implanted in the eye, could be affected by the corneal aberrations of each individual or by the Stiles–Crawford effect, among other factors. The conclusions to be drawn from our results are that the effective added power that can be obtained with the enhanced model depends directly on the pupil size and the power of the IOL implanted. The higher additions were achieved with the higher nominal IOL powers. Furthermore, the relationship between the pupil diameter, the corneal aberration of the patients, and the power profile of these IOLs could have a crucial implication on the far distance and the final effective addition. However, it is important to note that these findings should be clinically validated through the implantation of these models in patients’ lenses.