Enhanced Depth of Focus Intraocular Lenses: Through Focus Evaluation of Wavefront-Shaping versus Diffractive Optics

Enhanced Depth of Focus (EDoF) (IOLs) had been successfully introduced some years ago for cataract surgery or refractive lens exchange. Usually, with EDoF IOLs [1], patients will benefit from spectacle independence for distance and intermediate vision. According to the American Academy Task Force’s definition [1], with these IOLs, monocular depth of focus needs to be at least 0.5 D greater than that of monofocal IOLs at logMAR 0.2 in the defocus curve, distance-corrected intermediate visual acuity has to be superior to monofocals, and 50% of the eyes have to achieve at least logMAR 0.2 in the intermediate distance (66 cm).

The first EDoF IOL worldwide was the Tecnis Symfony ZXR00 by AMO, now Johnson & Johnson Vision [2, 3], with a sophisticated echelette design and a correction of longitudinal chromatic aberration. However, some halos and glare are always present in any diffractive optics [4, 5]. A new kind of nondiffractive, wavefront-shaping EDoF IOLs has been introduced very recently to cope with that disadvantage.

We assessed the through focus modulation transfer function (MTF) of two latest nondiffractive, wavefront-shaping EDoF IOLs on the optical bench: Alcon AcrySof IQ Vivity DFT015 and Bausch & Lomb LuxSmart Crystal. Both IOLs were compared to the diffractive Johnson & Johnson Tecnis Symfony ZXR00. Measurement of the MTF is a common and standard assessment of the optical quality of an IOL. This investigation of the through focus performance of these IOLs is a subsequent assessment to our comparison of the through frequency MTF of the same IOLs in different conditions of alignment, published very recently [6]. Deductive to a thorough research in literature, such comparison of EDoF IOLs on the optical bench had not been done before but is of high importance for lens surgery. The new class of wavefront-shaping IOLs is supposed to extend the range of vision with a very new optical design. Diffractive optics with concentric rings, refractive optics with segmental near add producing some pseudo-coma, accommodating IOLs or pinhole approaches are commonly used to produce some depth of field [7‒9]. In contrast, the novel wavefront-shaping optics use increased spherical aberration [10‒13] to extend the range of vision. For Vivity and LuxSmart, spherical aberrations of different orders and sign [10] are generated by a complex surface modulation of the central optical zone resulting in a markedly increased negative spherical aberration. It is of high clinical interest for the cataract surgeon to assess and compare the through focus performance of these three different EDoF IOLs on the optical bench. With both kind of EDoF IOLs available now, a more specific decision can now be made which IOL will be implanted, in which patient, and in which individual eye. IOL optical design can be chosen up to patient’s expectations and needs, for example, working or driving in mesopic conditions. IOL design should be adapted to corneal topography and asphericity. Clinical evaluation and defocus curves have to be established, but a company-independent, standardized evaluation of IOL properties on the optical bench is crucial to understand the function of the IOL to be implanted.

All three single-piece IOLs had a nominal power of 22 diopters with a 6 mm optics. The properties of the IOLs are given in detail in Table 1, the functional design of both novel IOLs, Vivity and LuxSmart, in Figure 1 and 2, respectively.

For all IOLs tested, the complex aspherical design is applied to the anterior surface, while the achromatic echelette blaze of Symfony IOL is applied to the posterior surface. Through focus measurements were done by one single, experienced specialist at Trioptics GmbH, Wedel, Germany. Trioptics’ well-established OptiSpheric^® IOL PRO 2 imaging test bench was used according to the ISO 11979-2 standard [14, 15]. Power measurements can be performed with an accuracy of 0.3%.

We used an ISO 11979-2:2014, type 2 eye model (ISO-2, spherical aberration +0.28 µm for a 6-mm pupil; wavelength 546 nm) with NaCl (c = 0.9%, n = 1.337) which was heated to 35°C to mimic the conditions of the human eye. Apertures were 3 mm (ISO standard) and 4.5 mm to simulate photopic and mesopic pupils of an elder patient [16]. For both sizes of aperture, a through focus MTF scan was done at the spatial frequency of 50 line pairs per millimeter (lp/mm), which is equivalent to 14.9 cycles/degree. Sagittal and tangential values of MTF, that should be equal for rotational symmetric IOLs, were obtained and averaged for analysis. The “depth of focus” of an IOL was determined by the power difference between distance and intermediate peak of the through focus MTF curve.

With an aperture of 3 mm (Fig. 3), two peaks showed up in the through focus MTF curves for all three IOLs, marking the distance and the intermediate focus. For Symfony, both peaks were most distinct with a maximal dip in between and a power difference between both peaks of 1.25 D. The MTF of Vivity and LuxSmart had a power difference of 1.75 D and about 1.50 D, respectively, between primary and secondary focus, yet a reduced overall MTF compared to Symfony. MTF value at the secondary focus for LuxSmart was considerably superior to Vivity. While the peaks at the primary (distance) focus of Symfony and Vivity were more pronounced than the secondary peak, we found the inverse pattern for LuxSmart. For Vivity, a third minor peak showed up at the near focus. The single peak MTF values are shown in Table 2.

With an aperture of 4.5 mm (Fig. 4), the MTF values of all IOLs were markedly reduced only for the intermediate focus. The shape of the through focus curves were roughly similar to the aperture of 3 mm, but for LuxSmart, MTF value for both peaks became equivalent instead. For Symfony and Vivity, the power difference between the peaks at primary (distance) and secondary (intermediate) focus kept stable with larger aperture (Symfony 1.25 D, Vivity 1.75 D). For LuxSmart, however, a minor reduction in the depth of focus to 1.0 D was revealed. The single peak MTF values are shown in Table 2.

Distance and intermediate foci of Symfony were more distinct compared to both new wavefront-shaping IOLs and showed higher peak values. Through focus MTF of this diffractive IOL behaves like in a bifocal IOL. Our measurements are consistent with literature for similar settings [17, 18], with similar peak values but with a slightly lower depth of focus of Symfony IOL in our study. Both new wavefront-shaping IOLs had lower values of peak MTF but a markedly more continuous through focus behavior compared to the diffractive EDoF IOL. Depth of focus was most extended for Vivity while a marginal third focus even at near distance might be referred to the proprietary wavefront pattern of the Vivity optics [10]. Vivity and LuxSmart proved to feature the estimated extended range of focus of 1.75 D and 1.5 D, respectively, with the small aperture, corresponding to the classification as EDoF IOLs [1]. An aperture of 3 mm simulates a smaller pupil that can be seen in bright light conditions or in pupillary near reaction [19]. Even with larger aperture of 4.5 mm, simulating the mesopic pupil of an elder person [16], the range of focus still was about 1.25 D for Symfony, 1.75 D for Vivity, and somewhat reduced to 1 D for LuxSmart. This slight reduction for LuxSmart might be due to the aberration neutral periphery of this lens. Interestingly, LuxSmart IOL showed a higher second peak and thus seems to be designed to favor the intermediate vision with small pupils that may be useful for visual tasks in intermediate distance because of the pupillary near reaction. Vivity IOL, however, obviously has the dominant focus on the distance vision for both pupil conditions. Even in bright light, distance vision thus may be less blurred. A neglectable shift of the main focus in relation to their nominal power can be seen for both novel IOLs. This shift was not adjusted and may be due either to manufacturer’s tolerances or to distinct measurement defocus because of the different nominal spherical aberration designs in relation to the ISO-2 cornea.

It should be considered as well that the overall MTF performance of any IOL is significantly dependent of the spherical aberration. Our corneal spherical aberration (ISO-2) of +0.28 µm fits best for Tecnis Symfony IOL (−0.27 µm) [20]. The nominal lens spherical aberration for Vivity IOL is −0.2 µm instead and is neutral for LuxSmart, as indicated by the manufacturers.

The results on the optical bench may vary with IOL power. 22 D is a common IOL power. An evaluation of low- and high-diopter IOLs will be addressed in the future.

As this is the very first comparative optical bench investigation of these three EDoF IOLs, there is no other peer reviewed literature available we could refer to. A transfer of our MTF results on the optical bench to the clinical performance of these novel IOLs is limited, of course. Therefore, a high and broad through focus MTF does not necessarily reflect a large range of vision to intermediate or even near distance. In clinical investigations, polychromatic light is used and different optical aberrations of different eyes are present. Nevertheless, all three EDoF IOLs tested, the established diffractive, as well as the novel wavefront-shaping IOLs, could prove their performance for some extended depth of focus, as intended by their manufacturers. The new class of EDoF IOLs will be an important part of the future lens surgery portfolio.

The authors want to thank Christin Fuchs, Trioptics GmbH, for all modulation transfer function measurements with Trioptics’ OptiSpheric IOL PRO 2 optical bench and the graphical work.

This work is an optical bench analysis (in vitro study). An ethics statement was not required for this study type; no human or animal subjects or materials were used.

No conflict of interest and no financial interest or benefit that has arisen from our research.

No funding was received.

RS and AFB designed the study, discussed the results, and wrote the manuscript. All measurements, data collection, and graphics were performed by assignment by Trioptics company and confirmed by certificates.

All data analyzed in this study are included in this article. Further inquiries can be directed to the corresponding author.