Optical Design and Functional Classification of Contemporary Intraocular Lenses

INTRODUCTION

Currently there are several IOL categories in the market, and it's challenging to accurately classify them due to the vast number of products and differences in language and terminology used across IOL manufacturers.^1,2 Understanding the mode of action of different IOLs and their expected visual performance is essential to ensure appropriate IOL selection and ultimately postoperative patient satisfaction. In order to support surgeons in this delicate process, this article proposes a novel view of IOL classification, which does not only take into account optical principles, but is also focused on clinical performance and builds on the most updated International Organization for Standardization (ISO 11979-7:2024) requirements.

MONOFOCAL LENS DESIGN

Monofocal IOLs are designed to achieve optimal uncorrected visual acuity (VA) at a fixed viewing distance,¹ usually for far vision, and can present spherical, aspheric, or diffractive aspheric lens designs. Compared with spherical IOLs, aspheric monofocal lenses fully or partly compensate for the natural positive asphericity of the cornea to provide a better focus (average and unoperated corneas).³ Aspheric lenses themselves can be associated with slight differences in depth of focus, depending on their amount of spherical aberration correction and how their asphericity compares with that of the cornea. Some aspheric lenses have been shown to provide improved intermediate vision for patients compared to spherical IOLs.^4,5

Notably, there is a new wave of monofocal lenses entering the market that define themselves as “enhanced monofocal” or “mono-EDF”; these refer to monofocal lenses that apply minor changes in the optical design to extend the depth of focus from distance in order to improve intermediate vision.^6,7 The visual performance of these monofocal lenses should not be confused with that provided by EDF IOLs, which provide a greater and more consistent range of in-focus vision. Aspheric monofocal lenses claiming to be enhanced monofocal in design include TECNIS Eyhance (Johnson & Johnson, Santa Ana, CA, USA), IsoPure (BVI, Liège, Belgium) and RayOne EMV (Rayner, Worthing, West Sussex, England), among others.^7-9 Xact mono-EDF (Santen, Goleta, CA, USA) is the only marketed diffractive aspheric monofocal lens using diffraction rings.⁶

EXTENDED DEPTH OF FOCUS LENS DESIGN

EDF IOLs are part of the range of Simultaneous Vision IOLs (SVIOLs), which are indicated for surgical correction of presbyopia in patients undergoing cataract surgery or refractive lens exchange. They use various optical approaches to allow a continuous range of in-focus vision from distance to at least intermediate distances and often functional near vision.¹⁰

Compared with monofocal IOLs, EDF lenses provide greater spectacle independence but, depending on the optical approach, they may reduce contrast sensitivity and cause greater visual disturbances. Nevertheless, the aim of EDF is to minimize these distance visual quality losses relative to multifocal lenses, which provide distinct focal points at distance and near and tend to be associated with increased visual disturbances.^10,11

Several different optical technologies have been applied to EDF IOLs, in combination with aspheric surfaces, including refractive, diffractive, small aperture, and wavefront shaping (Table 1).

THE ISO STANDARDS

Recently, the International Organization for Standardization has technically reviewed and updated ISO 11979-7:2018 related to Ophthalmic implants, Intraocular lenses for the correction of aphakia.²² The main changes are related to the development of definitions of non-accommodative posterior chamber “Simultaneous Vision Range” (SVIOL) lenses that include the subtypes of MIOL (Multifocal), EDF (Extended Depth of Focus) and FVR (Full Visual Range) IOLs. Table 2 shows general requirements for simultaneous vision IOLs and specifically for EDF IOLs. The standard requires to test those criteria based on minimum clinical performance: the evaluation required a comparison with an aspheric monofocal IOL control in at least 100 subjects per group after 6 months follow-up.²² This ISO update is in alignment with previous ANSI criteria for EDF IOLs definition;^23,24 both propose to evaluate the clinical performance to classify IOLs as EDF or monofocal, instead of only focusing on the mechanism of action of those IOLs, which provides useful information for better patient selection and high postoperative patient satisfaction.¹

FUNCTIONAL CLASSIFICATION OF EXTENDED DEPTH OF FOCUS AND MONOCULAR LENSES

The ISO and ANSI criteria support IOL selection by identifying a lens as EDF, hence clinically differentiating it from a monofocal.^22-24 However, they have not yet found wide application due to the lack of published data following the ISO and ANSI criteria; only AcrySof IQ Vivity, IC-8 Apthera, and TECNIS Symfony show that the EDF criteria are met through the required clinical evaluation.^18,25,26

In the absence of dedicated studies that report clinical data based on the ISO or ANSI standards, we sought to propose a classification system that differentiates EDF and monofocal IOLs based on indication for use and published defocus curve data, building on the ISO approach, as detailed in Table 3 and summarized in Figure 1 (i.e. “No ISO standard”). In short, if an IOL has no published data per ISO criteria but has an indication of presbyopia correction, this is categorized as EDF if it maintains a monocular VA of at least 0.2 logarithm of minimum angle of resolution (logMAR; or a binocular VA of at least 0.1 logMAR) for a negative defocus of at least 1.5 D but less than 2.5 D; if the negative depth of focus is less than 1.5 D at 0.1 logMAR binocularly or 0.2 logMAR monocularly, the lens is considered monofocal. IOLs that do not meet the ISO standard and have no presbyopia correction indication per manufacturer direction for use (DFU) or instructions for use (IFU) are considered by default as monofocal IOLs. Published defocus curve data to develop our proposed classification system have been reviewed and categorized based on levels, depending on the design, duration, or sample size. To closely mimic the patient population included in the ISO-based studies, studies on patients with non-pristine eyes (e.g. with ocular comorbidities or history of ocular surgery) were excluded.

The rationale for the selection of our criteria can be articulated as follows:

• Using indication for presbyopia correction as the starting point for our system helps standardize the approach, thus overcoming the challenge of manufacturers using inconsistent lens terminologies.
• We have elected to center our system around defocus curve profiles, obtained by measuring VA through a range of spectacle lenses, as this parameter has become the gold standard method for assessing the ability of an IOL to correct a refractive error at a range of focal point(s)⁴⁶
• Including the options of evaluating defocus curves either monocularly or binocularly allows for the categorization of a higher number of IOLs currently used in cataract surgery practice

CONCLUSIONS

EDF can be achieved through different optical approaches, refractive, diffractive, including small aperture, or wavefront-shaping technologies in combination with aspheric properties. The classification system proposed in this article aims to provide strong support to interpret the clinical performance of monofocal and EDF IOLs whose data have not been generated following the ISO standards, with the ultimate goal to help the ophthalmology community achieve an optimal IOL selection and improve postoperative patient satisfaction. This article reflects the need to generate proper clinical trials (level 1a evidence- randomized clinical trials versus monofocal control) to demonstrate the benefits of IOLs clinically and classify them adequately based on their clinical performance.

AcrySof IQ Vivity Directions for use for a list of indications, contraindications, and warnings. Available at https://ifu.alcon.com/

©2024 Alcon Inc. 6/24 IMG-VIV-2400020