Current and future nomenclature and categorization of intraocular lenses

In 1949, Sir Harold Ridley implanted the first poly(methyl methacrylate) intraocular lens (IOL).1,2 We have come a long way in the field of cataract and refractive surgery since Ridley's intervention, and the strides in IOL development over the past 75 years have brought more than 100 IOLs to the market. Along with the vast choices come a variety of IOL classification systems, making it increasingly more complicated to understand which lens will perform in what desired function.3,4 The preoperative planning of IOL selection is significantly more time-consuming than most refractive procedures, in large part because the lenses are not easily comparable. Parameters such as targeted vision, optical design and function, and pupil dependence are helpful tools in navigating the selection process, but without consistency and agreement in labeling and without a standardized classification system, we may be comparing apples to oranges. We desperately need to adopt standardized language and methods of comparison so that we can more efficiently and accurately select the best lenses for our patients.5 In this issue, the ESCRS Functional Vision Working Group suggests an evidence-based functional classification of simultaneous vision IOLs (page 794). Nomenclature has the ability to clarify or confuse. Patients as well as physicians are often confused about what defines or differentiates specific IOLs. We should begin by defining and labeling the qualities of a lens in terms of design, how the light is focused, and functional outcomes. In January 2024, the International Organization for Standardization (ISO), a worldwide federation of standards, reviewed and updated its requirements and recommendations for IOL investigations and models in the treatment of aphakia.6 This updated document outlines 4 main categories of IOLs based on optical design and/clinical performance. When an IOL is designed to provide more than one type of refractive correction, that IOL will have to satisfy each of the separate requirements of those correction designs. Using the ISO approach provides physicians and patients with a consistent, measurable, and repeatable system of naming, categorizing, and understanding any given IOL. The 4 categories and IOL abbreviations are (1) monofocal (IOL), (2) toric (TIOL), (3) simultaneous vision (SVIOL), and (4) accommodating (AIOL). The basic monofocal IOL evolved into a multifocal lens in the 1990s and is identifiable by its single point of vision. They are designed to achieve optimal uncorrected visual acuity at a fixed distance. This technology merged into what has been referred to as monofocal plus, enhanced monofocal, or mono-extended depth of focus (EDOF). It has been questioned whether these IOLs should become standard of care (page 789). These IOLs are often mixed up with EDOF or even trifocal or panfocal lenses. However, these IOLs (enhanced monofocal IOLs) refer to monofocal lenses that apply minor changes in the optical design to extend the depth of focus to improve intermediate vision but should not be confused with the visual performance provided by a true EDOF IOL. Based on the design and performance of the lens, it can be categorized as an IOL or SVIOL, which can be broken down into 3 subcategories. The 3 SVIOL categories are: (a) multifocal (mIOL), (b) extended depth of focus (EDOF IOL), and (c) full visual range (FVR IOL). The mIOL emphasizes visual acuity at distance and shows improved optical and clinical performance at near focal distances compared with a monofocal lens. The EDOF IOLs demonstrate strong visual acuity at far and intermediate distances. FVR IOLs perform well at far and intermediate and up to near focal distances. A full description of these categories can be found in the ISO report.6 The area of greatest contention is the EDOF IOL. It is defined as a presbyopia-correcting lens that uses a single but elongated focal point to create more depth of focus.3 Within the EDOF IOL family there are 4 types of technologies: small aperture, bioanalogic, diffractive, and nondiffractive.7 Based on the recommendations of a task force from the American Academy of Ophthalmology (AAO), the U.S. Food and Drug Administration defined several clinical criteria related to EDOF IOLs in the American National Standards Institute (ANSI)/AAO Standard, Z80.35–2018, including minimum thresholds of good intermediate visual performance while maintaining distance vision and visual quality close to that of a monofocal IOL.8 The criteria outlined by ANSI/AAO give measurable guidelines as to what differentiates an EDOF IOL and a monofocal IOL: "EDoF IOLs should provide a monocular negative depth of focus at 0.2 logarithm of the minimum angle of resolution (logMAR) or Snellen 6/9.6 of at least 0.5 diopters (D) greater than a monofocal control. Additionally, monocular photopic distance-corrected intermediate VA (66 cm) should be superior to that of a monofocal control IOL, and at least 50% of eyes should achieve a VA of 0.2 logMAR (Snellen 6/9.6) or better.9,10 Finally, mean monocular photopic best-corrected distance VA should be noninferior to that of a monofocal control IOL."9,11 IOL design has come a long way. We currently have 5 types of IOL optic designs: (1) multifocal (including bifocal, trifocal and panfocal), (2) EDOF, (3) monofocal with enhanced depth of focus (mono-EDOF), (4) conventional monofocal aspherical, and (5) spherical. These categories do not address astigmatism but only presbyopia. Taking into consideration the optics and the outcomes are critical, but patient expectation must also equally factor into the decision when selecting the IOL.