A look at how EDOF technology can be categorised into five types
As life expectancy increases, the condition of presbyopia, which occurs naturally after cataract surgery, continues to be challenging. Nowadays, patients have higher expectations not only for distance vision but for an ideal intraocular lens (IOL) that provides superior far-, intermediate-, and near-vision performance without spectacles, and without adverse effects.
Indeed, in recent decades cataract surgery has undergone a tremendous evolution, transforming from a therapeutic method for restoring vision to a refractive procedure that enhances patients’ quality of life. As a result, there has been a significant advancement in the development of IOLs with new designs based on different optical principles to enable patients to see clearly at all distances without the need for glasses.
The subject of IOLs is constantly at the centre of attention. Premium IOLs, a diffuse term that includes multifocal, toric and now extended depth-of-focus (EDOF) lenses, are a broad area in which both the industry and the most enthusiastic and early-adopter surgeons have become more immersed in their development. Multifocal lenses have been well defined in extensive papers and books.1-6 The same can be said of toric IOLs.
EDOF lenses are a different topic, however, involving different fundamentals and myriad lenses, not all of them appropriate.
EDOF lenses are those that can stretch the focal point of the retina, creating an area in which the image is good enough to offer an adequate solution to the patient in terms of distance. In other words, these lenses can focus on different distances without being multifocal. In this way, EDOF IOLs differ from the multifocal IOLs, which show at each of their foci secondary out-of-focus images corresponding to the rest of the foci, which cause the dysphotopic phenomena. The EDOF lenses provide a continuous range of focus, avoiding the overlapping of images and thus eliminating the halo effect.
However, it should be noted that because EDOF lenses elongate focus, the consequence is a linear distribution of the light energy instead of all this energy going to one focus or different foci. Consequently, there is a potential decrease of contrast sensitivity with a degradation of the retinal optical quality of the image.
The way in which EDOF lenses perform this task differs from that of other lenses, and findings from previous studies and publications have described five types in this group of EDOF lenses.7,8 These are very well defined today, and the surgeon should be familiar with them in order not to be confused or biased due to inadequate commercial information or a lack of sufficient knowledge.
Type 1 EDOF lenses are those that are based on the changes in spherical aberration (SA). Inducing SAs in EDOF IOLs means that incoming light waves are extended in a longitudinal plane (Figure 1). The elongated focus eliminates the overlap of near and far images and theoretically eliminates the halo effect. The trade-off is a decrease in the quality of the retinal image.9 Thus, SA has limits in its ability to elongate the axis and create an adequate retinal image without a degradation of the visual quality. This explains why some of the models that have been offered that try to exceed 1 D of EDOF effect are, simply speaking, inadequate because the retinal image they provide is of poor quality. The consequences are dissatisfied patients and inadequate outcomes, and finally, some cases end in IOL explantation.
Type 2 lenses are those in which the pinhole principle is used (Figure 2). These lenses provide potential for those patients with irregular corneas; however, they should probably not be offered to those with normal eyes because of their lack of performance in the peripheral visual field (ie, the one that is measured from 30° to 60°). Because small-aperture implants have a fixed pinhole diameter that does not vary with ambient lighting conditions, they decrease the amount of light entering the eye, thereby decreasing the visual acuity of night vision.
Type 3 lenses have been offered as EDOF because the defocus curve is similar and they provide good intermediate vision. However, they are really multifocal lenses, either refractive or diffractive, endowed with a low near vision addition (Figure 3).
Even although these lenses can provide a defocus curve that masquerades that of the EDOF lenses, they are actually low-power multifocal lenses and are affected by the same problems and limitations as multifocal lenses but with fewer adverse effects because of the low power. Among these lenses, those that are based on segmental optics (rotational asymmetrical) are much better tolerated and more useful than those based on diffractive optics (diffractive multifocal lenses), in which halos and glare present because these are intrinsic to the diffractive technology.
Type 4 lenses are hybrid, multifocal, and EDOF (Figure 4). The industry is trying to compensate for some of the adverse effects that are created by the multifocality by changing the light distribution caused by the EDOF effect. These lenses have several examples in the market and as yet have not been completely investigated. They can probably add value to the multifocal lenses already available but their real difference and effectivity in terms of less postoperative photic phenomena and subjective complications remain to be demonstrated.
Type 5 lenses are those that modify the optical profile of the anterior IOL surface by creating an EDOF. In this group, we can distinguish between two subgroups:
These lenses have received special attention and are probably the most successful today and the most developed as well. Type 5A lenses are less sensitive to decentration, which, on the contrary, is the main pitfall of type 5B.
Medical evidence and data from independent publications are still lacking for most of these lenses. The surgeon should be aware of this and choose lenses for which there is sufficient information regarding the quality of the retinal image, the defocus curve and particularly the negative secondary patient-reported outcomes, such as of patient dissatisfaction in independent studies.
The future, in the short term, will define the position of each of these lenses as far as the practical surgeon is concerned. It is hoped that further evidence will clarify this topic and prevent many eyes from being implanted with lenses that ought not to be implanted in healthy human eyes with visual potential.
Jorge L. Alió, MD, PhD, FEBOphth, is professor and chairman of ophthalmology at Miguel Hernández University of Elche, and founder of Vissum Miranza Group in Alicante, Spain.
Elinor Megiddo-Barnir, MD, is affiliated with the Cornea, Cataract
and Refractive Surgery Unit at Vissum Miranza Group in Alicante, Spain.
The authors have no financial interests relevant to this article.