Longer Life Expectancy – Spectacle-Free Future & The EDOF IOL

Longer Life Expectancy – Spectacle-Free Future & The EDOF IOL

November 12, 2020

The EDOF IOL, or extended range of vision IOL, is a new technology in the treatment of presbyopia-correcting lenses. The basic optical principle is to create a single-elongated focal point to enhance the depth-of-focus, on the contrary to monofocal IOLs (in which light is focused on one single point) or MF IOLs (having 2 or 3 discrete points).

In light of the increase in life expectancies and lifestyle changes, an increasing number of patients are requesting spectacle-independent near and intermediate vision for their daily activities, aside from excellent distance vision.

There are better solutions now for people with Presbyopia. Presbyopia -correcting IOLs are also a treatment option for presbyopic patients who are not candidates for laser refractive surgery and do not want to rely on reading glasses. Over recent years, a wide spectrum of multifocal (MF) IOLs has been developed.

These Presbyopia-correcting IOLs are divided into three broad categories such as; MF IOLs (including diffractive or refractive designs), extended depth of-focus (EDOF) IOLs and accommodative IOLs (intracapsular or sulcus placed). EDOF IOLs, or extended range-of-vision IOLs, are a fairly new technology in the treatment of presbyopia.

The basic optical principle is to create a single-elongated focal point to enhance the depth of focus, on the contrary to monofocal IOLs (in which light is focused on one single point) or MF IOLs (which has two or three discrete points).

The idea of EDOF is not new and its history goes back to 30 years ago. Nakazawa and Ohtsuki reported apparent accommodation in 39 eyes implanted with spherical IOLs. The authors found that the depth of field in these cases was inversely proportional to the pupillary diameter. Since then, several optical strategies have been used to extend the depth of focus at both the cornea and lens plane.

However, the term EDOF should be limited to those IOLs in which a manipulation has been made in their aberrometry profile to enlarge the depth of field. Those IOLs which have a MF design and, on top of it, also offer a manipulated aberrometry optical profile, should be called ‘hybrid EDOF IOLs’.

The first so-called EDOF IOL, which is really a hybrid EDOF (Tecnis Symfony, Johnson and Johnson Vision) was approved for use in the United States in 2016. Since then, several EDOF-labelled IOLs, many of them not really based on the EDOF principle, have been released.

Optical Models

Spherical aberration (SA) is associated with focal length difference between the central and marginal ray where the light enters in the lens.

For any given eye, the Zernike coefficients may vary widely, but a mean value of corneal spherical aberration is +0.31 ± 0.135 μm for a 6-mm pupil size.4 It was shown that aspheric IOLs which effectively reduce spherical aberration improve the optical quality over spherical IOLs.

 

On the other hand, higher positive SA in eyes having received an aberration-free IOL results in a better distance-corrected near visual acuity than that following implantation of negativeSA IOLs (that have reduced corneal SA). Similarly, intentional induction of SA within the IOL design can increase the depth of focus.

Chromatic aberrations (CAs) are associated with focal length difference between the visible spectrum of different colors of light. The human cornea produces a CA, in which blue light is diffracted more than red light.

The optical design also has an impact: a refractive optic maintains the same CA of the cornea, so with this lens the final ocular CA will increase, as will the dispersion of the wavelengths. On the contrary, diffractive IOLs can reverse CA: red blends more than blue. So, diffractive IOLs can minimize the CA in every eye.

Achromatization does not bring an extended depth-of-field improvement but rather an improvement in the contrast sensitivity function; thus, diffractive could lead to an improvement of the contrast sensitivity and the quality of vision.

The pinhole effect is another concept which allows a greater depth-of-focus to be obtained. In general, it could be stated that the smaller the pupil size, the greater the depth of field (and depth of focus). Following this principle, the use of an opaque pinhole mask in a monofocal IOL enhances the depth of focus.

Moreover, the Stiles-Crawford effect could be additionally possible; it is believed that when an equal intensity of light enters near the center of the pupil, it produces a greater photoreceptor response compared with the light entering the eye near the edge of the pupil.

EDOF IOLs provide a continuous range of focus without a clearly asymmetric IOL power distribution. This elongated focus is introduced to eliminate the overlapping of near and far images caused by traditional multifocal IOLs and the halo effect; ideally EDOF IOLs should enhance intermediate and near visual performance, while minimally affecting distance vision.

In this way, EDOF IOLs differ from multifocal IOLs, in which the secondary out-offocus images correspond to the additional foci and might induce halos. Today, there is a relevant interest in the new models of presbyopic IOLs. Several IOLs are marketed as EDOF IOLs.

We encounter a confusion in the terminology; some of the so-called EDOF lenses are really MF lenses with low near-add power, in which part of the additional optical power has been withdrawn to avoid the overlapping of images and the consequent halos and glare. We believe that it is necessary to clarify the current nomenclature and differentiate two types of EDOF IOLs: pure EDOF IOLs and hybrid MF-EDOF IOLs.

Pure EDOF IOLs are based on the spherical aberration-based optics or the pinhole effect, but have no multifocality. To name a lens a pure EDOF IOL, the optical profile has to be continuous, without a change in the optical transition of the optical profile.

 

 

This rule is equal for both the refractive and diffractive MF IOL models. All the lenses that employ correction of CAs, have a diffractive/diffractive-hybrid profile or an additional power to increase the near vision are not pure EDOF IOLs.

The trade-off of inducing certain amounts of ocular aberrations is the potential degradation of quality of vision. This also limits their performance; their near vision capability is usually limited to about 1 D. Those IOLs that have attempted to provide more are either no longer on the market, no longer available because the originator company has closed or they have produced very bad results.

The reason for the latter is that for the first time we are able to see the retinal image quality with pyramidal aberrometry and the results are very different. On the other hand, multifocality and EDOF characteristics are not exclusive of each other.

A bifocal IOL may exhibit EDOF characteristics, likewise with an aspheric monofocal IOL or even a diffractive or refractive trifocal IOL. For example, the Tecnis Symfony is an IOL which combine an EDOF with multifocality.

We propose naming such lenses ‘hybrid MF-EDOF IOLs.” Hybrid MF-EDOF IOLs include diffractive-EDOF IOLs, refractive-EDOF IOLs and diffractive-refractive-EDOF IOLs. Some IOLs named as ‘EDOF’ are really only MF lenses with a low near-vision add and no EDOF component.

Clinical Outcomes

Several optical bench reports have shown that the EDOF lenses provide better optical quality on the whole addition range than MF and monofocal lenses. Nevertheless, assessment of the quality of vision and optical/refractive performance of EDOF IOLs can be challenging due to the wide array of procedures available for evaluation of these lenses.

In some cases, even if the optical laboratory benchmark study showed that an IOL has supreme optical properties, the results were not always correlated with patient satisfaction and spectacle independence in clinical trials. For several other IOL types, the currently available clinical evidence is limited.

An American Academy of Ophthalmology Task Force consensus statement on EDOF lenses requires: > A minimum of 100 patients with EDOF lenses; > Depth of focus defined as the interval of nonpositive defocus values with a mean visual acuity of at least 0.2 logMAR; and > A depth of focus set at least 0.5 D wider than for the monofocal control group at 0.2 logMAR.

However, none of the currently published studies fulfills all the aforementioned requirements. In practice, EDOF lenses provide excellent intermediate vision, but inadequate quality of vision for near distance. We believe EDOF lenses should be used as monofocal lenses with a minor improvement for near vision; they can be expected to provide bad quality of near vision, while intermediate vision can be adequate.

One of the ways to compensate the insufficiency in near visual acuity in patients with EDOF lenses is mini-monovision, or mix-and-match strategies with diffractive low-add lenses; nevertheless, using the mini-monovision may cause a decrease in far vision and additional halos from the low myopia in the contralateral eye.

 

If a patient requires good near vision with an IOL, we believe that they should receive a MF lens (a refractive and diffractive model) making the choice dependent on the patient profile and preferences. It is not an easy task to make the IOL choice and this is why objective information based on aberrometer internal component information should be required.

Neuroadaptation is a major concern in MF or EDOF IOLs; this process is time consuming and dependent on individual factors (of which some are unknown).

Pure EDOF lenses with a relevant amount of aberrations to improve near vision may be poorly tolerated by the patient, since although the brain is adapted up to a certain amount of aberrations over time, a sudden increase in aberrations may be problematic.

Regarding photic phenomena the evidence is scarce, however, some studies have reported a lower intensity of photic phenomena in EDOF IOLs when compared with MF IOLs. CONCLUSION There is a wide range of IOLs available on the market.

A careful and thorough patient examination, taking into account IOL selection based on lifestyle and visual needs, is essential to avoid patient misunderstandings about the expected outcomes. In our recent article,11 there is a biased used of the term EDOF, as some of the IOLs marketed with this name are only MF or hybrid MF/EDOF lenses.

We propose an alternative terminology and naming lenses that have combined optical designs as “hybrid IOLs.” Clinicians and surgeons should be aware of this misleading use of the EDOF concept. As a result, future research will continue towards finding a balance between quality of vision, extended depth of focus and dysphotopsias. Counselling and advice is needed in order to ensure satisfactory outcomes–both for the patient and the surgeon.