INTRODUCTION

At present, the prevalence of visually significant cataract is around 2.5% at the age of 40-49 years, increasing with age and reaching 68% before the age of 80 years1.

Multifocal IOLs have side effects such as halos, glare2 and loss of contrast sensitivity (CS) in scotopic conditions, especially in high spatial frequencies (due to the distribution of light in different foci of the IOL)3. Currently, different optical principles are being combined to improve uncorrected near visual acuity (UNVA), uncorrected intermediate visual acuity (UIVA) and uncorrected distance visual acuity (UDVA), as well as to reduce the secondary effects derived from the implantation of these lenses. The impact on the quality of vision of patients after the implantation of an IOL, must be understood and analyzed, to choose the appropriate IOL for each individual4.

The steps that are carved on the surface of the diffractive IOLs produce the simultaneous formation of a distance focus (refractive effect) and a near focus. They generate good visions for far and near but leave a valley in the intermediate vision where the vision is not so good. This aspect has been tried to solve with the design of bifocal lenses of low addition that moved away the near focus to narrow the valley of intermediate vision, although the optical quality between both focuses still was not sufficiently good.

There is a new category of multifocal IOLs of extended focus (EDOF, Extended Depth Of Focus) that would achieve an extension of the light source with fewer dysphotopic phenomena. The concept of extended depth of focus (EDOF) has been developed in recent years, with the new advances in technological research, being increasingly popular among the population.

EDOF has been incorporated into IOLs, but it has already been used to correct presbyopia through the use of multifocal contact lenses or with the PresbyLASIK surgical technique.

The concept of depth of focus is complementary to the concept of depth of field that we usually handle in clinical practice. For a given configuration of an optical system (or a static state of accommodation in the eye), the depth of field is defined as the distance along which we can move an object without causing a loss of sharpness considering a certain level of tolerance. The depth of focus is defined as the distance ahead or behind the image focus (retina) along which we can move the image without causing a loss of sharpness considering a certain level of tolerance.

An EDOF IOL will be one that having the same material and paraxial power as a monofocal IOL will provide greater depth of field. In order to extend the depth of focus, a decrease in the amount of energy that the main focus of the IOL has to be produced, the tolerance level being the minimum energy that the focus has to have in order for visual acuity (VA) or CS of the intervened patient to reach the visual expectations of that patient.

In the market there are different types of IOL that produce an extension of the focus to improve near vision, through different mechanisms that we will be discussing in the following sections.

SYMFONY LENS (Abbott Medical Optics, Santa Ana, California)

The Symfony lens is characterized by two patented diffraction technologies:

• Echellette diffraction net that introduces a diffraction pattern of light that lengthens the focus of the eye, thus achieving greater range of vision.
• Achromatic technology that reduces chromatic aberration to improve CS.

It compensates the corneal spherical aberration (SA) thanks to its "wavefront" aspheric front surface of -0.27 microns. Corneal chromatic aberration correction sharpens the focus of light; when this is combined with correction of the SA, the quality of the retinal image increases, without negatively affecting the depth of focus. All the technical characteristics of this IOL are described in Table 1.

In a comparative prospective study carried out on 80 eyes comparing patients operated with a monofocal IOL (Tecnis ZCB00 Abbott Medical Optics Inc., Santa Ana, CA) to patients operated on with the Symfony IOL, a significant improvement was observed in distance, near and intermediate monocular and binocular VA (p=0.013) of the patients operated with the Symfony lens in front of the monofocal lens5. However, no significant differences on CS were observed between the groups (p ≥ 0.156), nor in the ocular optical quality parameters (p ≥ 0.084). Therefore, the Symfony IOL achieves a better distance, intermediate and near VA than the aspherical monofocal IOL, maintaining the same level of visual quality.

A multicenter study carried out in 411 patients, on whom a bilateral implantation was performed with the Symfony lens, analyzes the results based on two study groups: micro-monovision (112 patients) and intentional emmetropia (299 patients)6. The uncorrected near and intermediate VA were better in the micro-monovision group (p = 0.011 and p = 0.003 respectively). Therefore, a mild micro-monovision will help to reinforce the optimal result of near vision in the postoperative period without affecting distance vision in patients operated on with this intraocular lens.

MINIWELL INTRAOCULAR LENS (SIFI Medtech SRL, Lavinaio, Italy)

This is a new EDOF IOL design that offers a multifocality by varying the SA in the central optical zone.

It is an aspheric IOL with positive SA in the central zone of 2 mm, a negative SA in the pericentral ring of 1 mm and a monofocal outer aspherical zone. This combination increases the depth of focus. In contrast to other diffractive IOLs, it has three different optical zones with transition zones with a smoothed transition profile. Thanks to these features you get a good VA and visual quality even in low light conditions. All the technical characteristics of this IOL are described in Table 2.

The published results of several in vitro studies show that this IOL offers good optical quality in distant, intermediate and near distances and offers performance benefits compared to other trifocal IOLs and in the concept of extended vision range7,8.

The new design of this IOL was tested in optical bench to investigate the depth of focus and sensitivity to tilt, decentration and Kappa angle9. The results of the study indicate that the modulation transfer function (MTF) for a 3 mm pupil was from 0.35 to 0.40 for far vision and from 0.25 to 0.30 for intermediate and near vision. These values ​​were not influenced by tilting up to ± 2.5°, by decentration up to ± 0.5 mm or by the Kappa angle up to 9°. The simulations indicated a good VA of up to 2.00 D of pseudo-accommodation. Therefore, it offers a good MTF in a wide dioptric range, which suggests a good VA between 4 and 50 cm.

Laboratory measurements show that with the increase in pupillary size, the level of higher order aberrations (HOAs) of this IOL remain within a physiological range. This assessment distinguishes this IOL from others that correct presbyopia, but that induce an increase in HOA as the pupil size increases. This suggests that it has a reduced potential to cause photopic phenomena and disturbances in night vision.

LENS IC-8 (AcuFocus Inc., Irvine, California)

This is a lens that extends the depth of focus by combining the small aperture technology with a monofocal lens. Clinical studies indicate that it provides a continuous and uninterrupted range of vision, good image quality throughout the range of vision, less symptoms such as glare or halos, good near and intermediate vision without compromising far vision and compensation for the effects of astigmatism up to 1.5 D.

It is based on the same aperture as the KAMRA™ inlay. It incorporates a non-diffractive opaque mask 3.23 mm in diameter with a central opening of 1.36 mm embedded within a single-piece hydrophobic acrylic lens of 6.0 mm.

The opaque mask creates a pinhole effect, which provides almost 3.0 D of extended depth of focus by blocking out-of-focus peripheral light rays and isolating the central and paracentral rays more centered through the central aperture. All the technical characteristics of this IOLs are described in Table 3.