INTRODUCTION

Cataract surgery is one of the most accomplished surgical procedures and has become increasingly safe and reproducible since the onset of phacoemulsification. The traditional approach consists of replacing the lens with a monofocal intraocular lens (IOL), allowing the restoration of visual acuity (VA) for distance, implying, however, the use of glasses for a good VA for near¹. The goals of cataract surgery are to improve the quality of life of the patient, allowing the independence of glasses for all distances. Reducing the need for glasses has become a growing expectation among patients undergoing cataract surgery, particularly among those who maintain an active lifestyle. This is why the approach of pseudophakic presbyopia has become such an important topic in the current practice of cataract surgery². Multifocal Intraocular Lenses (MIOLs) were introduced into clinical practice in the early 1990s as an optical solution to meet the visual needs of patients who wanted glasses independence, while promoting good visual acuity both for far and near^3,4.

MIOLs were designed to generate two focal points separated along the optical axis, simulating the equivalent of accommodation¹. Implementing the concept of simultaneous vision, there is the formation of overlapping images in the retina: the brain selects the sharpest image and suppresses the others¹. MIOLs project multiple images into the retina, which can result in undesirable visual phenomena, such as reduced contrast sensitivity, glare and halos⁵. Positive dysphotopsias (glare and halos) are 3.5 times more frequent with MIOLs than with monofocal IOLs and are the main reason for postoperative dissatisfaction. MIOLs are associated with lower contrast sensitivity (vs. monofocal), especially in mesopic conditions².

MIOLs – OPTICAL PROPERTIES

MIOLs use a refractive, diffractive or combined design.

Refractive Lenses

Refraction consists of changing the direction of the light rays due to the thickness, curvature, and optical density of the transmitting material⁶. The refractive MIOLs present two or more spherical zones of different radii of curvature, that is, concentric circles of different refractive powers (Figure 1).

A constant refractive zone allows creating a focus for distance and another refractive zone provides a focus for near. The lens design is based on the assumption that when viewing a nearby object, the inherent miosis hides the peripheral part of the lens and, consequently, only the central zone is effective, allowing a clear view of a nearby object. When viewing a distant object, the pupil increases in size, allowing a sufficiently large area of ​​the periphery of the lens to be effective to allow viewing of the object. The interface between the refractive zones causes unwanted halos and glare. The main disadvantage of refractive MIOLs is the dependence on pupil size: a pupil smaller than 2 mm does not allow the creation of a focus for distance vision^1,6.

Other disadvantages are high sensitivity to lens centering, kappa angle intolerance, potential for halos and glare and loss of contrast sensitivity².

Diffractive Lenses

Diffraction occurs when light meets an obstacle in the material in which it travels and is diverted in a different direction⁶. The diffractive IOLs use the Huygen-Fresnel principle: when light passes through a narrow slit it is deflected; if two adjacent slits are illuminated by the same light source, the resulting deviated light waves overlap, producing interference (Figure 2).

Depending on the phase of the two wavefronts, light waves may be constructive, reinforcing each other, or may be destructive, mutually weakening. Thus, the “steps” on the surface of the MIOL act as a diffraction gradient (dividing light into several foci) creating different foci through constructive or destructive interference from incident light¹.

The diffractive MIOLs have concentric rings that cover the anterior and posterior surfaces and serve as a phase gradient, leading to diffraction of light and producing the two focal points of vision for near and far (Figure 3).

Hybrid lenses

Hybrid MIOLs combine refractive and diffractive concepts. For their development, two guiding principles were necessary: ​​near vision is less important in scotopic conditions when the pupil has a larger size; the minimization of the perception of halos and glare in scotopic conditions is fundamental.

The apodization is defined by the gradual reduction of the height and width of the "steps" of the IOL surface, from the center to the periphery, radially, balancing the distribution of light energy for the two primary foci¹. This gradual change is very important optically, since it avoids any sudden optical limits, reducing the loss of light due to high diffractive orders and allowing a smooth transition in the distribution of light.

The first hybrid lens approved in Portugal was Acrysof ReStor (Alcon). The base curvature of the lens allows distance vision using its refractive shape. In addition, there are 12 diffraction discontinuities or "steps" that were incorporated into the anterior surface to provide the addition power. These discontinuities cover the central diameter of 3.6 mm of the IOL, while the peripheral ring, from 3.6 mm to the 6.0 mm edge, consists of a refractive surface dedicated to distance vision (Figure 4). This type of MIOL offers a reduction in undesirable optical phenomena, an increase in contrast sensitivity, better intermediate vision, but poorer near vision, and in some cases glasses may be required for longer readings¹.

EXTENDED FOCUS LENSES

The MIOLs marketed in Portugal with this feature (extended focus) are Symfony (Abbott) and Precizon Presbyopic (Ophtec). They have a diffractive design, independent of the pupil, that improves contrast sensitivity using the achromatic technology for the correction of the chromatic aberration. According to the manufacturer, the IOL creates an elongated and continuous focus regardless of the pupil size (i.e., a prolonged and continuous range of distant and near vision), with no defined planes⁸.

TRIFOCAL INTRA-OCULAR LENSES

Classical MIOLs are bifocal, and several studies have shown that refractive^9-12, diffractive^13-17 and hybrid^18-22 MIOLs provide good visual acuity for distance and near when implanted. However, the visual function for intermediate distance (computer work) is poor between these two focal points^23-26.

Tri-focal intraocular lenses (TIOLs) have been introduced to include a third focal point that uses light lost in the conventional diffractive (bifocal) lens. The goal of introducing a third focal point in TIOLs optics is to provide better VA for intermediate distances by keeping a good VA for far and near.

TIOLs can be achieved by combining two bifocal diffractive profiles on one surface of the IOL or using a trifocal diffractive profile combined or not with a bifocal diffractive pattern²⁷.

1^st generation

1. FineVision Micro F (Physiol SA, 2010)

AT LISA is a one-piece aspheric diffractive TIOL with a biconvex optical zone of 6.0 mm and a total diameter of 11.0 mm (Figure 6). The IOL optical design combines a central trifocal zone of 4.34 mm in diameter with a bifocal zone at the periphery (4.34-6.0 mm). The central zone provides an addition for near of +3.33 D and an intermediate addition of +1.66 D. The incident light is distributed asymmetrically: 50%, 20% and 30% to the foci for distance, intermediate and near vision, respectively⁶.

In the study of Alfonso et al⁴, at the sixth postoperative month, the mean monocular UDVA and CDVA were 0.11 ± 0.16 logMAR and 0.05 ± 0.10 logMAR, respectively, with 100% of the patients achieving a binocular CDVA of 20/25 or better⁴.

These results are similar to other recent studies⁶. Regarding close-up vision, monocular UNVA (0.17 ± 0.13 logMAR) and CNVA (0.14 ± 0.12 logMAR) in the last follow-up visit were of the same magnitude or better than those reported in previous studies with the same trifocal IOL. In addition, the study found good results for postoperative binocular CNVA (0.06 ± 0.10 logMAR), with 87% of patients achieving a CNVA of 0.1 logMAR or better (20/25) at 40 cm, which are considered adequate to achieve a high level of independence of glasses. The study had good results in VA for intermediate distances: the mean binocular VA for intermediate distance was 0.07 ± 0.11 logMAR (> 20/25), 0.09 ± 0.08 logMAR (> 20/25), and 0.11 ± 0.11 logMAR (approximately 20/25) at 50, 60 and 70 cm, respectively.

Mojzis et al reported better results for VA at intermediate distances (66 and 80 cm) with TIOL AT LISA compared to its bifocal predecessor²⁷.

The aforementioned studies^4,27 have shown that corrected and uncorrected VA for distant and near vision are comparable to those obtained with its bifocal predecessor (AT LISA 801, Carl Zeiss Meditec), suggesting that the creation of a third intermediate focus does not detract from the other two main focuses. As expected, the introduction of an intermediate focus on the optical design of IOL resulted in an improvement in VA for intermediate distances (compared to previous bifocal IOL models) without compromising performance for near vision and for long distances⁴. Contrast sensitivity in mesopic conditions is lower than under photopic conditions in all spatial frequencies analyzed, as in diffractive bifocal IOLs⁴.

The optical surface of the AT LISA presents fewer rings and absence of steep angles compared to the anterior bifocal IOL, resulting in less glare and halos. In the study by Alfonso et al⁴, the patients were satisfied with the visual results achieved, especially for intermediate distances, and no patient complained of undesirable optical phenomena after the surgical procedure⁴.

In summary, the TIOL AT LISA tri 839MP implementation provides stable and good results in VA at close range, with a satisfactory range of VA at intermediate distances⁴.

3. Comparison between the two 1^st generation lenses: FineVision vs. AT LISA

In the study by Marques and Ferreira⁶, after implantation of Finevision or AT LISA, there was no statistically significant difference in monocular or binocular VA uncorrected or corrected during follow-up. The two TIOLs provided excellent VA results for distance, intermediate and near vision. The UDVA was 0.3 logMAR or better (Snellen equivalent 20/40 or better) in 30 eyes (100%) in the Finevision group and in 29 eyes (97%) in the AT LISA group. UIVA at 80 cm was 0.3 logMAR or better in 29 eyes (97%) and 30 eyes (100%) in FineVision and AT LISA, respectively. The UNVA at 40 cm was 0.3 logMAR or better in all eyes in both groups. Although monocular VA for near and intermediate distances appear to be slightly better in the FineVision Micro F group, binocular VA for near and intermediate distances were similar in both groups.

The incidence of dysphotopic phenomena was low and comparable between the two groups of TIOLs. In all spatial frequencies tested, the values ​​of binocular contrast sensitivity were similar between the two groups⁶.

2^nd generation

1. AcrySof PanOptix (Alcon)

The AcrySof PanOptix TIOL is a one-piece lens, composed of a hydrophobic acrylate/methacrylate polymer, able to filter blue/ultraviolet light, mimicking the transmission of light through the crystalline lens (Figure 7). This TIOL uses non-sequential diffractive orders to create a focus at distance, an intermediate focus at 60 cm and a near focus at 40 cm. It has a central optical zone of 6.0 mm and two haptics, with a total diameter of 13.0 mm. The optical zone is biconvex with a diffractive structure in the central portion of 4.5 mm (with 15 diffractive zones) of the aspheric anterior surface²⁹. The dimension of the diffractive optical zone offers good VA for near and intermediate distances, even with dilated pupils, being less dependent on the size of the pupil³⁰. The light is distributed in 50%, 25% and 25% for distance, intermediate and near vision, respectively. The IOL creates a fourth focal point at 1.20 m (quadrifocal technology), however, this does not mean that a new focal point becomes accessible to the patient, since the light from the first focal point is diffracted to focus for distance, leading to a more natural transition of the intervals and increasing the luminous efficiency of the IOL up to 88%, culminating in better visual results compared to conventional MIOLs (bifocal)³⁰.

COMPARISON BETWEEN THE 3 TRIFOCAL LENSES

Carson D. et al compared the 3 TIOLs using optical performance tests. The far- and mid-focus modulation transfer function (MTF) values ​​corresponding to the 20/20 and 20/40 visual acuities on the Snellen scale were higher with PanOptix. The values ​​of the focus for near were higher with the AT LISA²⁹.

The maximum MTF values ​​in the near focus occurred at different distances for each IOL: 42 cm (PanOptix), 38 cm (AT LISA) and 40 cm (FineVision)²⁹. The maximum MTF values ​​for the intermediate focus occurred at different distances for each IOL: 60 cm (PanOptix) and 80 cm (AT LISA and FineVision)²⁹.

With regard to intermediate distance vision, PanOptix delivered a clear resolution at 60 cm, consistent with its intended design for visual performance at intermediate distances. The 20/20 line was achieved for distance and at the intermediate focus of 60 cm. Regarding near-visual acuity, the 20/20 line was slightly blurred at 40 cm, because the best focus for near is located at 42 cm.

For AT LISA, the 20/20 line was very sharp in focus at a distance and in focus for close to 40 cm. In the AV evaluation for intermediate distances, the letters below the 20/40 line were better visualized at 80 cm than at 60 cm, consistent with its design, for an optimal intermediate focus at 80 cm29.

For the FineVision IOL, the 20/20 line was sharp in the distance focus and the 40 cm near focus. Similar to AT LISA, at intermediate visual acuity, letters smaller than 20/40 were sharper at 80 cm than 60 cm, consistent with MTF results²⁹.

MTF measurements in near and far visual acuities showed that the second generation of TIOLs (PanOptix) presented values ​​comparable to the two first generation models (AT LISA and FineVision) but had a higher MTF value in the focus for intermediate distances.

This point is particularly important, since the intermediate distance of 60 cm is more adequate than 80 cm for the visualization of computer monitors²⁹.

Second-generation trifocal IOL (PanOptix) showed equivalent or better performance in image quality, resolution and optical phenomena compared to first-generation TIOLs (FineVision and AT LISA)²⁹.

The halos intensity was similar for PanOptix and FineVision IOLs, but slightly higher for AT LISA. The differences in intensity may be due to IOL designs. PanOptix and FineVision IOLs have a trifocal diffraction pattern, while the LISA AT has a trifocal center, but contains a bifocal periphery²⁹.

PanOptix showed optical performance equivalent to that of AT LISA and FineVision regarding image contrast, resolution and propensity for halos²⁹.