Introductory Chapter: Refractive Surgery

1. Introduction

Refractive surgery today includes surgical procedures by which it is possible to reduce or eliminate a certain type of refractive error [1]. To date, with advances in technology, it has evolved far beyond the standard keratorefractive surgery. Thanks to the development of femtosecond technology and lasers, the precision of the LASIK procedure has been raised to a new level, and new keratorefractive methods, such as SMILE, have been developed [2]. To address refractive errors in patients who do not meet the criteria for standard laser surgery, phakic lens implantation stands as a safe surgical treatment with the possibility of correcting extreme refractive errors. We also witnessed the development of surgical methods for the correction of presbyopia with new laser ablation profiles, intracorneal implants, and the introduction of new generations of presbyopia-correcting intraocular lenses (IOLs). Extensive developments in methods of corneal biomechanics and topography analysis have led to easier identification of patients who are potentially risky candidates due to the possible development of keratectasia [3, 4]. Currently, most ophthalmic practice works on the principle of validating preoperative data by physicians, while in the future we are likely to face an era of artificial intelligence and the implementation of machine learning as a more precise way of finding appropriate parameters or functions to classify input data from large amounts of training data. This would greatly simplify the method of detecting borderline candidates for keratorefractive procedure, discriminating keratoconus from normal corneas, and finding the best-suitable IOL for providing complete spectacle independence without compromising functional vision and optical quality [3, 4, 5].

2. Refractive surgery

2.1 Preoperative evaluation for refractive surgery

For patients seeking spectacle independence, detailed preoperative assessment plays a key role in determining a safe and effective outcome. Examination for refractive surgery begins with taking a detailed medical history that includes systemic status, medication, allergies, ocular status, and history of contact lens use. The examination itself consists of a detailed biomicroscopic examination of the anterior and posterior segments of the eye, and the measurement of intraocular pressure. Refraction is the most important part of the preoperative examination. Each patient needs to determine the manifest refraction, monocular uncorrected, and best-corrected visual acuity at distance and near. In addition to the examination, it is mandatory to record the pupil size in photopic, mesopic, and scotopic conditions, corneal topography and tomography with placido-based curvature topographic systems, pachymetry, biometry, wavefront aberrometry, evaluation of tear film, determination of ocular dominance, ocular motility, and specular microscopy [6, 7, 8, 9, 10]. In addition to standard corneal topography, evaluation of corneal biomechanics is a good clinical adjunct in process of detecting subclinical keratoconus among eyes clinically deemed to have seemingly normal topography. Next to standard topography, corneal biomechanics analysis, as at the Oculus CORVIS tonometer, is gradually being introduced into practice today [11, 12]. Newer models of high-resolution swept-source OCT (SS-OCT) have been generating corneal epithelial thickness maps with standard anterior segment metrics, which is going to play a role in planning keratorefractive surgery and identification of early keratoconus [13]. Currently, modern wavefront aberrometers are incorporating corneal topography systems to calculate the contribution of corneal aberrations (anterior and posterior), and internal aberration (from the crystalline lens) to complete ocular wavefront. Therefore, taking into account the available data, clinicians are able to decide on the type of personalized (customized) ablation profiles [14, 15]. In cataract surgery or CLE/RLE candidates, besides standard preoperative assessment, IOL power calculation is crucial data for ensuring an effective surgery result. Calculation formulas are undergoing continuous improvements, with the latest formulas having shown promising precision and less refractive surprises [16]. As previously mentioned, machine learning technologies could create classification models using algorithms trained from data for achieving better results. One of the examples that are already present in clinical practice is the Kane formula [17].

It is also important to discuss the reasons for undergoing refractive surgery to identify patients with unrealistic expectations. It is extremely important to explain to patients that refractive procedures primarily serve to reduce spectacle independence in everyday situations.

2.2 Keratorefractive surgery

2.2.1 History of keratorefractive surgery

The beginnings of refractive surgery date back to ancient times. The first written records of cataract surgery date back to ancient Egypt in the fifth century BC [18]. The development of modern refractive surgery began in the mid-twentieth century. Tsutomu Sato introduced anterior and posterior keratotomy into clinical practice, and in 1939 published his results [19, 20]. The method is being further developed by Russian scientists Beliaev, Durnev, Yenaliev, and Fyodorov, who eventually introduced the radial keratotomy procedure to correct myopia [21, 22, 23, 24]. Later on, José Barraquer in Colombia started developing the idea of lamellar corneal surgery to change the shape of the cornea. The idea arose from the observation that lamellar keratoplasty leads to a reduction in the cone in patients with keratoconus, and consequently to a reduction in myopia. In 1964, Barraquer described the principles of lamellar corneal surgery and called the procedure “keratomileusis,” which means the formation of the cornea [25, 26, 27]. The development of excimer lasers began in the 1970s with experiments on a combination of rare gases (such as argon and xenon) and halogen gases (such as fluorine and chlorine) used as laser media. Trokel and Srinivasan were the first to suggest that an excimer laser could have unique qualities for performing corneal surgery. In 1983, they suggested that such a laser could be used to remove tissue lamellae to change the curvature of the cornea and make precise incisions in the cornea [28, 29]. Theo Seiler was the first person to use an excimer laser on the human eye. In 1985 he performed astigmatic keratotomy, and in 1986 he performed the first phototherapeutic keratectomy (PTK) [30, 31, 32]. Munnerlyn et al. developed a computer-generated algorithm that links the diameter of the treatment zone to the depth of ablation to induce a specific diopter change in the cornea. An algorithm known as the Munnerlyn formula was used to develop laser patterns for inducing specific changes in corneal curvature to achieve the desired diopter change [33]. Laser in situ keratomileusis (LASIK) is a modification of Barraquer’s keratomileusis and automated lamellar keratoplasty [34, 35]. The first method was developed by Lucio Buratto and involved the creation of a corneal lenticule (free flap) with a microkeratome, then excimer laser ablation of the posterior surface of the cornea, and re-suturing of the corneal lenticule [36]. In the meantime, Ioannis Pallikaris was developing a method in another way. The method involved creating a lamellar corneal flap with a microkeratome of his design and using an excimer laser to remodel the remaining corneal stroma under the flap. Palikaris coined the term of the method “Laser in situ keratomileusis” (LASIK) [37, 38]. The first clinical femtosecond laser approved by the FDA for refractive surgery use was the IntraLase FS, launched in 2003 [39, 40, 41]. In 2007, new low pulse energy and high pulse frequency Fs laser was introduced by Ziemer – FEMTO LDV. Since 2009, versions of Fs laser systems for use in cataract surgery, such as the first LensX, have also begun to develop in practice [42]. The first clinical version of a lenticule extraction procedure was introduced in the clinical treatment of refractive surgery patients in 2007 [42] as “FLEx” (Femtosecond Lenticule Extraction). A refined surgical version, small-incision lenticule extraction (SMILE) was introduced by Carl Zeiss Meditec and in a short period has replaced FLEx in clinical use [43]. Currently, novel laser systems for SMILE procedure are introduced by ATOS from SCHWIND eye-tech solutions (SmartSight procedure) and ZIEMER LDV Z8 from Ziemer Ophthalmic Systems AG (CLEAR procedure, Corneal Lenticule Extraction for Advanced Refractive correction).

2.2.2 Photorefractive keratectomy (PRK)

Photorefractive keratectomy (PRK) involves the use of an excimer laser on the anterior surface of the cornea to change the refractive status of the eye by changing the curvature of the cornea [44, 45, 46]. Except for refractive purposes, excimer laser surface ablation is used in the treatment of corneal scars and dystrophies when it is called phototherapeutic keratectomy (PTK) [47]. PRK is considered the method of choice, both refractive and therapeutic, in patients with basal membrane dystrophy, given that postoperatively better epithelial adherence occurs [48]. It is indicated in myopia from 1.0 to 6.0D, hyperopia up to 3.0D, and astigmatism up to 6.0D. Treatment of higher corrections is not recommended due to the risk of postoperative corneal opacity [49]. The surgical technique involves removal of the epithelium by excimer laser (transepithelial PRK), knife, 18–20% ethanol alcohol, or sponge. After epithelial removal, excimer laser ablation is performed. After excimer laser ablation, 0.02% mitomycin C is optionally applied to prevent corneal clouding. Postoperative recovery includes postoperative discomfort caused by epithelial erosion and gradual recovery of visual acuity during epithelial healing (within 72 h).

2.2.3 Laser-assisted subepithelial keratectomy (LASEK/epi-LASIK)

LASEK was firstly performed by Dimitri Azar in 1996 and he called it PRK with “Alcohol assisted flap PRK” [50, 51]. The method was named LASEK by Massimo Camellin in 1999, who popularized the technique [52, 53]. The technique involves applying 20% ethanol to the epithelium for 30 seconds to weaken the hemidesmosomal connections between the epithelium and the Bowman’s membrane, leading to the formation of an epithelial sheet that is easily removed before excimer laser ablation and repositioned at the original position. Epi-LASIK was described by Palikaris et al. [54]. The technique involves the use of an automated knife, similar to a microkeratome, to remove epithelium without the use of alcohol. Disadvantages of the method are the possibility of treating only myopia, and the inability to predict the level of postoperative pain and prolonged epithelial healing.

2.2.4 Laser in situ keratomileusis (LASIK)

Laser in situ keratomileusis (LASIK) is the most commonly performed surgical technique for the correction of most refractive errors [55, 56]. LASIK is performed in two steps and combines lamellar surgery with excimer laser application. The first step involves the formation of the anterior corneal flap, its lifting to expose the stroma of the cornea. Today, two technologies are available for flap formation—mechanical microkeratomes and femtosecond lasers, known as femtosecond LASIK (FsLASIK). The second step consists of applying an excimer laser to the stroma of the cornea to change the curvature of the corneal anterior surface. Upon completion of the excimer laser action, the flap is repositioned to its original position [37, 57, 58]. The advantages of LASIK over superficial ablations are the ability to treat a wider range of refractive errors, faster vision recovery, less postoperative discomfort, and lower incidence of postoperative corneal or scar fogging in higher refractive errors. The main disadvantages of the method are the complications related to the creation of the flap, and the risk of iatrogenic keratectasia [59, 60, 61].

2.2.5 Femtosecond refractive lenticule extraction (RELx) and small-incision lenticule extraction (SMILE)

Femtosecond refractive lenticular extraction (RELx) is a corneal refractive procedure based on intrastromal refractive lenticular extraction. In the RLEx procedure, the lenticule was accessed by creating a front corneal flap similar to the LASIK flap, while in the SMILE procedure, the lenticule, located under a 120–130 μm thick cap is accessed through a small 2–4 mm incision on the anterior surface of the cornea. The shape and size of the lenticule are based on a mathematical calculation for the correction of a specific refractive error, and the location and amount of tissue extracted are similar to that of LASIK. The advantages of the method are related to the absence of possible complications related to the formation of the flap, less impact on the biomechanical stability of the cornea, less pronounced dryness of the eye, and less induced aberrations of higher order. The main disadvantages of the method are the possibility of treating only myopia and lower amounts of astigmatism. In case of residual refractive error, currently, only a surface ablation procedure is advised for correction [62, 63, 64, 65].

2.2.6 Multifocal laser ablation profiles

Multifocal laser ablations for the treatment of presbyopia are still in the developmental stage. The multifocal cornea produces a simultaneous image on the retina, and the brain selects the appropriate image depending on whether the person is looking into the distance or at close range while the other image remains blurred. Potential side effects of these procedures are dysphotopsia and monocular diplopia. In this type of ablation protocol, the laser is used to create a multifocal surface on the cornea (changing the strength of the refractive gradient over the pupil) to correct ametropia at a distance and near. A central hyper-positive zone is created for proximity correction, leaving the middle corneal periphery for distance correction. Since some studies reported an unacceptable rate of losing CDVA using this kind of protocol, the procedure did not get widespread clinical use [66, 67, 68]. In recent times, the correction of presbyopia aims the change corneal asphericity and thus, using spherical aberration, increasing the depth of focus. The protocol is called Laser Blended Vision (LBV) and currently has been reported far better tolerated than multifocal ablation procedures [69].

2.3 Corneal implants/inlays

Spectacle independence is all the more sought after, so new surgical treatments are being invented to provide glasses-free life. The idea of keratophakia brought new light to presbyopia treatment [70, 71]. One of the introduced treatments initiated by this idea was corneal inlays [72, 73, 74, 75]. Raindrop is a corneal inlay shaped like a clear lenticule made of hydrogel, which is permeable to oxygen, fluids, and nutrients. The lens is 2 mm wide, 32 μm thick in the center with decreasing thickens to about 10 μm in the periphery, and has no refractive power; therefore, it induces hyperpolate corneal shape allowing good near and intermediate vision with negligibly affected distance vision. It is placed in the non-dominant eye under the corneal flap or intracorneal pocket at 120–200 μm depth at the center of the light constricted pupil [72, 73, 74]. Until 2017 around 4000–6000 Raindrops were implanted worldwide but in November 2018 the manufacturer asked for a device recall due to postoperative haze [76, 77]. Kamra inlay uses the pinhole principle to facilitate near vision. This opaque ring shaped inlay is made of polyvinylidene fluoride and carbon. It is 6 µm thick, 3.6 mm diameter wide with a central 1.6 mm aperture. The inlay is placed in the nondominant eye 250 μm deep into the lamellar corneal pocket. If LASIK is done earlier, the inlay is placed 100–110 μm below the corneal flap and centration is based on the first Purkinje image. Around 20,000 Kamra inlays have been implanted and generally, there was a high satisfaction rate with both distant and near vision [75, 78, 79].

Presbia Flexivue Microlens is a refractive corneal inlay with a plano central zone surrounded by rings of varying additional power between +1.25D and + 3.5D. It is 3 mm wide, 15–20 μm thick, and is made of hydroxyethyl methacrylate and methyl methacrylate. It is placed over the first Purkinje image in a femtosecond creating a corneal pocket that is 280–300 μm deep. The overall satisfaction among patients was high but between other inlays, UCDVA showed a significant decrease from preoperative to postoperative values, but no changes in binocular UCDVA [74, 75, 80, 81, 82]. Best indications for corneal inlays are phakic, presbyopic patients, 41 to 65 years of age, who have low manifest refraction, who do not require correction for clear distance vision, but who do require near correction [78, 83]. Inlays are also indicated as a therapeutic model in keratoconus eyes. One of the most used ones is the Intacs corneal implant by Additional Technology Inc. It consists of two segments designed to be placed in the periphery of the cornea, at approximately two-thirds depth, and are surgically inserted through a small radial incision in the corneal stroma [84, 85]. They are composed of two clear segments made from polymethylmethacrylate (PMMA), each having an arc length of 150°, and are available in six thicknesses—0.210, 0.250, 0.300, 0.350, 0.400 and 0.450 mm [86]. Aimed populations that can benefit from Intacs are patients with keratoconus older than 21 years of age who have progressive vision deterioration, clear corneas with at least 450 μm corneal tissue at the proposed incision site and transplantation as the only remaining treatment option. Contraindications for Intacs are corneas below 449 μm at the incision site, patients with autoimmune and immunodeficiency disorders, pregnant and nursing women, patients with recurrent corneal erosion and other corneal dystrophies, and patients taking isotretinoin or amiodarone hydrochloride [86, 87].

2.4 Phakic intraocular lenses (pIOLs)

Phakic intraocular lenses (pIOLs) are one of the available surgical option for the treatment of ametropia [88]. When the natural crystalline lens is clear and usually has retained its accommodative function pIOLs are used. They are an effective and relatively safe option for surgical treatment of refractive errors with a special emphasis on very high refractive errors in both myopic and hyperopic eyes [89]. It has been generally accepted that refractive surgery is an effective and safe way of treating refractive errors. The first choice for surgical treatment is the cornea, the ubiquitous and commoditized nature of excimer lasers today has popularized surgical options [90]. The issue is when a refractive surgeon is met with a challenge of very high prescriptions or other confounding factors, such as thin corneas, or other factors that increase the risk for an adverse outcome for corneal refractive surgery. The most commonly accepted range for laser vision correction in corneal refractive surgery is between 10 diopters of myopia to 5 diopters of hyperopia with up to 5 diopters of cylindrical correction. For patients with high motivation and a clear crystalline lens with no presbyopia, that fall outside these limits or have other factors connected to a potentially adverse outcome, phakic IOLs present a great option [89]. There are two refractive pIOLs approved for correcting refractive errors—anterior chamber and posterior chamber pIOLs [91].

2.4.1 Anterior chamber pIOLs

Anterior chamber pIOLs in use today are made by Ophtec, a Dutch company, their anterior chamber lens is called ArtiLens, there are two types—a flexible version ArtiFlex and a rigid PMMA version Artisan [92]. These lenses were at one time distributed by AMO, now Johnson & Johnson Vision, and many surgeons will use them under their old names Veriflex and Verisyse. The ArtiFlex is a foldable pIOL that is inserted through an opening of 3.2 mm, it is fixated on the iris using an enclavation technique, the powers range from −14.5 to −2.0, there is a toric version available with powers from −13.5 to −1.0 and cylinder ranging from −5.0 to −1.0. The Artisan is a rigid PMMA pIOL that has an optic diameter of 5 or 6 mm the powers available range from −15.5 to +12.0. These pIOLs are also fixated on the iris but as they are not foldable they require a larger incision either 5 or 6 mm depending on the optic diameter [92]. There was another option, Alcon CACHET, an angle-supported anterior chamber pIOL was an additional variant, but was discontinued during 2014 due to endothelial cell loss as a serious side effect. Endothelial cell loss occurs due to contact of endothelium layer and pIOL surface, causing corneal endothelial decompensation which leads to corneal edema (overhydration), and in the advanced stage, bullous keratopathy [93].

2.4.2 Posterior chamber pIOLs

Posterior chamber phakic IOLs are a different approach that wants to avoid endothelial cell loss by moving the pIOL behind the iris plane. But by moving the pIOL behind the iris and just above the crystalline lens, there are new potential issues that can arise. The first issue is angle closure glaucoma, as the pIOL can reduce the aqueous fluid outflow by pushing the iris angle, inducing very high IOP. Previous generations of pIOL designs required a small iridotomy creation to facilitate aqueous fluid outflow [94]. The second issue is that the pIOL could block the flow of fluid around the optic and into the anterior chamber, the iridotomy was also beneficial in these cases. The third issue was in case of pIOL touching the capsule of the crystalline lens, an early onset cataract can form [95]. The three big issues today are mostly avoided by the use of modern diagnostic tools and surgical experience. The new posterior chamber pIOLs have very strict sizing guides to adjust the size of the pIOL to the sulcus of the patient to avoid the lens closing the iridocorneal angle, proper sizing also ensures a large enough vault between the pIOL and crystalline lens, and the block of fluid flow is rectified by new pIOLs with a center hole for unobstructed flow [96]. Currently, there is only one generally adopted posterior chamber pIOL, STAAR Surgical Visian ICL. STAAR Surgical has patented the design and material of their lenses, these are very soft collamer-based pIOLs that are implanted with minimally invasive 3.2 mm injectors, and the folded lens is placed in the sulcus after it unfolds in the iris plane. Visian ICL comes in spherical powers from −18.0 to +10.0 diopters, and there is a toric variant from +0.5 to +6.0 diopters of the cylinder. The ICL is produced in four sizes, 12.1 mm, 12.6 mm, 13.2 mm, and 13.7 mm to fit the size of the sulcus of the patient as best as possible [92, 97]. STAAR Surgical Visian ICL is a great option for patients that are not suitable candidates for corneal refractive surgery and are not ready for refractive lens exchange due to their age.

2.5 Cataract surgery and refractive lens exchange

2.5.1 History of cataract surgery and evolution of intraocular lenses

The first records of cataract surgery date back to antiquity, where couching was the only method of resolving optical path opacity, but without replacing the refractive property (power) of the removed crystalline lens [18, 98]. At a later age, about 600 BC a primitive version of extracapsular cataract extraction was described by an Indian surgeon Sushruta [99]. It was not until the 18th century, in 1947, that the forerunner of modern cataract surgery—extracapsular cataract extraction (ECCE) was performed by Jacques Daviel [100, 101]. A few years later, in 1753 Samuel Sharp performed intracapsular cataract extraction (ICCE) [102]. During the next decade, ICCE was considered as a method of choice for cataract surgery. The main difficulties of these procedures were related to complications, such as high risks of postoperative infection, prolonged wound healing (10–12 mm), vitreous prolapse, and retinal ablation. In 1961 Tadeusz Krwawicz invented cryoextraction, a freezing method for removing the cataractous lens [103]. The introduction of phacoemulsification in 1967 by Dr. Charles Kelman was the basis and beginning of today’s modern cataract surgery [104]. Concurrently, the idea of replacing the cataractous lens with artificial optics was developing, starting from Sir Harold Ridley who observed in the 2nd World War that one of the pilots had a plastic shrapnel eye injury, without causing foreign body reaction. Guided by that, he developed the first intraocular lens (IOL) made of polymethylmethacrylate (PMMA) for insertion in the eye after cataractous lens removal [105]. Various materials have been tried for IOLs, and finally in the late 1970s flexible, silicone lens was brought into use. The primary aim of introducing flexible IOLs was to avoid the disadvantages of PMMA, such as larger incisions and consequent postoperative astigmatism. Silicone IOLs rapidly adopted and conquered the market during the 1980s [106]. In 1989, the first commercially available three-piece silicone IOL was introduced (PhacoFlex SI-18 by AMO, now Johnson & Johnson Vision) [107]. Further on, in the 1980s, Barret developed the first hydrogel IOL made of soft hydrophilic material (IOGEL PC-12) implanted in 1983 [108]. Following closely, hydrophobic acrylic IOLs were developed, which represent the most common implanted foldable IOL today [109, 110]. The combination of innovations, such as the phacoemulsification technique, foldable IOL, and even the use of topical anesthesia [111], has ensured the development of modern cataract surgery. Over the next few decades, attempts were made to introduce lasers into ophthalmic surgery. Bille and Schanzlin were the first to propose ultrashort laser pulses for treating cataracts back in 1993 [112] and the first clinical results of femtosecond laser use in cataract surgery (Femtosecond-Laser-Assisted cataract surgery - FLACS) was reported by Nagy et al. [113]. In parallel with the development of surgical methods, the evolution of an IOL design has moved in the direction of correcting all working distances, trying to minimize or even remove spectacle independence [114]. With the introduction of multifocal lenses, an era began in which cataract surgery became refractive surgery. Since the 1980s, bifocal, trifocal, quadrifocal IOLs have been designed, and toric IOLs have been introduced to correct astigmatism [115]. As refractive surgery has developed widely, with increasing needs of working patients, occasional refractive surprises became a problem in clinical practice due to patient dissatisfaction. In this name, supplementary IOLs, such as SulcoFlex by Rayner, have been developed as one of the possible surgical correction options [116, 117, 118, 119]. Accommodative IOL was developed to provide better distance corrected near visual acuity and higher levels of spectacle independence than standard monofocal IOLs but also producing minimal unwanted visual disturbances, such as halos and glares and contrast sensitivity compared with multifocal IOLs. The first accommodative FDA-approved IOL was CrystaLens by Bausch & Lomb Inc. [120, 121]. In an attempt of overcoming the drawbacks of multifocal and accommodative IOLs, EDOF design was developed, with the first FDA approval for Tecnis Symfony by Johnson & Johnson Vision [122]. The main principle of EDOF design is a single elongated focal point that enhances the depth of focus (range of vision), and therefore significantly reduces potential halos and glares induced by multifocal IOL by eliminating the overlapping of near and far images [123, 124, 125].

2.5.2 Monofocal intraocular lenses

Monofocal intraocular lenses are the most common type of IOL used in cataract surgery. They are designed to correct a patient’s visual acuity for far distances, with the need for an optical aid for near vision. Lenses are usually indicated in patients with extremely high myopic or hyperopic refraction, amblyopia, macular degeneration, dry eye syndrome, history of previous ocular surgery, ocular trauma, autoimmune diseases, or connective tissue diseases.

Currently, monofocal IOLs are mainly represented as hydrophobic acrylic lenses with an aspheric surface design. Aspheric types of IOLs are eliminating positive spherical aberration of the prolate cornea, improving functional vision and reducing side effects, such as low contrast sensitivity or low night-driving performance. The functional benefit and optical advantages of aspheric IOL technology are related to pupil size, depth of focus, IOL centration, and customization. Since it is pupil-size dependent, some studies have shown that aspheric IOLs offer little or almost no benefit in smaller pupils [126, 127, 128, 129]. Therefore, when it comes to customization, preoperative assessment is extremely important, which in addition to standard measurements includes corneal topographic analysis, and the values of corneal aberrations, especially spherical aberration. According to the obtained parameters, the final decision on the type of IOL is given.

2.5.3 Presbyopia-correcting intraocular lenses

With the development and increase in cataract surgical treatment, the patient’s expectation regarding postoperative outcomes is also increasing, as they seek to achieve independence from spectacles and favorable visual outcomes at both near and far distances to meet the needs of everyday activities [130, 131, 132]. Nowadays, in a presbyopia-correcting pool of IOLs, multifocal and extended range of vision (EDOF) IOLs are most prevalent in clinical use. Multifocal IOLs in the initial variant had a diffractive design, and afterward, refractive design was introduced. Diffractive IOLs had one variant with or without apodization, where the central (near) area is surrounded by concentric rings with decreasing heights [133] and the second variant with an aspheric anterior surface and a posterior surface with diffractive rings [134, 135]. Refractive IOL design has an asymmetrical shape of the central (near) segment to provide a sort of transition between zones of IOL [136, 137]. Trifocal diffractive IOL was introduced to improve intermediate vision with a third focus, at 80 cm. The first one in clinical use was FineVision IOL by Physiol, further followed by At LISA tri by Zeiss, and RayOne Trifocal by Rayner. In comparison with a traditional trifocal IOL, quadrifocal IOL has three added powers for near and intermediate vision, providing more continuous vision. One example of IOL with quadrifocal design is PanOptix by Alcon, which uses a specific optical technology to redirect the focal point at 120 cm to the distance focal point for amplified performance [138].

After introducing a technology designed to improve the range of vision, especially at intermediate distances, EDOF IOLs gained high popularity in refractive cataract surgery. An EDOF technology development arose from the necessity to obviate drawbacks of monofocal and multifocal IOLs – providing better vision for intermediate distance without compromising functional vision, reducing contrast sensitivity, or inducing disturbances, such as halos and glares. Currently, there are four different EDOF technologies [139]—diffractive optics IOL (Tecnis Symfony and Synergy IOL, Tecnis Eyhance IOL by Johnson & Johnson Vision, and AT LARA by Zeiss- hybrid multifocals) [125, 140] non-diffractive optics IOL (AcrySof IQ Vivity IOL by Alcon and SiFI Mini WELL IOL by SIFI MedTech Srl.) [141, 142], small-aperture IOL (IC-8 IOL by AcuFocus Inc.) [143], and bioanalogic IOL (Wichterle IOL-Continuous Focus - WIOL-CF by Medicem) [144].

Conflict of interest

The authors declare no conflict of interest.

Acronyms and abbreviations

LASIK

laser in situ keratomileusis

SMILE

small-incision Lenticule extracton

IOL

intraocular lens

SS-OCT

swept-source optical coherence tomography

CLE

clear lens extraction

RLE

refractive lens exchange

PTK

photo therpeutic keratectomy

FLEx

femtosecond lenticule extraction

PRK

photo refractive keratectomy

LASEK

laser assisted sub-epithelial keratectomy

UCDVA

uncorrected distance visual acuity

PMMA

polymethyl methacrylate

PIOL

phakic intraocular lens

ECCE

extracapsular cataract extraction

ICE

intracapsular cataract extraction

FLACS

femtosecond laser assisted cataract surgery

EDOF

extended depth of focus