**7. Intraocular Lenses (IOLs)**

Nowadays, cataract surgery is not synonymous with lens extraction; it evolved in a more refined procedure due to advances in phacoemulsification procedure and intraocular lens technology [39]. The most frequent and the cheapest intraocular lenses are monofocal. After their implantation most patients need spectacles, at least for near vision. The goal of premium IOL design is to enable the best refractive outcome with restoration of vision for distance and near without spectacles. Multifocal and accommodative intraocular lenses, as well as toric IOLs for corneal astigmatism compensation, are considered premium IOLs. The aim of multifocal and accommodative premium IOLs is to allow the presbyopic patient to regain the ability to accommodate. The capacity of the eye to actively change its refractive power to create a sharp image on the retina of distant, intermediate, and near objects is called accommodation [40].

#### **7.1. Multifocal IOLs**

**Figure 3.** Diaphragm pump

**Figure 1.** Peristaltic pump

190 Advances in Eye Surgery

**Figure 2.** Venturi pump

Multifocal IOLs focus light in more than one point. They are described as refractive, diffractive, and combinations of both optical principles. Also, they can be spherical and aspherical. They were first introduced in the 1980s [41, 42]. They consist of multiple circular, concentric areas that provide a continuous variation of the refractive power. Diffractive multifocal artificial lenses are based on the HuygensFresnel principle [43] presenting concentric rings that result in two or more coexisting retinal images and are independent of pupil size.

On the other hand, refractive IOLs are dependent on pupil size. Refraction is based on a change in direction of the light ray due to a change in the optical density of the material transmitting the light ray. Refractive IOLs provide usually good near vision but are mostly insufficient regarding very small prints [44]. Recent studies report very good results in most cases after implantation of a multifocal IOL, diffractive [45-47], refractive [46, 47], or hybrid diffractiverefractive [48]. Aspheric multifocals decrease higher-order aberrations of the ocular optical system, primarily by compensating for the increased positive spherical aberration of the cornea in older subjects [49, 50].They provide a significantly better near visual acuity compared to spherical multifocal IOLs, with no significant influence on night vision symptoms and contrast sensitivity [51]. Various studies have shown that uncorrected near vision is improved by implantation of a multifocal IOL in comparison to monofocal IOL, resulting in lower levels of spectacle dependence for near tasks without compromising distance visual acuity [52-58]. In general, multifocal IOLs are able to provide patients with excellent uncorrected distance and near visual acuity resulting in high levels of spectacle independence [49]. Although, the preceding discussion makes the multifocals very appealing, they are not devoid of problems. One of the main reasons for patient dissatisfaction is dysphotopsia [59, 60]. The most reported phenomena after multifocal IOL implantation are halos and glare [61, 62], especially in refractive multifocal IOLs [46]. In general, multifocal IOLs are associated with lower contrast sensitivity than monofocal IOLs [46]. Regarding that, diffractive multifocal IOLs appear to be equal or superior to refractive multifocal IOLs [63, 64]. Also, multifocal IOLs have been associated with higher levels of high order aberrations (HOAs) than monofocal IOLs [65]. Refractive multifocal IOLs performed better at intermediate than near distance [51, 66]. Diffractive multifocal IOLs with lower near additions have increased visual acuity at inter‐ mediate distance without decreasing near and distance visual acuity [48, 67]. Lately, it has been shown that intermediate visual acuity is increased with trifocal diffractive IOLs [68]. In preoperative evaluation, it is of great importance careful selection of patients, individual approach, patient s education and consideration of all benefits and side-effects of multifocal IOLs [69, 70].

#### **7.2. Accommodative IOLs**

An alternative to multifocal lenses are accommodative lenses, which are able to change position or shape in response to the accommodative reflex. They were designed to avoid the optical side effects of multifocal IOLs and to offer the best solution for presbyopia: an IOL of high-amplitude variable focality [71]. They are classified according to design into single-optic, dual-optic, and curvature change IOLs [72]. Accommodative lenses act like monofocal lenses, but provide better visual acuity for intermediate and distance vision. In comparison to multifocals, they are independent on pupil size and therefore provide less disphotopic effects and do not decrease contrast sensitivity. On the other hand, there is a variability of the postoperative outcome, greater risk for capsular contraction and opacification, and the need for further near vision correction [73]. Ideal IOL would allow the presbyopic patient to regain his or her ability to accommodate. Experiments have been conducted with refilling the capsular bag with a clear and elastic substance in order to achieve desirable accommodation, but unfortunately unsuccessfuly [74]. Also, different attempts with the change in position of the IOL or parts of it within the optical system in order to change the optical power of the optical system and to restore the patient's accommodation in that way have not resulted as expected [75]. Some of the ultrasound studies showed changes in the position of accommodating IOLs within the optical system in response to physiological or pharmaceutical stimuli [76], while others did not provide evidence of significant movement of those IOLs [77, 78]. Looking into a clinical practice, accommodating IOLs seems to be insufficient to result in large changes in the power of the optical system [75, 79].

#### **7.3. Toric IOLs**

implantation of a multifocal IOL, diffractive [45-47], refractive [46, 47], or hybrid diffractiverefractive [48]. Aspheric multifocals decrease higher-order aberrations of the ocular optical system, primarily by compensating for the increased positive spherical aberration of the cornea in older subjects [49, 50].They provide a significantly better near visual acuity compared to spherical multifocal IOLs, with no significant influence on night vision symptoms and contrast sensitivity [51]. Various studies have shown that uncorrected near vision is improved by implantation of a multifocal IOL in comparison to monofocal IOL, resulting in lower levels of spectacle dependence for near tasks without compromising distance visual acuity [52-58]. In general, multifocal IOLs are able to provide patients with excellent uncorrected distance and near visual acuity resulting in high levels of spectacle independence [49]. Although, the preceding discussion makes the multifocals very appealing, they are not devoid of problems. One of the main reasons for patient dissatisfaction is dysphotopsia [59, 60]. The most reported phenomena after multifocal IOL implantation are halos and glare [61, 62], especially in refractive multifocal IOLs [46]. In general, multifocal IOLs are associated with lower contrast sensitivity than monofocal IOLs [46]. Regarding that, diffractive multifocal IOLs appear to be equal or superior to refractive multifocal IOLs [63, 64]. Also, multifocal IOLs have been associated with higher levels of high order aberrations (HOAs) than monofocal IOLs [65]. Refractive multifocal IOLs performed better at intermediate than near distance [51, 66]. Diffractive multifocal IOLs with lower near additions have increased visual acuity at inter‐ mediate distance without decreasing near and distance visual acuity [48, 67]. Lately, it has been shown that intermediate visual acuity is increased with trifocal diffractive IOLs [68]. In preoperative evaluation, it is of great importance careful selection of patients, individual approach, patient s education and consideration of all benefits and side-effects of multifocal

An alternative to multifocal lenses are accommodative lenses, which are able to change position or shape in response to the accommodative reflex. They were designed to avoid the optical side effects of multifocal IOLs and to offer the best solution for presbyopia: an IOL of high-amplitude variable focality [71]. They are classified according to design into single-optic, dual-optic, and curvature change IOLs [72]. Accommodative lenses act like monofocal lenses, but provide better visual acuity for intermediate and distance vision. In comparison to multifocals, they are independent on pupil size and therefore provide less disphotopic effects and do not decrease contrast sensitivity. On the other hand, there is a variability of the postoperative outcome, greater risk for capsular contraction and opacification, and the need for further near vision correction [73]. Ideal IOL would allow the presbyopic patient to regain his or her ability to accommodate. Experiments have been conducted with refilling the capsular bag with a clear and elastic substance in order to achieve desirable accommodation, but unfortunately unsuccessfuly [74]. Also, different attempts with the change in position of the IOL or parts of it within the optical system in order to change the optical power of the optical system and to restore the patient's accommodation in that way have not resulted as expected [75]. Some of the ultrasound studies showed changes in the position of accommodating IOLs within the optical system in response to physiological or pharmaceutical stimuli [76], while

IOLs [69, 70].

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**7.2. Accommodative IOLs**

Approximately 20% to 30% of patients who have cataract surgery have corneal astigmatism of 1.25 diopters (D) or higher and approximately 10% of patients have 2.00 D or higher [80-82]. Toric intraocular lenses are a predictable method of astigmatic correction with minimal impact to the cornea. Based on current evidence, it appears that a minimal amount of corneal astig‐ matism of approximately 1.25 D should be present before toric IOL implantation is considered [83]. They provide an opportunity for corneal astigmatism correction at the time of cataract surgery and for spectacle independence. But, effectiveness of torics is dependent on its orientation [84, 85]. A 10-degree error in rotation results in a 35% residual error in the magni‐ tude of astigmatism. Since the beginning of the first implanted foldable toric intraocular lens in 1994 [86-89], many improvements in IOL material and design have been made to improve postoperative rotational stability and therefore improved visual outcomes following toric IOL implantation. Toric IOLs are made of hydrophobic acrylic, hydrophilic acrylic, silicone, or polymethylmethacrylate (PMMA) biomaterial. The IOL biomaterial plays a major role in the postoperative rotation of the IOL. After implantation of a toric IOL in the capsular bag, the anterior and posterior capsules fuse with the IOL, preventing IOL rotation [90]. Adhesions of the IOL to the capsular bag are thought to prevent IOL rotation. Lombardo et al. found that hydrophobic acrylic IOLs showed the highest adhesive properties, followed by hydrophilic acrylic IOLs, PMMA IOLs, and finally silicone IOLs [91]. Oshika et al. showed the strongest IOL–capsular bag adhesions for acrylic IOLs, followed by PMMA and silicone IOLs [92]. Acrylic IOLs generally form the strongest adhesions with the capsular bag [90]. Also, IOL design contributes to the stability of the lens in the capsular bag and avoidance of postoperative IOL rotation. It has been shown that the overall IOL diameter and haptic design are major factors in the prevention of IOL rotation [93-95]. Currently available toric IOLs have a total IOL diameter ranging from 11.0 mm to 13.0 mm. The lens with longer IOL diameter was found to have much better rotational stability than the lens with shorter diameter [94]. Two haptic designs are available: plate-haptic and loop-haptic. When comparing the postoperative rotation of plate versus loop-haptic silicone IOLs, some studies found significantly higher postoperative rotation with loop-haptic IOLs than with plate-haptic IOLs [93]. Others com‐ pared plate-haptic and loop-haptic acrylic IOLs and did not find a significant difference in postoperative rotation, suggesting that in acrylic IOLs, plate and loop haptics have both good rotational stability [96]. Toric IOLs are most effective in the correction of regular astigmatism, but they have been shown to be effective in patients with irregular corneal astigmatism, including keratoconus (only if the risk for progression is minimal) [97], pellucid marginal degeneration [98], and post-keratoplasty eyes [99, 100]. Not suitable for torics are patients with potential capsular bag instability, like those with pseudoexfoliation syndrome or traumainduced zonulysis because zonular weakness affects IOL stability and may result in rotation or decentration of a toric IOL [83]. Multifocal toric IOLs gain for spectacle independence regarding distance, intermediate, and near vision. Evidence has been provided that the presence of 1.00 D or higher astigmatism in eyes with a multifocal IOL compromise distance and near visual acuity, showing the importance of an optimal astigmatism correction in these patients [101].
