**2.3 Small incision lenticule extraction**

The femtosecond laser corneal procedure known as small incision lenticule extraction (SMILE) was originally described by Sekundo et al. and became clinically

**Figure 1.**

*Comparison of change in spherical correction over time between T-PRK and LASIK.*

#### **Figure 2.**

*Comparison of change in astigmatism correction over time between T-PRK and LASIK.*


#### **Table 1.**

*Comparison of change in high-order aberrations over time between T-PRK and LASIK.*

#### **Figure 3.**

*Polar diagram showing target and surgically induced astigmatic values for the SMILE group. The concentric semicircles reduce in 0.50 D steps from −2.00 DC (outermost semicircle) toward zero (central point) in 0.50 DC steps. From right to left, the 0 to 90 to 180° axes are shown in 30° steps. The target and surgically induced astigmatism data points are shown as empty circles and filled dots, respectively.*

available in 2011 [15]. The procedure does not require the creation of a flap: two precise intrastromal planar sections are created using a single femtosecond laser to form an intrastromal lenticule. The intrastromal lenticule is dissected from the pocket, grasped with a forceps, and manually extracted through a small incision. The incision is placed at the superior temporal/nasal quadrant, usually angled at 70°, and 2–5 mm in length. The removal of the intrastromal lenticule alters the shape of the cornea, thereby correcting myopia and astigmatism. Since Bowman's layer remains intact, the procedure offers greater biomechanical stability, especially in the treatment of higher levels of myopia [15]. The flapless property of SMILE obviates the risks associated with LASIK including adverse events at flap creation and dislocation [16].

The tensile strength of the cornea may reduce by 55% after a SMILE procedure when the lenticule is formed and extracted from the anterior half of the stroma. Loss of tensile strength is less profound when the lenticule is extracted from deeper regions of the stroma. Thus, the exact change in the biomechanical properties of the cornea will depend on the amount of ablation and the location where the lenticule is formed [13].

Regarding the available data, and our experience, LASIK and SMILE are comparable procedures in terms of visual quality and reduction of myopia; however, in treating astigmatism LASIK still offers better precision (**Figures 3** and **4**).

#### **2.4 Indications and preoperative preparation for refractive surgery**

A detailed review of the patient's condition before surgery and informing the patient about the results, benefits, and disadvantages of the procedure are the most important steps for a successful outcome of refractive surgery [17].

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**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.85644*

situations, for an indefinite time period.

ing pregnancy and lactation [25].

The examination should include detailed medical history (systemic status, medications intake, allergies, ocular status, information about previous ocular surgeries—especially in the case of refractive lens exchange—and information about contact lens wear) and reasons/motivations for refractive surgery to identify patients with unrealistic expectations [18, 19]. It is important for patients to understand that refractive surgery primarily serves to reduce spectacle dependence and contact lens use, and it is not meant to completely remove all optical aids in all

*astigmatism data points are shown as empty circles and filled dots, respectively.*

*Polar diagram showing target and surgically induced astigmatic values for the FsLASIK group. The concentric semicircles reduce in 0.50 D steps from −2.00 DC (outermost semicircle) toward zero (central point) in 0.50 DC steps. From right to left, the 0 to 90 to 180° axes are shown in 30° steps. The target and surgically induced* 

Patients should discontinue contact lens use before the examination (for soft contact lenses, at least a week prior to the examination, and for rigid gas permeable contact lenses, at least 2–3 weeks prior) since corneal topography and biometry measurement can be severely affected by the corneal changes induced by contact lens wear. In the case of corneal warpage syndrome (corneal irregularities caused by contact lenses), contact lenses should be discontinued for at least 4–6 weeks [20]. The preoperative evaluation must include monocular manifest refraction, cycloplegic refraction, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), pupillometry, tonometry, anterior chamber depth (ACD) measurement, corneal topography/tomography, pachymetry, aberrometry, tear film quality and quantity, determining the dominant eye, ocular motility, and a fundus examination [18, 21]. Cycloplegic refraction is recommended to exclude the accommodation effect, while in patients in/or close to presbyopia age near visual acuity should be checked also. It is mandatory to check the patient's refractive stability during the time, which can most often be obtained by inspecting the patient's eyeglasses or by reviewing the previous ophthalmological documentation [21]. Contraindications for refractive surgery may relate to systemic or ocular disorders. Absolute systemic contraindications are poorly controlled systemic immune diseases (e.g., rheumatoid arthritis, systemic lupus erythematosus, polyarteritis nodosa), as well as poorly controlled diabetes and AIDS. Such patients have a higher risk of complications associated with prolonged inflammation or corneal healing after refractive surgery [18, 22–24]. Surgical procedures are not recommended dur-

Ocular absolute contraindications are considered to be poorly controlled or untreated eye inflammation (blepharitis, dry eye syndrome, atopy/allergy), poorly controlled glaucoma, clinically significant lens opacities, Stevens-Johnson syndrome, ocular pemphigoid, and chemical burns of the eye surface [26, 27]. Instability of refraction (i.e., a change greater than 0.50 D within a year) is

#### **Figure 4.**

*Intraocular Lens*

**Figure 3.**

**Table 1.**

*Polar diagram showing target and surgically induced astigmatic values for the SMILE group. The concentric semicircles reduce in 0.50 D steps from −2.00 DC (outermost semicircle) toward zero (central point) in 0.50 DC steps. From right to left, the 0 to 90 to 180° axes are shown in 30° steps. The target and surgically induced* 

available in 2011 [15]. The procedure does not require the creation of a flap: two precise intrastromal planar sections are created using a single femtosecond laser to form an intrastromal lenticule. The intrastromal lenticule is dissected from the pocket, grasped with a forceps, and manually extracted through a small incision. The incision is placed at the superior temporal/nasal quadrant, usually angled at 70°, and 2–5 mm in length. The removal of the intrastromal lenticule alters the shape of the cornea, thereby correcting myopia and astigmatism. Since Bowman's layer remains intact, the procedure offers greater biomechanical stability, especially in the treatment of higher levels of myopia [15]. The flapless property of SMILE obviates the risks associated

The tensile strength of the cornea may reduce by 55% after a SMILE procedure when the lenticule is formed and extracted from the anterior half of the stroma. Loss of tensile strength is less profound when the lenticule is extracted from deeper regions of the stroma. Thus, the exact change in the biomechanical properties of the cornea will depend on the amount of ablation and the location where the lenticule is formed [13]. Regarding the available data, and our experience, LASIK and SMILE are comparable procedures in terms of visual quality and reduction of myopia; however, in

A detailed review of the patient's condition before surgery and informing the patient about the results, benefits, and disadvantages of the procedure are the most

with LASIK including adverse events at flap creation and dislocation [16].

treating astigmatism LASIK still offers better precision (**Figures 3** and **4**).

**2.4 Indications and preoperative preparation for refractive surgery**

important steps for a successful outcome of refractive surgery [17].

*astigmatism data points are shown as empty circles and filled dots, respectively.*

*Comparison of change in high-order aberrations over time between T-PRK and LASIK.*

**104**

*Polar diagram showing target and surgically induced astigmatic values for the FsLASIK group. The concentric semicircles reduce in 0.50 D steps from −2.00 DC (outermost semicircle) toward zero (central point) in 0.50 DC steps. From right to left, the 0 to 90 to 180° axes are shown in 30° steps. The target and surgically induced astigmatism data points are shown as empty circles and filled dots, respectively.*

The examination should include detailed medical history (systemic status, medications intake, allergies, ocular status, information about previous ocular surgeries—especially in the case of refractive lens exchange—and information about contact lens wear) and reasons/motivations for refractive surgery to identify patients with unrealistic expectations [18, 19]. It is important for patients to understand that refractive surgery primarily serves to reduce spectacle dependence and contact lens use, and it is not meant to completely remove all optical aids in all situations, for an indefinite time period.

Patients should discontinue contact lens use before the examination (for soft contact lenses, at least a week prior to the examination, and for rigid gas permeable contact lenses, at least 2–3 weeks prior) since corneal topography and biometry measurement can be severely affected by the corneal changes induced by contact lens wear. In the case of corneal warpage syndrome (corneal irregularities caused by contact lenses), contact lenses should be discontinued for at least 4–6 weeks [20].

The preoperative evaluation must include monocular manifest refraction, cycloplegic refraction, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), pupillometry, tonometry, anterior chamber depth (ACD) measurement, corneal topography/tomography, pachymetry, aberrometry, tear film quality and quantity, determining the dominant eye, ocular motility, and a fundus examination [18, 21]. Cycloplegic refraction is recommended to exclude the accommodation effect, while in patients in/or close to presbyopia age near visual acuity should be checked also. It is mandatory to check the patient's refractive stability during the time, which can most often be obtained by inspecting the patient's eyeglasses or by reviewing the previous ophthalmological documentation [21].

Contraindications for refractive surgery may relate to systemic or ocular disorders. Absolute systemic contraindications are poorly controlled systemic immune diseases (e.g., rheumatoid arthritis, systemic lupus erythematosus, polyarteritis nodosa), as well as poorly controlled diabetes and AIDS. Such patients have a higher risk of complications associated with prolonged inflammation or corneal healing after refractive surgery [18, 22–24]. Surgical procedures are not recommended during pregnancy and lactation [25].

Ocular absolute contraindications are considered to be poorly controlled or untreated eye inflammation (blepharitis, dry eye syndrome, atopy/allergy), poorly controlled glaucoma, clinically significant lens opacities, Stevens-Johnson syndrome, ocular pemphigoid, and chemical burns of the eye surface [26, 27]. Instability of refraction (i.e., a change greater than 0.50 D within a year) is

#### *Intraocular Lens*

considered as an absolute contraindication, as well as insufficient corneal thickness or corneal irregularities suspicious for keratoconus [21, 26, 28, 29]. Precautions are also needed in patients with certain systemic therapies (isotretinoin, amiodarone, sumatriptan, colchicine) [23, 24, 30]. Caution is also required in functional monocular patients and in patients with well-controlled glaucoma. Other relative contraindications are history of uveitis, herpes simplex, and varicella zoster keratitis. In patients with epithelial basal membrane degeneration, LASIK is not recommended, but PRK is the procedure to consider [21, 31].

## **2.5 Limitations and complications of corneal refractive surgery**

Complications of corneal refractive surgery are considered rare. They can be divided in intraoperative and postoperative complications (which can be early or delayed).

Regarding the intraoperative complications, they are mainly correlated with corneal flap creation or excimer laser ablation. During the era of microkeratome, flap-related complications were encountered more often and fell within 3%; with the introduction of femtosecond lasers, they were almost nullified; however, some complications specific to femtosecond lasers appeared [32].

Flap-related complications include free or partial flap creation, incomplete and irregular flap creation, thin and perforated flaps, and corneal perforation. Those complications were mostly related to corneal anatomy (flat <41.00 D or steep >46.00 D corneas, small corneal diameter), inadequate suction, mechanical failure—a defect in the dissection blade or motor unit—and surgeon experience. Penetration into the anterior chamber is extremely rare and may occur during lamellar dissection or excimer laser photoablation usually on extremely thin corneas with old scars [33].

Femtosecond-related complications are closely correlated with cavitation bubbles and formation of the flap. They are presented in the form of confluent cavitation bubbles in the corneal lamellae or anterior chamber which can interfere with excimer laser systems and vertical gas breakthrough which is presented in the forms of incomplete buttonholes or difficulties in dissecting the flap due to tissue bridges [34]. Temporary hypersensitivity to light and rainbow glare are complications exclusively related to energy and pattern of femtosecond lasers characterized with normal visual acuity and photophobia without inflammation or light dispersion in low light conditions [35].

Laser-related complications include decentration of excimer laser ablation, irregular astigmatism, and formation of central islands. Those complications are clinically characterized by poor uncorrected and corrected distance visual acuity complaints of glare, "ghosting" around images and haloes, and refractive astigmatism in the axis of decentration [33].

Early postoperative complications include flap striae, diffuse lamellar keratitis, central toxic keratopathy, pressure-induced steroid keratitis, infectious keratitis, and epithelial ingrowth.

Flap striae are caused by misalignment of the flap; peripheral striae usually are asymptomatic; however, central location of the striae is associated with loss in corrected distance visual acuity and night vision disturbances [33, 36].

Diffuse lamellar keratitis (Sands of Sahara syndrome) is a sterile inflammation probably caused by the introduction of toxins in the flap interface [37, 38]. It is graded in four stages, with stage one and two being mild and visually unthreatening, while stage four can lead to corneal melting and permanent changes [33, 39]. In comparison to diffuse lamellar keratitis, central toxic keratopathy is a rare noninflammatory central corneal opacification linked to enzymatic degradation of keratocytes with spontaneous resolution and mild central opacification which often causes refractory hyperopic shift [40].

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*DOI: http://dx.doi.org/10.5772/intechopen.85644*

melting causing visual disturbances [43].

reduced vision quality [44].

astigmatism [45].

with brown iris [46].

Pressure-induced stromal keratitis is also easily mistaken with diffuse lamellar keratitis but is caused by postoperative steroid use which leads to increase in

Infectious keratitis after LASIK is extremely rare but can be quite serious since invading organisms are already implanted into the deep corneal stroma. The most often isolated organisms include *Streptococcus pneumoniae*, *Staphylococcus aureus*,

Epithelial ingrowth under the LASIK flap is reported to occur in merely 1–2% of patients and is caused by migration of epithelial cells under the flap. It is usually insignificant, but if epithelial cells continue to grow, it can cause flap distortion and

Late postoperative complications include dry eye, night vision problems, corneal

Dry eye syndrome is caused by denervation and cutting of nerve fibers during the formation of the flap, removal of corneal tissue by excimer laser, and changes in the shape of the cornea. Dry eye syndrome is usually transient and symptoms fade away after healing period. It causes discomfort, fluctuation in vision quality, slower healing, and epithelial damage and may lead to regression of refractive error and

Symptoms of impaired visual quality are usually more expressed during the night due to physiological pupil dilatation. The main causes of nighttime issues are the increase in spherical aberrations at the centrally flatted cornea, decentered ablations, too small optical zones, newly emerging lens opacities, and induced

Corneal haze reduces corneal transparency at variable degrees and is more common after PRK and correction of high myopia (>−6.00 D). Besides the ablation depth, it is correlated with an excessive ocular UV-B radiation, duration of the epithelial defect, postoperative steroid treatment, male sex, and certain population

Regression of refractive error is defined as return of part of the primary refractive error and is associated with increase in thickness and curvature of the cornea. Potential mechanisms include nuclear sclerosis, stromal synthesis (wound healing),

Postoperative ectasia is linked to biomechanical weakening of the cornea and is characterized with progressive corneal steepening, either centrally or inferiorly, resulting in severe progressive irregular astigmatism and decrease of both uncorrected and best-corrected visual acuity. The incidence of ectasia after LASIK has been estimated between 0.04 and 0.9% [48]. Risk factors include abnormal topographic findings, thin

Intraoperative complications of SMILE procedure are usually not sight threatening, and the procedure usually can be continued [13, 15, 50]. The most common complications are incision or cap tears, suction loss, cap perforation, black spots, and opaque bubble layer which lead to cap lenticular adhesions and retained lenticule. Regarding the postoperative complications of SMILE procedure, they are similar to all laser refractive procedures and include epithelial ingrowth, dry eye, diffuse lamellar keratitis, corneal haze, irregular astigmatism, minor interface

Two basic intraocular procedures exist: phakic intraocular lens (pIOL) implantation and refractive lens exchange (RLE) with posterior chamber IOL implantation.

compensatory epithelial hyperplasia, and iatrogenic keratectasia [47].

cornea, and high myopia together with young age at the time of surgery [49].

infiltrates, increased aberrations, and iatrogenic ectasia [50, 51].

**3. Intraocular correction of myopia**

intraocular pressure and represents as cystoid lamellar edema [41].

haze, regression of refractive error, and iatrogenic corneal ectasia.

*Mycobacterium chelonae*, and *Nocardia asteroides* [33, 42].

#### *Surgical Correction of Myopia DOI: http://dx.doi.org/10.5772/intechopen.85644*

*Intraocular Lens*

but PRK is the procedure to consider [21, 31].

sion in low light conditions [35].

tism in the axis of decentration [33].

causes refractory hyperopic shift [40].

and epithelial ingrowth.

**2.5 Limitations and complications of corneal refractive surgery**

complications specific to femtosecond lasers appeared [32].

considered as an absolute contraindication, as well as insufficient corneal thickness or corneal irregularities suspicious for keratoconus [21, 26, 28, 29]. Precautions are also needed in patients with certain systemic therapies (isotretinoin, amiodarone, sumatriptan, colchicine) [23, 24, 30]. Caution is also required in functional monocular patients and in patients with well-controlled glaucoma. Other relative contraindications are history of uveitis, herpes simplex, and varicella zoster keratitis. In patients with epithelial basal membrane degeneration, LASIK is not recommended,

Complications of corneal refractive surgery are considered rare. They can be divided

Flap-related complications include free or partial flap creation, incomplete and irregular flap creation, thin and perforated flaps, and corneal perforation. Those complications were mostly related to corneal anatomy (flat <41.00 D or steep >46.00 D corneas, small corneal diameter), inadequate suction, mechanical failure—a defect in the dissection blade or motor unit—and surgeon experience. Penetration into the anterior chamber is extremely rare and may occur during lamellar dissection or excimer laser photoablation usually on extremely thin corneas with old scars [33]. Femtosecond-related complications are closely correlated with cavitation bubbles and formation of the flap. They are presented in the form of confluent cavitation bubbles in the corneal lamellae or anterior chamber which can interfere with excimer laser systems and vertical gas breakthrough which is presented in the forms of incomplete buttonholes or difficulties in dissecting the flap due to tissue bridges [34]. Temporary hypersensitivity to light and rainbow glare are complications exclusively related to energy and pattern of femtosecond lasers characterized with normal visual acuity and photophobia without inflammation or light disper-

Laser-related complications include decentration of excimer laser ablation, irregular astigmatism, and formation of central islands. Those complications are clinically characterized by poor uncorrected and corrected distance visual acuity complaints of glare, "ghosting" around images and haloes, and refractive astigma-

Early postoperative complications include flap striae, diffuse lamellar keratitis, central toxic keratopathy, pressure-induced steroid keratitis, infectious keratitis,

Flap striae are caused by misalignment of the flap; peripheral striae usually are asymptomatic; however, central location of the striae is associated with loss in cor-

Diffuse lamellar keratitis (Sands of Sahara syndrome) is a sterile inflammation probably caused by the introduction of toxins in the flap interface [37, 38]. It is graded in four stages, with stage one and two being mild and visually unthreatening, while stage four can lead to corneal melting and permanent changes [33, 39]. In comparison to diffuse lamellar keratitis, central toxic keratopathy is a rare noninflammatory central corneal opacification linked to enzymatic degradation of keratocytes with spontaneous resolution and mild central opacification which often

rected distance visual acuity and night vision disturbances [33, 36].

in intraoperative and postoperative complications (which can be early or delayed). Regarding the intraoperative complications, they are mainly correlated with corneal flap creation or excimer laser ablation. During the era of microkeratome, flap-related complications were encountered more often and fell within 3%; with the introduction of femtosecond lasers, they were almost nullified; however, some

**106**

Pressure-induced stromal keratitis is also easily mistaken with diffuse lamellar keratitis but is caused by postoperative steroid use which leads to increase in intraocular pressure and represents as cystoid lamellar edema [41].

Infectious keratitis after LASIK is extremely rare but can be quite serious since invading organisms are already implanted into the deep corneal stroma. The most often isolated organisms include *Streptococcus pneumoniae*, *Staphylococcus aureus*, *Mycobacterium chelonae*, and *Nocardia asteroides* [33, 42].

Epithelial ingrowth under the LASIK flap is reported to occur in merely 1–2% of patients and is caused by migration of epithelial cells under the flap. It is usually insignificant, but if epithelial cells continue to grow, it can cause flap distortion and melting causing visual disturbances [43].

Late postoperative complications include dry eye, night vision problems, corneal haze, regression of refractive error, and iatrogenic corneal ectasia.

Dry eye syndrome is caused by denervation and cutting of nerve fibers during the formation of the flap, removal of corneal tissue by excimer laser, and changes in the shape of the cornea. Dry eye syndrome is usually transient and symptoms fade away after healing period. It causes discomfort, fluctuation in vision quality, slower healing, and epithelial damage and may lead to regression of refractive error and reduced vision quality [44].

Symptoms of impaired visual quality are usually more expressed during the night due to physiological pupil dilatation. The main causes of nighttime issues are the increase in spherical aberrations at the centrally flatted cornea, decentered ablations, too small optical zones, newly emerging lens opacities, and induced astigmatism [45].

Corneal haze reduces corneal transparency at variable degrees and is more common after PRK and correction of high myopia (>−6.00 D). Besides the ablation depth, it is correlated with an excessive ocular UV-B radiation, duration of the epithelial defect, postoperative steroid treatment, male sex, and certain population with brown iris [46].

Regression of refractive error is defined as return of part of the primary refractive error and is associated with increase in thickness and curvature of the cornea. Potential mechanisms include nuclear sclerosis, stromal synthesis (wound healing), compensatory epithelial hyperplasia, and iatrogenic keratectasia [47].

Postoperative ectasia is linked to biomechanical weakening of the cornea and is characterized with progressive corneal steepening, either centrally or inferiorly, resulting in severe progressive irregular astigmatism and decrease of both uncorrected and best-corrected visual acuity. The incidence of ectasia after LASIK has been estimated between 0.04 and 0.9% [48]. Risk factors include abnormal topographic findings, thin cornea, and high myopia together with young age at the time of surgery [49].

Intraoperative complications of SMILE procedure are usually not sight threatening, and the procedure usually can be continued [13, 15, 50]. The most common complications are incision or cap tears, suction loss, cap perforation, black spots, and opaque bubble layer which lead to cap lenticular adhesions and retained lenticule. Regarding the postoperative complications of SMILE procedure, they are similar to all laser refractive procedures and include epithelial ingrowth, dry eye, diffuse lamellar keratitis, corneal haze, irregular astigmatism, minor interface infiltrates, increased aberrations, and iatrogenic ectasia [50, 51].

#### **3. Intraocular correction of myopia**

Two basic intraocular procedures exist: phakic intraocular lens (pIOL) implantation and refractive lens exchange (RLE) with posterior chamber IOL implantation.

#### **3.1 Phakic intraocular lenses**

Phakic intraocular lenses (pIOL) provide a safe and effective alternative for young patients with moderate to high refractive errors who may not be suitable candidates for excimer laser procedures or who prefer a reversible form of vision correction with efficacy comparable to results of LASIK [52]. It has been established that attempted corrections of high myopia with excimer laser procedures induce more higher-order aberrations, affecting vision quality and creating problems such as glare, halos, and ghost imaging [53]. Additional advantages of intraocular procedures are a broader range of treatable ametropia, faster visual recovery, more stable refraction, and better visual quality. In addition, the pIOL implantation does not affect accommodation, and the procedure is reversible [52, 54].

Currently, there are two types of phakic intraocular lenses approved for correcting refractive errors: anterior chamber—iris fixated—and posterior chamber. Verisyse and Veriflex lenses are iris-fixated intraocular lenses. More than 160,000 of these lenses have been safely implanted worldwide [55]. The Verisyse pIOL is made from rigid, ultraviolet-absorbing polymethyl methacrylate (PMMA). This lens requires a 5.5–6.5-mm incision, depending on the optic size of the lens, whereas the Veriflex pIOL requires a 3.2-mm incision. The Verisyse pIOL is available for myopia, hypermetropia, and astigmatism. For myopia, the pIOL is available in powers from −1.00 to −23.50 D in 0.50 D steps with two optic diameters of 5.0 or 6.0 mm. The Veriflex pIOL is a foldable implant with 6.0 mm flexible optic made of hydrophobic polysiloxane and features a PMMA haptic. It is available only for myopia in powers ranging from −2.00 to −14.50 D in 0.50 D steps.

The Visian Implantable Collamer Lens (ICL) is a posterior chamber phakic intraocular lens resting in the ciliary sulcus. ICL is made from soft advanced collamer material and requires 3.2 mm incision. It is available for myopia, hypermetropia, and astigmatism. For myopia, the pIOL is available in powers from −0.50 to −18.00 D in 0.50 D steps with four lens diameters (12.1, 12.6, 13.2, 13.6 mm) and optical zone up to 6.1 mm.

#### *3.1.1 Preoperative examination and indications for phakic intraocular lens implantation*

The preoperative evaluation of a patient for pIOL is the same as for any kind of refractive procedure. Inclusion criteria are more than 21 years of age, refractive stability (<0.50 D of change) for at least 1 year, ACD ≥ 3.0 mm measured from endothelium, endothelial cell count >2300 cells/mm<sup>2</sup> (>2500 cell/mm<sup>2</sup> if <40 years of age, > 2000 cells/mm<sup>2</sup> if >40 years of age), irido-corneal angle ≥30° (at least grade II by gonioscopy examination), mesopic pupil size <6.00 mm, no anomaly of iris or pupil function, no evolving retinal pathology, absence of uveitis or any kind of ocular inflammation, and absence of glaucoma or any systemic immunological disorder [56, 57].

#### *3.1.2 Intraocular lens power calculation and diameter selection*

pIOL optic power is calculated with the software provided by the manufacturer. The calculation is based on the formula developed by van der Heijde [58]. The formula uses the patient's refraction at the 12-mm spectacle plane or the vertex refraction, the corneal keratometry dioptric power at its apex, and central ACD [59]. For Verisyse and Veriflex lenses, only one lens diameter is available, while for the ICL overall diameter depends on the ciliary sulcus diameter and should provide perfect stability with no excess of compression forces to the sulcus and allow correct vaulting. The ICL's diameter should be oversized 0.5–1.0 mm from the white-to-white (WTW)

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too small relative to the pupil size [61].

19.75% after 12 years [67].

stable ECD during this time (**Figure 5**).

measurements in myopic eyes and the same length or oversized 0.5 mm in hyperopic eyes. The internal diameter of the ciliary sulcus can be measured by ultrasound biometry (UBM) or can be approximated by horizontal WTW measurement obtained manually using a caliper or automatically by topographic or biometric devices [60].

The complications relating to pIOLs can, at times, be more disabling than those from keratorefractive surgery. Night vision problems, corneal decompensation, glaucoma, cataract formation, dyscoria, uveitis, and endophthalmitis are potential complications after pIOL implantation. Night vision problems such as glare, halos, and diplopia are related to decentration of the pIOL and/or an optic diameter that is

Surgically induced astigmatism is an issue primarily correlated with rigid Verisyse lenses and incision diameter. However, some investigators reported that the resulting surgically induced astigmatism (SIA) was less than expected [62, 63]. However, when compared with the Veriflex pIOL and ICL, the SIA was significantly higher [64].

Implantation of a pIOL, whether iris fixated or positioned in the posterior cham-

In our experience after ICL implantation, there is a linear decrease in ECD over a 3-year period, without any signs of exponential EC loss or reaching a plateau or

With modern pIOL designs, increased intraocular pressure (IOP) seems to be relatively uncommon after 3 months postoperatively and is typically thought to be related to corticosteroid response [68]. Posterior chamber pIOLs cause narrowing of anterior chamber angle due to its position in ciliary sulcus, and its sizing (too long lenses which cause excessive vaulting >750 μm) is closely correlated with possible angle-closure glaucoma, pupillary block glaucoma, or pigmentary dispersion glaucoma [69, 70]. Given the risk of pupillary block, peripheral iridectomy or iridotomy is carried out as a preventative measure in anterior pIOL procedures,

Pupil ovalization/iris retraction is mainly correlated with iris-fixated pIOL and can occur if fixation of the pIOL haptics is performed asymmetrically [61, 68, 71].

Formation of cataract due to the iris-claw pIOL is unlikely because the pIOL is inserted over a miotic pupil without contact with the crystalline lens [61]. The incidence of cataract formation was 1.1% for the iris-fixated pIOL. The overall incidence of cataract formation for posterior chamber pIOLs was 9.60%, which is significantly higher in comparison to iris-fixated pIOLs [72]. With various generations of the ICL, appearance of cataract formation is different. The less vaulted

while in newer models of ICL with aquaport, technology is not needed.

No progressive pupil ovalization has been reported.

ber, is associated with an accelerated decrease in endothelial cell density (ECD) [60]. Damage to the corneal endothelium may be due to the direct contact between pIOL and the inner surface of the cornea during implantation, from postoperative changes in pIOL position, or from subclinical inflammation, and direct toxicity to the endothelium. The magnitude of ECD loss after phakic intraocular lens implantation surpasses the expected natural annual decrease of 0.6% as reported in a 1997 benchmark study based on 42 adults [65]. Following implantation of an iris-claw phakic intraocular lens, the loss of ECD is highest during the first year varying between 0.75 and 7.2% [66]. Thereafter, the ECD loss continues without following an obvious pattern, to about 8.9% after 10 years. However, with an ICL the impact on the endothelium is claimed to be lower because the implant is placed in the posterior chamber further away from the endothelium itself. For the ICL the ECD loss is about 1.7% after 2 years [60] increasing to 6.2% after 8 years [54] and up to

*3.1.3 Limitations and possible complications of phakic intraocular lenses*

*Intraocular Lens*

**3.1 Phakic intraocular lenses**

Phakic intraocular lenses (pIOL) provide a safe and effective alternative for young patients with moderate to high refractive errors who may not be suitable candidates for excimer laser procedures or who prefer a reversible form of vision correction with efficacy comparable to results of LASIK [52]. It has been established that attempted corrections of high myopia with excimer laser procedures induce more higher-order aberrations, affecting vision quality and creating problems such as glare, halos, and ghost imaging [53]. Additional advantages of intraocular procedures are a broader range of treatable ametropia, faster visual recovery, more stable refraction, and better visual quality. In addition, the pIOL implantation does

Currently, there are two types of phakic intraocular lenses approved for correcting refractive errors: anterior chamber—iris fixated—and posterior chamber. Verisyse and Veriflex lenses are iris-fixated intraocular lenses. More than 160,000 of these lenses have been safely implanted worldwide [55]. The Verisyse pIOL is made from rigid, ultraviolet-absorbing polymethyl methacrylate (PMMA). This lens requires a 5.5–6.5-mm incision, depending on the optic size of the lens, whereas the Veriflex pIOL requires a 3.2-mm incision. The Verisyse pIOL is available for myopia, hypermetropia, and astigmatism. For myopia, the pIOL is available in powers from −1.00 to −23.50 D in 0.50 D steps with two optic diameters of 5.0 or 6.0 mm. The Veriflex pIOL is a foldable implant with 6.0 mm flexible optic made of hydrophobic polysiloxane and features a PMMA haptic. It is available only for myopia in powers

The Visian Implantable Collamer Lens (ICL) is a posterior chamber phakic intraocular lens resting in the ciliary sulcus. ICL is made from soft advanced collamer material and requires 3.2 mm incision. It is available for myopia, hypermetropia, and astigmatism. For myopia, the pIOL is available in powers from −0.50 to −18.00 D in 0.50 D steps with four lens diameters (12.1, 12.6, 13.2, 13.6 mm) and optical

*3.1.1 Preoperative examination and indications for phakic intraocular lens implantation*

The preoperative evaluation of a patient for pIOL is the same as for any kind of refractive procedure. Inclusion criteria are more than 21 years of age, refractive stability (<0.50 D of change) for at least 1 year, ACD ≥ 3.0 mm measured from

grade II by gonioscopy examination), mesopic pupil size <6.00 mm, no anomaly of iris or pupil function, no evolving retinal pathology, absence of uveitis or any kind of ocular inflammation, and absence of glaucoma or any systemic immunological

pIOL optic power is calculated with the software provided by the manufacturer.

The calculation is based on the formula developed by van der Heijde [58]. The formula uses the patient's refraction at the 12-mm spectacle plane or the vertex refraction, the corneal keratometry dioptric power at its apex, and central ACD [59]. For Verisyse and Veriflex lenses, only one lens diameter is available, while for the ICL overall diameter depends on the ciliary sulcus diameter and should provide perfect stability with no excess of compression forces to the sulcus and allow correct vaulting. The ICL's diameter should be oversized 0.5–1.0 mm from the white-to-white (WTW)

(>2500 cell/mm<sup>2</sup>

if >40 years of age), irido-corneal angle ≥30° (at least

if <40 years

not affect accommodation, and the procedure is reversible [52, 54].

ranging from −2.00 to −14.50 D in 0.50 D steps.

endothelium, endothelial cell count >2300 cells/mm<sup>2</sup>

*3.1.2 Intraocular lens power calculation and diameter selection*

zone up to 6.1 mm.

of age, > 2000 cells/mm<sup>2</sup>

disorder [56, 57].

**108**

measurements in myopic eyes and the same length or oversized 0.5 mm in hyperopic eyes. The internal diameter of the ciliary sulcus can be measured by ultrasound biometry (UBM) or can be approximated by horizontal WTW measurement obtained manually using a caliper or automatically by topographic or biometric devices [60].
