**2.1 Photorefractive keratectomy**

Photorefractive keratectomy (PRK) was the first excimer laser technique for the treatment of refractive errors. Seiler performed the first corneal ablation in a live patient in 1985, and McDonald treated the first human sighted eye in 1985 after extensive preclinical investigation [4]. The PRK procedure involves removal of the central corneal epithelium, most commonly performed mechanically (brush, crescent knife, or alcohol) or with excimer laser when it is referred as transepithelial PRK (T-PRK). The denuded anterior stroma is then reshaped by the excimer laser, with either central corneal flattening, steepening, or a torical pattern when treating myopia, hyperopia, or astigmatism, respectively. Due to significant postoperative pain, relatively slow visual recovery, epithelial defects due to irregular healing, and haze development, especially when treating high myopia [5, 6], different techniques of epithelial removal were introduced over time to solve these complications [7]. Recently the role of surface ablations has been reevaluated due to raised issues of potential flap complications, risk of iatrogenic corneal ectasia, and inability to treat thin corneas with laser in situ keratomileusis (LASIK) [8]. With surface ablation techniques, there is no flap involved, and more cornea tissue is preserved, and by some it is still considered the overall safest procedure for treatment of low to moderate myopia [9]. It is performed, especially in corneas with superficial scarring, epithelial dystrophies, or recurrent erosions, in thin corneas, after penetrating keratoplasty and for keratorefractive retreatments. The introduction of mitomycin C and modern surface ablation techniques has also increased the range of treatment and lowered the risk of haze and regression after PRK [10]. Therefore today surface ablation includes several sub-techniques such as epithelial LASIK (epi-LASIK), laser-assisted subepithelial keratectomy (LASEK), and T-PRK [11].

### **2.2 Laser in situ keratomileusis**

The term LASIK was first used in 1990 by Pallikaris [4]. The procedure is performed in two steps. The first step involves the formation of a front corneal flap and the lifting of the flap for the purpose of exposing the corneal stroma. The hinged flap consists of the corneal epithelium, the Bowman membrane, and superficial stroma. The second step is the application of the excimer laser on the stromal bed. Once the ablation with the excimer laser is finished, the flap is returned into its original position.

LASIK has now become the most common elective surgical procedure in the world, presumably because it is almost painless with fast visual recovery, as compared to PRK [4]. Nowadays, there are two techniques available for the formation of the flap—mechanical microkeratomes and femtosecond lasers. The use of femtosecond laser-assisted laser in situ keratomileusis (FsLASIK) offers greater precision in flap

**103**

**Figure 2.**

**Figure 1.**

*Surgical Correction of Myopia*

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

**2.3 Small incision lenticule extraction**

creation leading to better morphological stability of the flap compared to earlier bladed microkeratome keratomileusis. However, changes in the biomechanical strength of the cornea, induction of higher-order aberrations, and flap-related complications can still occur [12]. LASIK reduces the tensile strength of the stroma by about 35% when the ablation takes place between 10 and 30% of the stromal depth [13]. Regarding the available data, and our experience, there is no significant difference in shorter-term refractive stability and induction of high-order aberrations between T-PRK and LASIK (**Figures 1** and **2** and **Table 1**). However, when higher refractive errors are treated,

surface ablations pose more risk for haze development and regression [14].

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

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

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

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

*Intraocular Lens*

**2. Corneal refractive surgery**

**2.1 Photorefractive keratectomy**

**2.2 Laser in situ keratomileusis**

vision correction, or refractive corneal surgery.

of phakic intraocular lenses (pIOLs), and refractive lens exchange (RLE) with implantation of multifocal and monofocal intraocular lenses (IOLs). It is important to perform a detailed examination of each patient and assess their needs, wishes, and expectations. Doctors need to explain in as much detail as possible what the expected

Procedures which involve altering the shape of the cornea with excimer laser are collectively referred to as keratorefractive surgery, refractive keratoplasty, laser

Photorefractive keratectomy (PRK) was the first excimer laser technique for the treatment of refractive errors. Seiler performed the first corneal ablation in a live patient in 1985, and McDonald treated the first human sighted eye in 1985 after extensive preclinical investigation [4]. The PRK procedure involves removal of the central corneal epithelium, most commonly performed mechanically (brush, crescent knife, or alcohol) or with excimer laser when it is referred as transepithelial PRK (T-PRK). The denuded anterior stroma is then reshaped by the excimer laser, with either central corneal flattening, steepening, or a torical pattern when treating myopia, hyperopia, or astigmatism, respectively. Due to significant postoperative pain, relatively slow visual recovery, epithelial defects due to irregular healing, and haze development, especially when treating high myopia [5, 6], different techniques of epithelial removal were introduced over time to solve these complications [7]. Recently the role of surface ablations has been reevaluated due to raised issues of potential flap complications, risk of iatrogenic corneal ectasia, and inability to treat thin corneas with laser in situ keratomileusis (LASIK) [8]. With surface ablation techniques, there is no flap involved, and more cornea tissue is preserved, and by some it is still considered the overall safest procedure for treatment of low to moderate myopia [9]. It is performed, especially in corneas with superficial scarring, epithelial dystrophies, or recurrent erosions, in thin corneas, after penetrating keratoplasty and for keratorefractive retreatments. The introduction of mitomycin C and modern surface ablation techniques has also increased the range of treatment and lowered the risk of haze and regression after PRK [10]. Therefore today surface ablation includes several sub-techniques such as epithelial LASIK (epi-LASIK),

laser-assisted subepithelial keratectomy (LASEK), and T-PRK [11].

The term LASIK was first used in 1990 by Pallikaris [4]. The procedure is performed in two steps. The first step involves the formation of a front corneal flap and the lifting of the flap for the purpose of exposing the corneal stroma. The hinged flap consists of the corneal epithelium, the Bowman membrane, and superficial stroma. The second step is the application of the excimer laser on the stromal bed. Once the ablation with the excimer laser is finished, the flap is returned into its original position. LASIK has now become the most common elective surgical procedure in the world, presumably because it is almost painless with fast visual recovery, as compared to PRK [4]. Nowadays, there are two techniques available for the formation of the flap—mechanical microkeratomes and femtosecond lasers. The use of femtosecond laser-assisted laser in situ keratomileusis (FsLASIK) offers greater precision in flap

results and risk would be with for the selected surgical method.

**102**

creation leading to better morphological stability of the flap compared to earlier bladed microkeratome keratomileusis. However, changes in the biomechanical strength of the cornea, induction of higher-order aberrations, and flap-related complications can still occur [12]. LASIK reduces the tensile strength of the stroma by about 35% when the ablation takes place between 10 and 30% of the stromal depth [13]. Regarding the available data, and our experience, there is no significant difference in shorter-term refractive stability and induction of high-order aberrations between T-PRK and LASIK (**Figures 1** and **2** and **Table 1**). However, when higher refractive errors are treated, surface ablations pose more risk for haze development and regression [14].
