Introduction

Corneal refractive laser surgery aims to modify the optical properties of the eye by remodelling the profile of the corneal tissue. The cornea is the transparent tissue that covers the iris, pupil, and anterior chamber of the eye.

The cornea is made up of three overlapping layers, as illustrated in the figure below: the epithelium, the stroma and the endothelium. The excimer laser targets the central optical zone of the cornea; it remodels its profile and changes its refractive power in order to focus light rays on the retina without the need for additional optical lenses.

Refractive errors 

Refractive errors reduce visual acuity and cause blurry visions. There are various types of such errors: myopia, hyperopia, astigmatism, and presbyopia. The cause of refractive errors lies in anatomical imperfections of the ocular bulb or, as in the case of presbyopia, in the reduced functionality of an eye structure, the crystalline lens.

The blurring of images is caused by the fact that the light rays traversing the eye’s lenses (cornea, crystalline lens) are not projected onto the retina. 

Myopia (nearsightedness) is the commonest refractive error. It occurs when the eyeball is too long and light rays focus in front of the retina. 

In hyperopia (farsightedness), the eyeball is too short, but vision remains clear past a certain point thanks to the accommodation of the crystalline lens. In severe cases, however, vision is blurred at any distance. 

Astigmatism is a condition in which, due to the particular shape of the eyeball, the light rays traversing the eye’s lenses come into focus on two different planes. 

Presbyopia is the error that “forces” many people over the age of 45 to wear reading glasses. After a certain age, the eye is no longer able to focus on near objects due to a reduction in the accommodative activity of the crystalline lens.

When is surgery indicated?

The reasons that can lead patients suffering from myopia, hyperomia, or astigmatism to seek the surgical correction of their respective error are numerous, the first and foremost being the need for corrective eyewear (eyeglasses or contact lenses) in order to move about or do anything else.

Clinical motivations include anisometropia, a condition in which the two eyes have unequal refractive errors which cannot be adequately corrected with eyeglasses. 

As for any other surgical interventions, patients who seek photorefractive corrective surgery must meet certain criteria. 
The refractive error must be stable for at least one year, and since ophthalmologists know that errors commonly evolve until the age of 21, patients must be at least that age. 

Corneal thickness in the optical zone must be above 450 µm (average thickness 500-520 µm). 

Patients must be free of any acute or chronic ocular pathologies, such as keratoconus, glaucoma, or macular degenerations; they must also be free  of any systemic diseases with an immunitary pathogenesis.

Epidemiology

Approximately 94,000 interventions were carried out in 2012 with excimer laser in Italy, of which approximately 78% are carried out by PRK and 22% by LASIK.

Current techniques

Excimer laser surgery can be performed using various techniques: PRK, LASIK, LASEK, EPI-LASIK and PTK. 
The first (PRK: photorefractive keratectomy) is also the oldest, and is based on the ablation of the anterior stroma after the removal of the corneal epithelium, which will re-generate in the first few days after surgery. The intervention lasts a few minutes, including patient preparation with the anesthesia of the cornea using eye drops and the removal of the epithelium. At the end of the operation, a therapeutic contact lens is applied to the surface of the cornea, with the dual goal of favouring the regeneration of the epithelium and reducing pain. The lens is removed after three to five days. 

In LASIK (laser-assisted in-situ keratomileusis) surgery, the laser targets a deeper layer of the stroma, after the surgeon has created, through the use of a mechanical microblade, a flap of surface tissue that is then be folded back to reveal the stroma underneath. At the end of the operation the flap is repositioned and  will be able to perfectly adhere to the underlying tissue in a brief period of time. LASIK surgery also requires topical anesthesia, but lasts slightly longer than PRK. Currently, a femtoseconds laser called femtolasik or I-Lasik is used to cut the lamella, which involves the use of two different lasers, an excimer laser and a femtoseconds laser. SMILE (Small Incision Lenticule Extraction) is an innovative technique that consists in the use of a single femtoseconds laser for the correction of refractive defect.

The procedure consists in the laser sculpting a lens of adequate thickness, diameter and geometric characteristics to correct the patient's myopic or astigmatic refractive defect, which is then extracted by the surgeon. This technique reduces the time required for surgery, minimizes the extension of corneal incisions and allows a very fast visual recovery. LASEK and EPI-LASIK are similar PRK, the difference lying in the way the epithelium is removed. In LASEK an epithelial flap is created by applying an alcohol-based solution, while in EPI-LASIK the epithelium is removed with a blunt plastic microblade. Once the laser ablation of the anterior stroma has been completed, the epithelial flap is repositioned on the corneal tissue and protected with a therapeutic contact lens. 
The choice of surgical technique must be made taking into account the anatomical characteristics of the eyeball and the extent of the visual defect to be corrected. In addition, the surgeon should consider the patient's lifestyle and visual expectation and refer the patient to the most suitable technique on a case-by-case basis..