Introduction

LASIK was developed about 20 years ago by Dr Pallikaris in Greece and by Dr Buratto in Italy evolving from previous lamellar refractive surgery performed by Dr Barraquer and Dr Ruiz1,2. This chapter will go through the different steps of LASIK, thus giving information about its role in refractive surgery.

Epidemiologic considerations and terminology

LASIK is the most performed refractive procedure in the world, with several millions of cases, and is overtaking surface ablations in most countries. LASIK stands for ‘Laser assisted in situ keratomileusis’. The purpose of LASIK is to change the refraction of the eye through modifying the corneal curvature by excimer laser ablation of corneal tissue. Different from surface ablations, the laser energy is applied within the corneal stroma under a 100–180 µm corneal flap cut with a microkeratome or with a femtosecond laser. A hinge is left to allow flap reposition without suturing (Fig. 26.1). The maximum amount of dioptric correction is determined by balancing the pupil diameter, the ablation optical zone, the cut depth, and the corneal thickness. The upper limit is around −10 D for myopia, and around +5 D for hyperopia and for astigmatism in most eyes3.

Fig. 26.1 Scheme of LASIK surgery.

Clinical features, diagnosis, and differential diagnosis

Myopic ablations are obtained by flattening the central corneal surface. However, they increase the positive spherical aberration of the cornea, causing image blurring and contrast sensitivity loss especially with wide pupils, a problem reduced by aspheric ablations. Myopic ablations are deeper at the center, and especially with large optical zones. A small optical zone can be selected to reduce the depth of the procedure, but this will increase optical problems in the postoperative, especially with wide pupils.

Hyperopic eyes are treated by curving the central corneal surface. Frequently they have nasal decentration of fixation, and the relevant data from the topographer must be loaded into the laser to avoid decentration. Depth problems are rare with hyperopia, but the increase in negative spherical aberration induced by the treatment suggests one should not treat hyperopia above 5–6 D. High order aberrations are increased more by hyperopic than by myopic ablations4.

LASIK can correct astigmatism up to 5–6 D, provided no forme fruste keratoconus is present. The efficacy of the procedure can be limited by ablation imprecision, coupling effect, incorrect eye alignment, and eye cyclotorsion, therefore the need for re-treatment should be anticipated when astigmatism is higher than 3 D.

Recently, several LASIK procedures have been proposed to treat presbyopia. The most popular approach consists of increasing the spherical aberration of the central cornea, around a 2 mm central zone, obtained by superimposing hyperopic and myopic ablations. A second approach aims at curving the inferior central cornea. Although good results have been published5, these procedures are still controversial.

LASIK has been employed with success to correct refractive defects after previous corneal surgery: radial keratotomy, photorefractive keratectomy, penetrating and lamellar keratoplasty. The main issues in these eyes are corneal curvature, which could lead to free or incomplete flaps, and corneal instability, which limits the precision of the outcome.

Anatomical considerations

The hinge position can be nasal or superior, because lateral or inferior positions favor flap displacement. Ideally, the LASIK cut should lie on a single lamellar plane within the corneal stroma. In practice, it crosses different planes and usually it is thinner in the center, impairing the corneal biomechanics and producing minor refractive changes6. The average cut depth is around 160 µm. Thin flaps below 130 µm can help, leaving more room to ablation, but are less stable in the first postoperative period, often requiring bandage soft contact lenses for one day. The laser ablation is similar to that of photorefractive keratectomy, but with lower regression and inflammation. Flap adhesion in the immediate postoperative is assured mainly by the endothelial pump and is usually excellent.