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.

Fundamental principles

LASIK is based on a few assumptions: (i) the corneal curvature can be modified by cutting a flap, performing an excimer laser ablation and repositioning the flap; (ii) the cornea will remain transparent and mechanically stable in the postoperative; (iii) the optical properties of the cornea will remain of high quality. These assumptions proved true in uncomplicated cases, but corneal instability after surgery can develop especially if the procedure went too deep. Specifically in myopic LASIK care has to be taken to leave at least 250 µm of untreated tissue3.

Goals of surgery

The refractive goal of LASIK is to precisely correct the existing refractive defect. In addition, the high order optical aberrations should not be increased, at least within the optical zone. Aspheric treatments try to maintain the preoperative corneal spherical aberration, while wavefront-guided and topography-guided treatments are intended to correct the preoperative existing aberrations.

Indications for surgery

As for surface ablations, LASIK is indicated for stable refractive defects in patients willing their removal. LASIK can be performed in patients with cheloid tendency and in eyes where some low re-epithelialization problem can be anticipated. The depth of the ablation makes it contraindicated in thin corneas, and especially in forme fruste keratoconus that should be carefully investigated because of the increased risk of postoperative ectasia.

Preoperative assessment

Cycloplegic refraction and corneal topography are mandatory for the pre-operative evaluation, and for the surgical planning. The corneal diameter and the K readings determine the microkeratome setting. As a rule, steep corneas ‘protrude’ more than flat corneas, therefore smaller rings and shallower depths can be selected. Small corneas involve the risk of a free-cap, therefore smaller rings and deep cuts are suggested. Deep-set eyes are a challenge for microkeratomes, and surface ablation could be suggested to avoid flap problems. In case of doubt, a trial session could be set a few days before surgery.

Before admitting the patient, the excimer laser and the microkeratome have to be tested according to the manufacturer’s instructions. The instrument tray includes a pair of scissors, the speculum, the corneal marker, the irrigating cannula and syringe, and absorbing sponges. Corneal forceps and needle holder are always sterile and available should a suture be required.

Anesthesia

Patient preparation is fundamental, because we need his/her cooperation during surgery. Sedative drugs are administered if required. Anesthetic drops (proparacaine or oxybuprocaine) are applied 5 minutes before surgery, as excessive drug use leads to epithelial defects. Anesthetic drops are repeated in case of pain after applying the speculum, and always in the second eye before surgery in bilateral cases.

Operation techniques

The patient is dressed with a gown and a cap. Once on the surgical bed, the eyelashes are covered with plastic adhesive drape to reduce bacterial contamination and to avoid any interference with the microkeratome. The speculum is applied and the head tilted for the best eye exposure. The conjunctival sac is dried with absorbing sponges and the cornea is marked to help later flap alignment. The microkeratome suction ring is applied onto the eye and its centration is controlled before suction is activated, as suction induces tissue folds and conjunctival edema that prevent position change. Usually, with side-cutting microkeratomes the ring is slightly decentered nasally, while with down-up microkeratomes the surgeon looks for perfect centration.

After hearing the tone indicating proper suction, the Barraquer tonometer can be used to check the intraocular pressure is above 65 mmHg. The cornea must be irrigated with BSS to help flap slipping and to reduce the risk of epithelial defect. The cutting head is placed in position, and the cut can start. Side-cutting microkeratomes usually have heavy handpieces that must be held by the surgeon, gently pulling the eye rather than pressing on it. With down–up microkeratomes we hold the microkeratome ring rather than the cutting head, without tilting the ring to reduce the risk of suction loss. The flap is usually lifted by the irrigating cannula, and folded on itself to expose the corneal epithelium and not the inner flap surface to possible ablation. The corneal bed is gently dried with a sponge, and a wet sponge is also used to protect the flap during the ablation.

The laser ablation is probably the simplest part of the procedure, yet attention is required in keeping eye alignment. Eye trackers referring to the iris plane cannot avoid decentration at the corneal plane if the patient’s gaze is not aligned. During the ablation, the corneal bed is dried when necessary, e.g. after vasoconstrictor eye drop application to control bleeding from limbal vessels.

Flap repositioning can be performed by the irrigating cannula. Prolonged irrigation of the interface removes debris and fat. Usually the flap is easily replaced by controlling the alignment marks and the keratectomy gutter, without need of re-lifting and re-positioning. A wet sponge is all that is needed to express the fluid from the interface and to check flap adherence after 2–3 minutes. The eye is medicated with steroids, antibiotics, and lubricants. A contact lens can be used to avoid flap folding in the first postoperative hours, especially with dry eyes or thin flaps. The patient is discharged 20–30 minutes later after slit-lamp control. No patch is applied, but plastic shields are given to prevent eye rubbing, especially at night.

Femto-LASIK employs a femtosecond laser to obtain the corneal flap. Femtosecond lasers differ for energy, repetition rate, shape of the suction cup, required intraocular pressure, and delivery pattern. The disposable suction cup is applied to the eye, sometimes flattening the cornea according to the various models. The cut follows. A thin blunt spatula has to be employed to separate and to lift the flap because of the many small bridges remaining in the interface. Femtosecond flaps are considered to be more precise and consistent in depth as compared with microkeratomes7. In addition, the hinge position can be selected for each individual case, although only superior and nasal hinges offer postoperative stability. In contrast, flap manipulation is higher than with microkeratomes, and the cut surface is rougher.

Epi-LASIK is a superficial ablation performed under a flap made only by corneal epithelium. This is a variation of photo-refractive keratectomy accredited with lower postoperative pain.

Intraoperative complications

Incomplete flaps result from microkeratome stop because of suction loss or because of interference with lids, lashes, drapes, or speculum. Buttonhole flaps often result because of inadequate suction, and more often in eyes with steep corneas. In these cases the ablation is not performed, and a new and deeper cut is attempted 2 months later. A free cap can occur in flat corneas or with thin flaps, but also with extremely low corneal diameter. It is better to leave the flap within the microkeratome head, perform the ablation, and then reposition the flap after hydrating it. Mattress 10-0 nylon sutures can be employed together with a contact lens to fix the flap, but they should be removed early to avoid corneal striae. Ablation on the flap can occur especially with large optical zones. This complication is usually patent to the surgeon, and can be prevented by protecting the flap or by reducing the treatment area. Bleeding from limbal vessels can happen in contact lens wearers, and it is usually controlled by application of vasoconstrictor eye drops.

Postoperative care

The patient is advised he will perceive moderate pain for a few hours, but no pain is expected for the night after surgery. Vision is usually recovered the next day, but photic phenomena can last several weeks. Most postoperative protocols include steroid and antibiotic eye drops q.i.d. for one week, and lubricants up to 3–4 months. Dry eye sensations are common, but induced blurred vision is rare. Postoperative visits are suggested on day 1, day 7, and day 30. Swimming is not permitted for 15 days, cosmetic make-up is allowed after 1 week. The obtained refraction can be considered stable after 4 months.

Postoperative complications

Dry eye sensations are very common in the first postoperative period, depending on corneal nerve cutting. Flap striae are due to flap lifting and reposition, flap movement during the first postoperative hours, or high ablations. Thin striae are common and do not affect vision, but thick striae make it necessary to lift the flap and ‘iron’ it in the very first postoperative day. Flap displacement is rare, but involves careful cleaning of the debris and of the epithelial cells that immediately spread over the cut surfaces. Diffuse lamellar keratitis (0.2–0.5% of cases)8 takes place at the interface, and it is caused by material remaining there after ablation. If recognized early from patient pain, corneal flattening, and slit-lamp appearance, flap lifting and aggressive treatment by steroid and antibiotic eye drops can avoid corneal scarring and loss of visual acuity. Corneal infection is very rare, probably less than 0.02%, and can occur both as early infection due to common conjunctival contaminants, and as late infection due to slow growing bacteria. Epithelial ingrowth within the interface is also rare and seldom requires treatment.

The worst late complication is corneal ectasia (0.66% or less), a structural failure of the cornea that manifests with curvature steepening, tissue thinning, irregular astigmatism, and loss of vision. Corneal ectasia has been demonstrated to be associated with preoperative corneal irregularities, high myopia, and excessive thinning. Useful prevent on protocols have been published3, but we still do not know why some eyes develop ectasia while the majority do not. Glare and halos are perceived especially at night when the effective optical zone is much too small, or the pupil much too wide, although the relations among pupil diameter, optical zone, and visual symptoms have never been clarified. As for possible retinal breaks or detachment, after 20 years and several studies we can conclude that LASIK does not favor retinal detachment, leaving the risk typical of myopic eyes unchanged9.

Assessment of surgery: self-evaluation; results of surgery

The results of LASIK are assessed in terms of efficacy (% of eyes with visual acuity over 20/40, or within 0.5 D from intended refraction) and of safety (% of eyes with visual acuity equal to or better than the preoperative). Results are better for low refractive corrections: up to 98% of eyes with myopia <3 D attain full uncorrected vision10. Over-corrections and under-corrections may depend on incorrect preoperative assessment, or on ablation problems. In most eyes a re-operation can be carried out by simply lifting the flap and performing the required new ablation, but with increased risk of epithelial ingrowth. Flap re-cutting is preferred after 2 years, and mandatory after 3 years. In case, of insufficient corneal tissue, superficial ablation can be employed if the attempted correction is low.