LASIK for myopia, hyperopia, and astigmatism

Published on 08/03/2015 by admin

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Last modified 08/03/2015

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CHAPTER 26 LASIK for myopia, hyperopia, and astigmatism

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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.

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