Corneal Effects of Keratorefractive Surgery
All keratorefractive procedures induce refractive changes by altering corneal curvature; however, the method by which the alteration is accomplished varies by procedure and by the refractive error being treated. Treatment of myopia requires a flattening, or decrease, in central corneal curvature, whereas treatment of hyperopia requires a steepening, or increase, in central corneal curvature. Corneal refractive procedures can be performed using a variety of techniques, including incisional, tissue addition or subtraction, alloplastic material addition, collagen shrinkage, and laser ablation (see the section Laser Biophysics for discussion of laser ablation).
Overall patient satisfaction after refractive surgery depends largely on the successful correction of refractive error and creation of a corneal shape that maximizes visual quality. The natural shape of the cornea is prolate, or steeper centrally than peripherally. In contrast, an oblate cornea is steeper peripherally than centrally. The natural prolate corneal shape results in an aspheric optical system, which reduces spherical aberration and therefore minimizes fluctuations in refractive error as the pupil changes size. Oblate corneas, such as those resulting from myopic treatments, increase spherical aberration. Common concerns in patients with substantial spherical aberration include glare, halos, and decreased night vision.
Incisions perpendicular to the corneal surface alter its shape, depending on the direction, depth, location, length, and number of incisions (see Chapter 4). All incisions cause a local flattening of the cornea. Radial incisions lead to flattening in both the meridian of the incision and the one 90° away. Tangential (arcuate or linear) incisions lead to flattening in the meridian of the incision and steepening in the meridian 90° away that may be equal to or less than the magnitude of the decrease in the primary meridian (Fig 1-17); this phenomenon is known as coupling (see Chapter 3, Fig 3-3).
Figure 1-17 Schematic diagrams of incisions used in astigmatic keratotomy. Flattening is induced in the axis of the incisions (at 90° in this case), and steepening is induced 90° away from the incisions (at 180° in this case).
(Illustrations by Cyndie C. H. Wooley.)
Reducing the optical zone of the radial incisions increases their effect; similarly, by placing tangential incisions closer to the visual axis, the greater is the effect. In addition, increasing in the length of tangential incision, up to 3 clock-hours, increases the effect.
For optimum effect, an incision should be 85%–90% deep to retain an intact posterior lamella and maximum anterior bowing of the other lamellae. Nomograms for number of incisions and optical zone size can be calculated using finite element analysis, but surgical nomograms are typically generated empirically (eg, see Chapter 3, Table 3-1). The important variables for radial and astigmatic surgery include patient age and the number, depth, and length of incisions. The same incision has greater effect in older patients than it does in younger patients. IOP and preoperative corneal curvature are not significant predictors of effect.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.