postoperative astigmatism after penetrating keratoplasty (PKP). Complex peripheral patterns may result in a refractive meridian of astigmatism that is not aligned with the topographic meridian. Conventional, wavefront-optimized, wavefront-guided, or topography-guided ablations may be considered in post-PKP eyes after all sutures have been removed and the refraction has stabilized, depending on the resulting refractive error and corneal shape.

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 increase spherical aberrations. Common complaints in patients with substantial spherical aberration include glare, halos, and decreased night vision.

Incisional Techniques

Incisions perpendicular to the corneal surface predictably alter its shape, depending on the direction, depth, location, 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 3-5).

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.)

The closer the radial incisions approach the visual axis (ie, the smaller the optical zone), the greater their effect; similarly, the closer tangential incisions are placed to the visual axis, the greater is the effect. The longer the tangential incision, up to 3 clock-hours, the greater 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 numbers of incisions and optical zone size can be calculated using finite element analysis, but surgical nomograms are typically generated empirically (eg, see 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.

Tissue Addition or Subtraction Techniques

With the exception of laser ablation techniques (discussed in the section Laser Biophysics), lamellar procedures that alter corneal shape through tissue addition or subtraction are primarily of historical interest only. Keratomileusis for myopia was originated by Barraquer as “carving” of the anterior surface of the cornea. It is defined as a method to modify the spherical or meridional surface of a healthy cornea by tissue subtraction. Epikeratoplasty (sometimes called epikeratophakia) adds carved donor tissue to the surface to induce hyperopic or myopic changes. Keratophakia requires the addition of a tissue lenticule or synthetic inlay intrastromally (see Chapter 4). There is, however, recurring interest in femtosecond laser techniques to excise intrastromal lenticules to alter corneal curvature without the need for excimer laser ablation. These procedures are termed refractive lenticule extraction (ReLEx), femtosecond lenticule extraction (FLEx), and small-incision lenticule extraction (SMILE). Although early results are promising, these procedures are currently under clinical investigation.

Alloplastic Material Addition Techniques

The shape of the cornea can be altered by adding alloplastic material such as hydrogel on the surface or into the corneal stroma to modify the anterior shape or refractive index of the cornea. For example, the 2 arc segments of an intrastromal corneal ring can be placed in 2 pockets of the stroma to directly reshape the surface contour according to the profile of the individual rings (Fig 1-18). For further discussion, see Chapter 4.

Figure 1-18 Schematic illustrations showing placement of intrastromal corneal ring segments. (Illustrations by Jeanne Koelling.)

Collagen Shrinkage Techniques

Alteration in corneal biomechanics can also be achieved by collagen shrinkage. Heating collagen to a critical temperature of 58°–76°C causes it to shrink, inducing changes in the corneal curvature.

Thermokeratoplasty and conductive keratoplasty (CK) are avoided in the central cornea because of scarring but can be used in the midperiphery to cause local collagen contraction with concurrent central corneal steepening (Fig 1-19; also see Chapter 7).