on keratorefractive procedures), corneal imaging for refractive surgery, and the effects of keratorefractive surgery on the cornea. It includes review of the optical principles discussed in BCSC Section 3, Clinical Optics; refractive errors (both lowerand higher-order aberrations); corneal biomechanics; corneal topography and tomography; wavefront analysis; laser biophysics and laser– tissue interactions; corneal biomechanical changes after surgery; and corneal wound healing.
Corneal Optics
The air–tear-film interface provides the majority of the optical power of the eye. Although a normal tear film has minimal deleterious effect, an abnormal tear film can have a dramatic impact on vision. For example, either excess tear film (eg, epiphora) or altered tear film (eg, dry eye or blepharitis) can decrease visual quality.
The optical power of the eye derives primarily from the anterior corneal curvature, which produces about two-thirds of the eye’s refractive power, approximately +48.00 diopters (D). The overall corneal power is less (approximately +42.00 D) as a result of the negative power (approximately –6.00 D) of the posterior corneal surface. Standard keratometers and Placido-based corneal topography instruments measure the anterior corneal radius of curvature and estimate total corneal power from these front-surface measurements. These instruments extrapolate the central corneal power (K) by measuring the rate of change in curvature from the paracentral 4-mm zone; this factor takes on crucial importance in the determination of IOL power after keratorefractive surgery (see Chapter 11). The normal cornea flattens from the center to the periphery by up to 4.00 D (this progressive flattening toward the peripheral cornea is referred to as a prolate shape) and is flatter nasally than temporally.
Almost all keratorefractive surgical procedures change the refractive state of the eye by altering corneal curvature. The tolerances involved in altering corneal dimensions are relatively small. For instance, changing the refractive status of the eye by 2.00 D may require altering the cornea’s thickness by less than 30 µm. Thus, achieving predictable results is sometimes problematic because minuscule changes in the shape of the cornea may produce large changes in refraction.
Refractive Error: Optical Principles and Wavefront Analysis
One of the major applications of the wave theory of light is in wavefront analysis (see also BCSC Section 3, Clinical Optics, Chapter 6). Currently, wavefront analysis can be performed clinically by 4 methods: Hartmann-Shack, Tscherning, thin-beam single-ray tracing, and optical path difference. Each method generates a detailed report of lower-order aberrations (sphere and cylinder) and higherorder aberrations (spherical aberration, coma, and trefoil, among others). This information is useful both in calculating custom ablations to enhance vision or correct optical problems and in explaining patients’ visual symptoms.
Measurement of Wavefront Aberrations and Graphical Representations
Although several techniques are available for measuring wavefront aberrations, the most popular in clinical practice is based on the Hartmann-Shack wavefront sensor. With this device, a low-power laser beam is focused on the retina. A point on the retina acts as a point source, and the reflected light is then propagated back (anteriorly) through the optical elements of the eye to a detector. In an aberration-free eye, all the rays would emerge in parallel, and the reflected wavefront would be a flat
plane. In reality, the wavefront is not flat. To determine the shape of the reflected wavefront, an array of lenses samples parts of the wavefront and focuses light on a detector (Fig 1-1A). The extent of the divergence of the lenslet images from their expected focal points determines the wavefront error (Fig 1-1B). Optical aberrations measured by the aberrometer can be resolved into a variety of basic shapes, the combination of which represents the total aberration of the patient’s ocular system, just as conventional refractive error is a combination of sphere and cylinder.
Figure 1-1 A, Schematic of a Hartmann-Shack wavefront sensor. As can be seen, the reflected wavefront passes through a grid of small lenses (the lenslet array), and the images formed are focused onto a charge-coupled device (CCD) chip. The degree of deviation of the focused images from the expected focal points determines the aberration and thus the wavefront error. B, An example of the images formed after the wavefront passes through the lenslet array. (Part A redrawn by Mark Miller
from a schematic image courtesy of Abbott Medical Optics Inc.; part B courtesy of M. Bowes Hamill, MD.)
Currently, wavefront aberrations are most commonly specified by Zernike polynomials, which are the mathematical formulas used to describe the surfaces shown in Figures 1-2 through 1-6. Each aberration may be positive or negative in value and induces predictable alterations in the image quality. The magnitude of these aberrations is expressed as a root mean square (RMS) error, which is the deviation of the wavefront averaged over the entire wavefront. The higher the RMS value is, the greater is the overall aberration for a given eye. The majority of patients have total RMS values less than 0.3 µm. Most higher-order Zernike coefficients have mean values close to zero. The most important Zernike coefficients affecting visual quality are coma, spherical aberration, and trefoil.
Figure 1-2 Zernike polynomial representation of defocus. Arrows indicate z axis (arrow emerging from cone) and zero axis.
(Courtesy of Tracey Technologies.)
Figure 1-3 Zernike polynomial representation of astigmatism. (Courtesy of Tracey Technologies.)
Figure 1-4 A, Zernike polynomial representation of spherical aberration. B, A schematic diagram of spherical aberration. Parallel rays impacting a spherical lens are refracted more acutely in the periphery than in the center of the lens. (Part A courtesy
of Tracey Technologies; part B developed by M. Bowes Hamill, MD.)
Figure 1-5 Zernike polynomial representation of coma. (Courtesy of Tracey Technologies.)