CHAPTER 6
Optical Considerations in Keratorefractive Surgery
This chapter provides an overview of the optical considerations specific to keratorefractive surgery. Refractive surgical procedures performed with the intent to reduce refractive errors can generally be categorized as corneal (keratorefractive) or lenticular. Keratorefractive surgical procedures include radial keratotomy (RK), astigmatic keratotomy (AK), photorefractive keratectomy (PRK), laser subepithelial keratomileusis (LASEK), epithelial laser in situ keratomileusis (epi-LASIK), laser in situ keratomileusis (LASIK), implantation of intracorneal ring segments and corneal inlays, laser thermal keratoplasty (LTK), and radiofrequency conductive keratoplasty (CK). Lenticular refractive procedures include cataract and clear lens extraction with intraocular lens implantation, phakic intraocular lens implantation, multifocal and toric intraocular lens implantation, and piggyback lens implantation. Although all of these refractive surgical techniques alter the optical properties of the eye, keratorefractive surgery is generally more likely than lenticular refractive surgery to produce unwanted optical aberrations. This chapter discusses only keratorefractive procedures and their optical considerations. For a discussion of optical considerations in lenticular refractive surgery, see BCSC Section 11, Lens and Cataract.
Various optical considerations are relevant to refractive surgery, both in screening patients for candidacy and in evaluating patients with vision complaints after surgery. The following sections address optical considerations related to the change in corneal shape after keratorefractive surgery, issues concerning the angle kappa and pupil size, and the various causes of irregular astigmatism.
Corneal Shape
The normal human cornea has a prolate shape (Fig 6-1), similar to that of the pole of an egg. The curvature of the human eye is steepest in the central cornea and gradually flattens toward the periphery. This configuration reduces the optical problems associated with simple spherical refracting surfaces, which produce a nearer point of focus for peripheral rays than for paraxial rays— a refractive condition known as spherical aberration. Corneal asphericity, the relative difference between the pericentral and central cornea, is represented by the factor Q. (Note that the asphericity Q factor is a geometric factor, distinct from the Q factor that characterizes a resonator such as a laser cavity.) In an ideal visual system, the curvature at the center of the cornea would be steeper than at the periphery (ie, the cornea would be prolate), and the asphericity factor Q would have a value close to
–0.50; at this value of negative Q, the degree of spherical aberration would approach zero. However, in the human eye, such a Q value is not anatomically possible (because of the junction between the cornea and the sclera). The Q factor for the human cornea has an average value of –0.26, allowing for a smooth transition at the limbus. The human visual system, therefore, suffers from minor spherical aberrations, which increase with increasing pupil size.
Figure 6-1 An example of meridional (tangential, left) and axial (right) maps of a normal cornea. (Used with permission from
Rob erts C. Corneal topography. In: Azar DT, ed. Gatinel D, Hoang-Xuan T, associate eds. Refractive Surgery. 2nd ed. St Louis: Elsevier-Mosb y; 2007:103–116.)
Following keratorefractive surgery for the treatment of myopia, the cornea becomes less prolate and has a shape resembling that of an egg lying on its side. The central cornea becomes flatter than the periphery. This flattening results in a change in the spherical aberration of the treated zone.
To demonstrate this change, consider the point spread function produced by all rays that traverse the pupil from a single object point. Generally, keratorefractive surgery for myopia reduces spherical refractive error and regular astigmatism, but it does so at the expense of increasing spherical aberration and irregular astigmatism (Fig 6-2). Keratorefractive surgery moves the location of the best focus closer to the retina but, at the same time, makes the focus less stigmatic. Such irregular astigmatism is what underlies many visual complaints after refractive surgery.
Figure 6-2 Examples of the effects of (A) coma, (B) spherical aberration, and (C) trefoil on the point spread functions of a
light source and a Snellen letter E. (Courtesy of Ming Wang, MD.)
A basic premise of refractive surgery is that the cornea’s optical properties are intimately related to its shape. Consequently, manipulation of the corneal shape changes the eye’s refractive status. Although this assumption is true, the relationship between corneal shape and the cornea’s optical properties is more complex than is generally appreciated.
Ablative procedures, incisional procedures, and intracorneal rings change the natural shape of the cornea to reduce refractive error. Keratometry readings in eyes conducted before they undergo keratorefractive surgery typically range from 38.0 D to 48.0 D. When refractive surgical procedures are being considered, it is important to avoid changes that may result in excessively flat (<33.0 D) or excessively steep (>50.0 D) corneal powers. A 0.8 D change in keratometry value (K) corresponds to approximately a 1.00 D change of refraction. The following equation is often used to predict corneal curvature after keratorefractive surgery:
Kpostop = Kpreop + (0.8 × RE)
where Kpreop and Kpostop are preoperative and postoperative K readings, respectively, and RE is the refractive error to be corrected at the corneal plane. For example, if a patient’s preoperative keratometry readings are 45.0 D (steepest meridian) and 43.0 D (flattest meridian), then the average K value is 44.0 D. If the amount of refractive correction at the corneal plane is –8.50 D, then the predicted average postoperative K reading is 44.0 + (0.8 × –8.50 ) = 37.2 D, which is acceptable.
The ratio of dioptric change in refractive error to dioptric change in keratometry approximates 0.8:1 owing to the change in posterior corneal surface power after excimer ablation. The anterior corneal surface produces most of the eye’s refractive power. In the Gullstrand model eye (see Table 2-1), the anterior corneal surface has a power of +48.8 D and the posterior corneal surface has a power of –5.8 D, so the overall corneal refractive power is +43.0 D. Importantly, standard corneal topography instruments and keratometers do not measure corneal power precisely because they do not assess the posterior corneal surface. Instead, these instruments estimate total corneal power by assuming a constant relationship between the anterior and posterior corneal surfaces. This constancy is disrupted by keratorefractive surgery. For example, after myopic excimer surgery, the anterior corneal curvature is flattened. At the same time, the posterior corneal surface remains unchanged or, owing to the reduction in corneal pachymetry and weakening of the cornea, the posterior corneal surface may become slightly steeper than the preoperative posterior corneal curvature, increasing its negative power. The decrease in positive anterior corneal power and the (minimal) increase in negative posterior corneal power cause an increase in the relative contribution to the overall corneal refractive power of the posterior surface.
The removal of even a small amount of tissue (eg, a few micrometers) during keratorefractive surgery may cause a substantial change in refraction (Fig 6-3). The Munnerlyn formula approximates these 2 parameters:
where t is the depth of the central ablation in micrometers, S is the diameter of the optical zone in millimeters, and D is the degree of refractive correction in diopters.
Figure 6-3 Comparison of a 43 D cornea with a 45 D cornea. Numbers below the vertical arrows indicate distance from the optical axis in millimeters; numbers to the right of the horizontal arrows indicate the separation between the corneas in micrometers. A typical pupil size of 3.0 mm is indicated. A typical red blood cell has a diameter of 7 µm. Within the pupillary space (ie, the optical zone of the cornea), the separation between the corneas is less than the diameter of a red
blood cell. (Courtesy of Edmond H. Thall, MD. Modified b y C. H. Wooley.)
An ideal LASIK ablation or PRK removes a convex positive meniscus in corrections of myopia (Fig 6-4A) and a concave positive meniscus in simple corrections of hyperopia (Fig 6-4B). A toric positive meniscus is removed in corrections of astigmatism. In toric corrections, the specific shape of the ablation depends on the spherical component of the refractive error.