Corneal Topography and the New Wave Stephen D. Klyce, Ph.D. The normal prolate ellipsoidal corneal front surface with its 8–10-m thin and optically smooth tear film accounts for about two-thirds of the refractive power of the eye. Disruptions in the tear film and induction of irregularity in corneal shape can, there- fore, degrade the optical quality of the eye and reduce visual acuity. Such distortions in the corneal surface can be so subtle as to escape detection with the usual biomicroscope examination. Retinoscopy is a subjective diagnostic procedure that can reveal fairly slight distortions in the retinal image that can arise from the cornea. However, retinoscopy does not reveal the nature and/or locus of the aberrating medium, for corneal back surface, lenticu- lar, and retinal anomalies can distort vision as well. Twenty-five years ago, corneal specialists used retinoscopy and photokeratos- copy to diagnose mild keratoconus and to manage astigmatism in corneal grafts. Just a few years later, refractive surgery took its first halting steps into clinical practice; it was this scenario that provided the impetus for the development of improved methods for analyzing corneal shape. Corneal topography as a routine clini- cal examination was born, and its acceptance can be judged by the explosive growth of publications on this subject in the peer- reviewed literature (Fig. 1). THE PHOTOKERATOSCOPE In the mid 1970s, corneal topography was evaluated with the photokeratoscope, and the two models generally available were the NIDEK PKS-1000 1 (NIDEK Corporation, Gamagori, Japan) and the Corneascope (International Diagnostic Instruments, Tulsa, OK, U.S.A.). 2 These devices combine an illuminated Placido disk tar- get and an instant film camera to capture an image of the Placido mires reflected from the cornea. Initially, clinicians interpreted photokeratoscope photographs by visual inspection, recognizing that corneal steepening was represented by minification of the mires, that corneal flattening was represented by a magnification of the mires, and that irregular astigmatism was represented by irregularities in the mire pattern. Further, corneal cylinder repre- sented by elliptically shaped mires and keratoconus often revealed itself as pear-shaped mires. Although this technology was useful, particularly for trying to manage the sizable astigmatism present in corneal transplants, 3 it was humbling to realize that no significant clinical advance had been made since 1880 when Antonio Placido first introduced the Placido disk. When keratorefractive surgery made its entrance in the late 1970s, it became clear that a more sensitive and quantitative method for corneal shape analysis was in order. Visual inspection of Placido mires can fail to detect up to 3 diopters (D) of corneal cylinder, mild keratoconus, and, in refrac- tive surgery and corneal transplants, the etiology of postoperative visual distortions. One of the most successful analytical approaches to providing quantitative measures from photokeratoscopy derived from the work of Doss et al. 4 who, in 1981, scanned Corneascope photo- graphs and proposed a mathematical method for the conversion of mire size and shape to corneal power. These data were presented in the form of a numerical plot. Subsequently in 1984, Klyce 5 proposed a method for reconstructing corneal shape and power by digitizing mires from NIDEK photokeratoscope photographs. In this work, graphical plots using three-dimensional wire mesh mod- els were used to depict corneal topography, as condensing the thousands of data points collected from the photokeratoscope pho- tographs was necessary to permit clinical use. The final graphical presentation form of this data, which has become the international standard, was the color-coded contour map of corneal powers pre- sented by Maguire et al. 6 in 1987. Corneal power or curvature had been measured clinically for nearly a century before the advent of corneal topographers. Oph- thalmometers (such as the ophthalmometer Javal and Schioetz in- troduced in the 1890s), predecessors to the keratometers, very accurately measure the curvature of the corneal front surface. They are calibrated so that a surface with a 7.5-mm radius of curvature would correspond to 45 D of refractive power. This leads to the convenient, but artificial, refractive index gradient of 7.5 × 45  337.5, which is termed the keratometric index. Importantly, this convention yields an accurate measure of the radius of curvature for the front surface of the cornea for contact lens fitting; however, this relation cannot be accurately used to predict corneal refractive power changes, particularly when only the anterior surface curva- ture is modified. 7,8 Nevertheless, for consistency of clinical inter- pretation, the keratometric index has been carried over for the expression of corneal power in corneal topographers. On the av- erage, normal adult human corneal power is about 43 D with this convention. For the 43-D cornea, anterior surface curvature, thus, is 7.85 mm (337.5/43). THE COLOR-CODED MAP With the introduction of the color-coded contour map of corneal powers, the concept of color association conveyed whether corneal power was higher or lower than the average 43-D norm. A color spectrum was chosen so that powers near the norm showed as Submitted February 16, 2000. Revision received May 17, 2000. Ac- cepted May 18, 2000. From the Lions Eye Research Laboratories, LSU Eye Center, Louisiana State University Medical Center School of Medicine, New Orleans, Loui- siana, U.S.A. Address correspondence and reprint requests to Dr. Stephen D. Klyce, LSU Eye Center, 2020 Gravier Street, Suite B, New Orleans, LA 70112, U.S.A. E-mail: sklyce@lsumc.edu Cornea 19(5): 723–729, 2000. © 2000 Lippincott Williams & Wilkins, Inc., Philadelphia 723