SURVEY OF OPHTHALMOLOGY
CURRENT EDWARD
COTLIER
VOLUME 35. NUMBER 4 * JANUARY-FEBRUARY 1991
RESEARCH AND ROBERT
WEINREB,
EDITORS
Advances in the Analysis of Cornea1 Topography STEVEN
E. WILSON,
M.D.,’ AND STEPHEN
D. KLYCE,
PH.D.*
‘Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas, and ‘LSU Eve Center, Louisiana State University Medical Center School of Medicine, New Orleans, Louisiana
Abstract. Recent advances in topographic
analysis have provided powerful tools for detecting subtle, but clinically significant, alterations of cornea1 contour. This article compares keratometry, keratoscopy, and computer-assisted topographic analysis and provides specific examples of the sensitivity of computer-assisted systems in revealing topographic alterations that were not previously discernable. Quantitative descriptors ofcorneal topography such as the surface asymmetry index, the surface regularity index, and simulated keratometry value augment the information provided by color-coded topographic maps. (Sure Ophthalmol 35:269-277, 1991)
Key words.
keratometry
contact lens-induced cornea1 warpage cornea1 topography keratoscopy keratoconus l- radial keratotomy l
l
l
l
raphy were the keratometer and the keratoscope. Each instrument, however, provides only a limited amount of information about the contour of the anterior cornea1 surface. The keratometer (also called the ophthalmometer) was first described in 1854 by Helmholtz.” It provides illuminated object mires that are reflected from the surface of the cornea which acts as a convex mirror. The virtual image formed by the cornea is minified and upright and appears to be located within the anterior chamber of the eye. The radius of curvature of the anterior cornea1 surface is determined from four reflected points that are evaluated as two pairs, based on the assumption that the cornea1 surface is spherocylindrical.5”4 The distance between the two points of a pair is measured to determine the radius of curvature in that meridian. This distance varies from approximately 2.6 to 3.7 mm depending on the cornea1 curvature.5’J4 Most commercially available instruments convert the radius of curvature to dioptric power using the standard keratometric index of 0.3375. This value
The systematic evaluation of radial keratotomy, astigmatic keratotomy, epikeratophakia, penetrating keratoplasty, and other cornea1 manipulations has been constrained by limitations in our ability to determine accurately and reproducibly the shape of the anterior cornea1 surface. For example, several clinical studies of radial keratotomy have noted a poor correlation between refraction and keratometry data for individual patients.4*6*46 Discrepancies such as these hinder efforts to improve the predictability of radial keratotomy and other refractive Recently, the emergence of surgical procedures. computer-assisted topographic analysis has provided a sensitive tool for assessing normal and pathologic cornea1 contour and for monitoring changes that occur following surgery. This article will describe the development and application of this technology.
I. Keratometry and Keratoscopy Until the mid 198Os, the only modalities that were widely available for monitoring cornea1 topog269
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1991
Fig. 1. Mire image of the Nidek PKS 1000 photokeratoscope. Note that the mires do not cover a large area of the central and peripheral cornea.
Fig. 2. Mire image of the CMS collimated videokeratoscope. Information is provided from rings extending into the central cornea and into the peripheral cornea essentially to the limbus.
represents
image which appears to be located within the anterior chamber of the eye. Information regarding the power of the anterior cornea1 surface is derived from visual inspection of the size of the mire; flatter corneas produce an image with mires of relatively larger diameter than those produced by a steeper surface. Information regarding the radius of curvature of a localized area of the cornea can be obtained by observing the separation between the mires reflected from that area of the cornea. Irregularity of the surface is displayed as distortion of the mires. The major advantage of photokeratoscopes, such as the Nidek PKS 1000 or the Kera Corneoscope, over the keratometer is that they provide
a combined
anterior
and posterior
tometer
determines
steepest
meridian
refractive cornea1
the power
for
The
and location
and the power
90” away. The instrument
index
surfaces.
the
keraof the
of the meridian
has the capacity
to meas-
ure a regular surface with an accuracy ofbetter than 0.25 diopters. The major limitations of the keratometer are that it assumes that the cornea is a spherocylindrical surface, it provides no information regarding the topography central or peripheral to the points of measurement, and mild cornea1 surface irregularity causes mire distortion that precludes meaningful measurement. Mande113’-33 was the first to demonstrate that the normal cornea is aspheric and flattened from the center to the periphery. The assumption of central spherocylindricity is a useful approximation for most normal corneas; this makes the keratometer particularly well suited for tasks such as fitting contact lenses on normal corneas. Diseased and postsurgical corneas, however, rarely approximate a spherocylindrical surface and keratometer measurements that are performed in these cases are, at best, of limited value. Any particular set of keratometer values could be associated with an unlimited number of cornea1 shapes. This follows from the wide variations that may occur in the topography central and peripheral to the keratometer measurement points. This limitation of the keratometer is likely a major determinant of the poor correlation that has been noted between keratometer and refraction measurements in clinical studies of refractive surgical procedures.‘,‘,4v6,44,46 Both Placido22 and Gullstrand” have been credited with the development of the first photokeratoscope. The photokeratoscope projects a series of concentric circular mires that also form a virtual
topographic map of a patient with 3 diopters of with-the-rule astigmatism. The central area without information corresponds to the lack of mires in the center of the Nidek PKS 100 photokeratoscope image. There is also no information provided for the far peripheral cornea.
Fig. 3. LSUCTS color-coded
ANALYSIS OF CORNEAL TOPOGRAPHY information from a larger portion of the cornea1 surface.gg Photokeratoscopes that are commercially available, however, provide limited information regarding the central cornea because that area is not covered by the mires (Fig. 1). The collimated video-
271 keratoscope component of the Cornea1 Modeling System” (Fig. 2) provides information from a larger portion of the cornea1 surface extending to the visual axis, but it is presently not available as an independent instrument providing photokeratoscopy
Fig. 4. CMS absolute and normalized
scale color-coded topographic maps ofa normal cornea with 3 diopters of with-therule astigmatism characterized by the vertically oriented bow-tie pattern. The absolute color scale (left) has fixed dioptric intervals that are 1.5 diopters in the central power range and 5.0 diopters in the high and low power ranges. Each individual color always represents a specific power interval. In the normalized color scale (right), the power intervals represented by individual colors are smaller and vary depending on the power range of the cornea being analyzed.
Fig. 5. Videokeratoscope image (left) and CMS absolute scale color-coded topographic map (right) of the left eye of a patient with advanced keratoconus in the opposite eye, demonstrating that early changes noted with computer-assisted topographic analysis may not be detectable by visual inspection of the videokeratoscope mires.
Fig. 6. CMS absolute scale color-coded topographic maps of central (left) and peripheral (right) cones of keratoconus. The same pattern is always noted in the two eyes ofan individual patient, even when there is a large difference in the stage of progression between the two eyes.
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KLYCE
Fig. 7. Left: CMS normalized
scale color-coded map of a cornea with rapid changes in power within the optical zone following radial keratotomy. This multifocal topography is characteristic of many corneas that have had radial keratotomy and accounts for the clinical finding of the maintenance of best spectacle-corrected visual acuity at a specified is uncertain in distance over a significant dioptric range during cycloplegic refraction. 35 Since the refractive endpoint such cases, multifocality is also a factor in the phenomenon ofbetter uncorrected visual acuity in patients following radial keratotomy compared with unoperated controls with similar refractive errors. Right: Videokeratoscope image that was of the mire pattern does not reveal the extent topographic map. Visual inspection analyzed to generate this color-coded
Fig. 8. Top left: Example of absolute scale color-coded topographic map of a patient with contact lens-induced cornea1 warpage caused by a rigid polymethylmethacrylate lens with a superior resting position during inter-blink intervals. Note that there is relative flattening of the superior cornea1 contour underlying the decentered resting position. The initial topographic pattern is similar to that noted in some patients with keratoconus (Fig. 5). There was no evidence of Top right: One week after stopping contact lens wear. Despite considerkeratoconus, however, on slit lamp examination. able change, the topographic pattern remains abnormal with a high degree of asymmetry. Bottom left: Videokeratoscope image analyzed one week after stopping contact lens wear. Note that visual inspection ofthe mire pattern does not reveal the extent of residual topographic abnormalities. Bottom right: Six weeks after stopping contact lens wear, the topography has returned to a normal, symmetrical, with-the-rule astigmatism.
ANALYSIS OF CORNEAL TOPOGRAPHY alone without computerized analysis. Photokeratoscopes are for the most part qualitative; studies have shown that clinically significant alterations of cornea1 contour are commonly not detectable with these instruments. p5*48,50Photokeratoscopes, however, continue to be useful and inexpensive instruments for the evaluation of cornea1 topographic alterations that are not subtle. For example, recent publications have used the photokeratoscope to monitor changes in cornea1 topography occurring following radial keratotomy,40 after adjustment of a single running suture following penetrating keratoplasty,“’ and as a guide to selective suture removal to decrease astigmatism following penetrating keratoplasty.‘5 However, it is generally accepted that clinically significant amounts of cornea1 cylinder (up to 3D) can be present in a cornea and can escape detection with traditional keratoscopy.
II. Computer-assisted Topographic Analysis A. TOPOGRAPHIC
ANALYSIS SYSTEMS
Rowsey and coworkers developed some of the earliest methods to provide quantitative information from photokeratoscope images by comparing the mire diameters from corneas to those reflected from standard reference spheres4’ and by measuring hemichord lengths from the center of the mire pattern to each keratoscope ring along several hemimeridians.g~4’ These methods have not, however, found widespread acceptance, primarily because of inaccuracy and difficulty in correlating the quantitative information provided by numerical plots of the power at various points on the photokeratoscope mires with clinical patterns of cornea1 topography. These methods were, however, instrumental in the formulation of some of the earliest principles of keratorefractive surgery by Rowsey.“g Computer-assisted analysis of photokeratoscope images was enhanced by Klyce in 1984.” Interpretation of shape anomalies and clinical utility were augmented with the introduction of color-coded topographic maps JOthat allow the clinician to discern subtle variations in power distribution on the anterior cornea1 surface and to recognize specific patterns of pathologic cornea1 topography. A brief description of instruments that provide computer-assisted topographic analysis is provided below and the current commercial instruments known to the authors are listed in Table 1. The details of the computerized algorithms that allow an accurate three dimensional reconstruction of the cornea1 surface from photokeratoscope data obtained by these devices are beyond the scope of this article and are covered elsewhere where avail-
273 TABLE 1 Clinical Automatic Keratoscope Cornea1 Topoprafihy Analyzers
Company Computed Anatomy EyeSys Visioptic
able.
Device Topographic Modeling System Cornea1 Analysis System Computerized Cornea1 Topographer
cost $24,950 -$20,000 $29,950
18.19.45.53
The LSU Corneal Topography System (LSIJCTS) analyzes images produced by the 1 l-ring Nidek PKS 1000 photokeratoscope (Fig. l).53 Manual digitization is used to capture data for computerized analysis. The HIPAD (Houston Instruments, Austin, TX) manual digitizing equipment used with this system is capable of 2000 lines per frame resolution, which translates to a resolution of 0.3 diopters on the cornea1 surface. The major limitations of the LSUCTS are that manual digitization is tedious and no information is obtained from the central and far peripheral cornea because these areas are not covered by the mires of the PKS 1000 photokeratoscope (Fig. 1).30 Topographic information is provided in the form of color-coded topographic maps (Fig. 3) that facilitate clinical and research interpretation of cornea1 topographic alterations. At present, the most widely used instrument is the Cornea1 Modeling System (CMS) manufactured by Computed Anatomy, Inc (New York, NY).‘3.53 The CMS uses a 32-ring collimated videokeratoscope that provides a central fixation point as a reproducible reference for the computerized statistical analysis of the power points on the videokeratoscope rings. On each videokeratoscope ring, 256 points are evaluated; therefore, the analysis includes thousands of individual power points and covers nearly the entire cornea1 surface from the visual axis to the far periphery (Fig. 2). The instrument features operator-monitored automated digitization with approximately 500 lines per frame resolution. Statistical procedures are utilized by the computerized algorithms to provide resolution on the cornea1 surface of less than 0.25 diopters. Topographic information is provided in the form of several different color-coded topographic maps (Fig. 4).‘” The CMS system is unable to analyze severely distorted keratoscope mires, but the diagnosis in such cases, however, is usually obvious. The accuracy and precision of this instrument in measuring the curvature of calibrated spherical surfaces has
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been validated.14 Precision and accuracy have yet to be published for radially asymmetric aspheric reference standards that simulate complex cornea1 shapes. A recent study confirmed that the original LSUCTS algorithm was accurate in modeling the central cornea, but was less accurate in the periphery.45 This study also reported new algorithms for improving the accuracy of the LSUCTS, but it is not certain whether this work will have an impact on the accuracy of the CMS, since that device uses independently developed reconstruction algorithms. CMS technology is now available in an instrument called the Topographic Modeling System (TMS-1). The Computerized Cornea1 Topographer EH270 (Visioptic Inc., Houston, TX), designed by El Hage,” duplicates the provision of cornea1 contour displays based on computer-assisted analysis of photokeratoscope images. This instrument provides several video displays that include a cornea1 contour map, a meridional contour map, and an astigmatism map. ” The accuracy and precision of this instrument, however, have not yet been validated by independent study. Additionally, there have been no studies reported in which this system was used to monitor alterations of cornea1 topography. Another videokeratoscope-based instrument currently available is the Cornea1 Analysis System from EyeSys Laboratories (Houston, TX).*’ This is a 16 ring videokeratoscope-based device with fast image processing time (3 seconds) and color-coded contour map plots in addition to a host of other data presentation schemes. Some data regarding the accuracy and precision of this instrument are available in the literature;2’ however, independent validation is not yet available. Additional instruments that use laser interferometry or rasterstereography’* for reconstructing the cornea1 surface have been reported. These instruments, however, are not commercially available and, again, their accuracy and precision have not been validated. B. PARAMETRIC DESCRIPTORS OF COBNEAL TOPOGRAPHY While the qualitative information obtained from color-coded maps and other visual presentation schemes has been shown to provide a unique understanding of cornea1 topographic alterations, the clinical utility and the application of computer-assisted topographic analysis to research would be augmented by the availability of quantitative descriptors of cornea1 topography. The Simulated Keratometry value (Sim K), Surface Asymmetry Index (SAI), and the Surface Regularity Index (SRI) are discussed below. These parametric descriptors are provided during topographic analysis with the
WILSON,
KLYCE
CMS instrument and could be incorporated into other topographic analysis systems. Additional descriptors are under development. The Sim K value5’ provides the power and location of the steepest and flattest meridian from a reconstructed cornea1 surface analogous to values provided by the keratometer. This descriptor allows the clinician to correlate the topography seen on the color-coded map with another common parameter and obviates the need to obtain separate keratometer measurements. The value is obtained from all power points on photokeratoscope rings 7, 8, and 9. These mires were selected because their position on the cornea1 surface approximates the location at which standard keratometry measurements are obtained.5 Both spherocylindrical and nonspherocylindrical values are provided. The spherocylindrical Sim K value provides the power and the location of the steepest meridian and the meridian 90” away. The nonspherocylindrical Sim K value also provides the power and location of the actual flattest meridian regardless of the angle between the steepest and flattest meridian. Pathologic and postsurgical corneas commonly have shapes that do not approximate a spherocylindrical surface and, therefore, the steepest and flattest meridians are often separated by less than 90”. Prospective studies have demonstrated a high correlation between keratometry and spherocylindrical Sim K values.21~“0 The SAI is a centrally weighted summation of differences in cornea1 power between corresponding points 180” apart on 128 equally spaced meridians that cross the four central photokeratoscope mires.8~‘8~2’~53 For example, if the power on ring 4 at 5” is 44.0 and the power on ring 4 at 185” is 46.0, the difference, 2.0, is entered into the centrally weighted summation. Similar differences are entered for all corresponding pairs of points on the four central mires. SAI approaches zero for a perfectly radially symmetrical surface and increases as the contour becomes more asymmetric. Since the normal cornea usually has a high degree of central radial symmetry,’ the SAI is a useful quantitative parameter for monitoring changes that occur in patients with contact lens-induced cornea1 warpage,50*5’ following penetrating keratoplasty,23 and in other cornea1 disorders that cause an alteration of cornea1 symmetry (e.g., off-center keratoconus apices). The SRI is determined from a summation of local fluctuations in power along 256 equally spaced hemimeridians on the 10 central mires4* This index approaches zero for a normally smooth cornea1 surface and increases directly with increasing irregular astigmatism. In a prospective clinical study,48 there was a high correlation between the SRI and best
ANALYSIS OF CORNEAL
TOPOGRAPHY
spectacle-corrected visual acuity (r = 0.80, p < 0.001). Therefore, the SRI value can be used to predict the optical performance that might be expected in a particular patient based on the cornea1 topography, if other components of the visual system such as the lens and the macula are functioning normally. This index would also be of value for predicting visual acuity following experimental refractive surgical procedures in animals.
275
ate the cornea1 curvature from only four points that are separated by approximately 3 mm, they would not detect the multifocal lens effect. Therefore, there can be a poor correlation between the change in keratometer measurements and the change in refraction following radial keratotomy. Visual inspection of photokeratoscope images is not sensitive enough to detect the multifocal topography in the majority of cases (Fig 7). A recent study by McDonnell et al”” reported a correlation between cornea1 topography and flucIII. Clinical and Research Applications tuation in visual acuity following radial keratotomy. of Computer-assisted Topographic Ninety-one percent of eyes with fluctuating vision Analysis had a topography characterized by the presence of Computer-assisted topographic analysis has a split or dumbbell-shaped optical zone. Eyes withbeen used to study normal cornea1 topography’ out fluctuation of visual acuity had either a round and to demonstrate the effects of disease,25~‘8~98~4g~“2 optical zone (80%) or a band-like zone (20%). Studsurgical procedures,26~‘7~2q~‘5~47post surgical maies such as this should provide valuable insights into nipulations,23*24 and contact lenses on cornea1 tocorrelations between cornea1 topography and visupography.“‘.” The examples below illustrate the al function following refractive surgical procedures. utility of the technique for providing information Computer-assisted topographic analysis has also that was not available prior to the development of provided new insights into the changes in cornea1 this method. topography induced by contact lenses.5”B5’ These Several studies have shown that in early keratostudies have demonstrated that there is frequently a conus topographic changes that are noted with correlation between the resting position of a rigid contact lens on the cornea1 surface and the topocomputer-assisted topographic analysis may not be graphic pattern in contact lens-induced cornea1 detected using the keratometer or the keratoscope (Fig. j!j),P5.9xs49 warpage (Fig. 8). Additionally, the time required in Early detection of the disorder allows the clinician to better advise and treat the patient some eyes for the topographic abnormalities to resolve may be far longer (6 or more months) than and may allow the etiology to be better defined by was previously appreciated. confirming the presence of subtle changes in family Color-coded topographic maps and recently demembers during molecular genetic investigations.“8 It has also been possible to demonstrate that the veloped quantitative descriptors of cornea1 topography have made it possible to better document topographic patterns of the two corneas of a patient changes in cornea1 contour that occur during corwith keratoconus show a high degree of mirror imneal surgical procedures. Examples of the usefulage symmetry, despite the fact that large differences ness of computer-assisted topographic analysis are in apex power are usually noted.4g Topographic provided by recent studies on the effect of running patterns in keratoconus patients can be divided into suture removal following penetrating keratoplasty’” two groups: those with central cones and those with and postsurgical manipulation of a single running 1Operipheral cones (Fig. 6).4g The same pattern is al0 nylon suture to reduce astigmatism following peneways noted in the two eyes of an individual patient, trating keratoplasty.‘” although the extent of progression of keratoconus These examples illustrate the usefulness of comis usually different. puter-assisted topographic analysis. Future studies Previous studies on the effect of radial keratotwill likely provide additional insights into the effects omy on cornea1 topography have noted a lack of of disease and surgery on cornea1 contour that correlation between changes in keratometer meawould not have been noted without advances in surements, refractions, and uncorrected visual acureconstruction of the anterior cornea1 surface. 4~6.4”.46 Computer-assisted topographic analysis ity. has demonstrated that one factor contributing to these findings is a multifocal central cornea1 conIV. The Ideal Cornea1 Topographic tour that frequently is present following radial kerAnalysis Instrument atotomy (Fig. 7).27.‘15,47The progressive change in Computer-assisted topographic analysis is useful power within the optical zone in such cases allows clinically and for research. Clinicians must decide, the patient to maintain focus during a cycloplegic based on the requirements of their practice, whethrefraction over an interval that can exceed 10 er they need the advanced analysis capabilities prodiopters. Since keratometer measurements evalu-
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vided by modern topographic analysis systems or if a keratometer or a keratoscope will be satisfactory. At present, there are three computer-assisted topographic analysis instruments commercially available (Table 1) and it is pointed out that not all of these may be equal in either utility or functionality. It is possible that other computerized analysis systems will become available. Current and future instruments should be evaluated on the basis of tttility, accuracy, precision, and affordability.*’ The clinician should have a working understanding of the operation and data interpretation of the device under consideration prior to committing to purchase. A real-time, hands-on evaluation of the instrument will help the potential buyer to determine whether the technology will be practical for operation by staff in the clinic. Currently, the only unbiased method to verify the accuracy and precision of an instrument under consideration is through the availability of independent studies published in peer-reviewed journals. Only a limited number of reports have been published for commercially available instruments,14 although other studies will likely become available in the future. If inexpensive aspheric reference surfaces were available, these would allow the individual clinician or investigator to directly analyze an instrument under consideration for purchase. It is hoped that surfaces such as these will become generally available. The cost of computer-assisted topographic analysis instrumentation has decreased sharply in the past few years from well over $100,000 for a laser scanning device developed in Europe to amounts shown in Table 1. Obviously, the price for any instrument can only be evaluated after carefully considering utility, accuracy, precision, and the needs and resources of the individual practice.
V. Future Developments in Cornea1 Topographic Analysis In the future, systems will likely be developed that will allow affordable, high-resolution, realtime, quantitative monitoring of cornea1 topography during surgical manipulations as an adjunct to surgical keratometers.‘* This will undoubtedly improve the predictability of all keratorefractive techniques. For example, one may envision a system in which a photoablative laser is combined with a realtime topographic analysis system. With such a system it might be possible for the surgeon to specify the desired cornea1 topography and for parameters to be monitored automatically by the instrument so that the ablation is performed accordingly. Helmholtz, Placido, and Gullstrand would certainly have been impressed!
WILSON, KLYCE
1991
References 1. Arffa RC, Klyce SD, Busin M: Keratometry and epikeratophakia. J Refract Surg 2:61-64, 1986 2. Arffa RC, Marvelli T, Morgan K: Keratometric and refractive results of pediatric epikeratophakia. Arch Ophthalmol 103:1656-1659, 1985 3. Arffa RC, Warnicki JW, Rehkopf PG: Cornea1 topography usingrasterstereography. Refractive Cornea1 Surg5:414-417, 1989 4. Arrowsmith PN, Marks RG: Visual, refractive, and keratometric results of radial keratotomy. Arch Ophthalmol 105: 76-80, 1987 5. Dabezies OH, Holladay JT: Measurement of cornea1 curvature: Keratometer (ophthalmometer), in Dabezies OH, Jr, Cavanagh HD, Farris RL, Lemp MA (eds): Contact Lenses: The CLAO Guide to Basic Science and Clinical Practice. Orlando, Grune and Stratton, 1986, pp I-17, 29 6. Deitz MR. Sanders DR, Marks RG: Radial keratotomy: An overview of the Kansas City study. Ophthalmology 91:467-78, 1984 7. Dingeldein SA, Klyce SD: The topography of normal corneas. Arch Ophthalmol 107:512-518, 1989 8. Dingeldein SA, Klyce SD, Wilson SE: Quantitative descriptors of cornea1 shape from computer-assisted analysis of photokeratographs. Refractive Cornea1 Surg5:372-378, 1989 9. Doss JD, Hutson RL, Rowsey JJ, Brown R: Method for calculation of cornea1 profile and power distribution. Arch Ophthalmol 99:1261-1265, 1981 10. Duke-Elder S: System of Ophthamology, Vol V. St Louis, CV Mosby, 1970, pp 127 11. El Hage SC: Acomputerized cornea1 topographer for use in refractive surgery. Refractive Cornea1 Sure 5:4 181124. 1989 ofsurgi12. Frantz JM, Re\dyJJ, McDonald MB: Actmparison cal keratometers. Refractive Cornea1 Surg 5:409-4 13, 1989 13. Gormley DJ, Gersten M, Koplin RS, Lubkin V: Cornea1 modeling. Cornea 7:30-35, 1988 14. Hannush SB, Crawford SL, Waring GO, et al: Accuracy and precision of keratometry, photokeratoscopy, and cornea1 modeling on calibrated steel balls. Arch Ophthalmol 107:123{-1239, 1989 15. Harris DJ, Waring GO, Burk LL: Keratography as a guide to selective suture removal for the reduction of astigmatism after penetrating keratoplasty. Ophthalmology 96:15971607, 1989 16. Helmholtz H von: Handbuch derphysiologicschen Optik. Hamburg, Germany, Leopold Voss, 1909 17. Klyce SD: Computer-assisted cornea1 topography. Highresolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci 25:1426-1435, 1984 18. Klyce SD, Wilson SE: Methods of analysis of cornea1 topography. Refracttve Cornea1 Surg 5:368-37 1, 1989 19. Klyce SD, Wilson SE: Imaging, reconstruction, and display of cornea1 topography. Proceedings of the SPIE 33rd Annual International Technical Symposium of Optical and Optoelectronic Applied Science and Engineering, New Methods in Microscopy and Low Light Imaging. SPIE 1161:409-416, 1989 20. Klyce SD, Wilson SE, Kaufman HE: Cornea1 topography comes of age. Refractive Cornea1 Surg 5:359-361, 1989 21. Koch DD, Foulks GN, Moran CT, Wakil JS: The cornea1 EyeSys system: Accuracy analysis and reproducibility of first-generation prototype. Refractive Cornea1 Surg 5: 424429, 1989 22. Levene JR: The true inventors of the keratoscope and photokeratoscope. BrJ Hist Sci 2:324, 1965 23. Lin DTC, Wilson SE, Klyce SD, lnsler MS: Topographic changes that occur with 10-O running suture removal following penetrating keratoplasty. Refractive Cornea1 Surg 6.21-25, 1990 24. Lin DTC, Wilson SE, Reidy JJ, et al: An adjustable running suture technique to reduce postkeratoplasty astigmatism A preliminary report. Ophthalmology 97:934-938, 1990 25. Maguire LJ, Bourne WM: Cornea1 topography of early keratoconus. Am J Ophthalmol 108:107-l 12, 1989 26. Maguire LJ, Bourne WM: Cornea1 topography oftransverse
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27.
28.
29.
30.
31
32.
33.
34. 35.
36.
37.
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39. 40.
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4”.
TOPOGRAPHY
keratotomies for astigmatism after penetrating keratoplasties. Am J Ophthalmol 107:323-330, 1989 Maguire LJ, Bourne WM: A multifocal lens effect as a complication of radial keratotomy. Refractive Cornea1 Surg 5:394-399, 1989 Maguire LJ, Klyce SD, McDonald MB, Kaufman HE: Corneal topography of pellucid marginal degeneration. Ophthnlmolog?l 94:5 19-524, 1987 Maguire LJ, Klyce SD, Sawelson H, et al: Visual distortion after myopic keratomileusis: Computer analysis of keratoscope photographs. Ophthalmic Surg 18:352-356, 1987 Maguire LJ, Singer DE, Klyce SD: Graphic presentation of computer-analyzed keratoscope photographs. Arch Ophthalmol 105:223-230, 1987 Malldell RB: Methods to measure the peripheral cornea1 curvature. Part one: photokeratoscopy. J Am Oplom&c As.wr 33:137-139, 1961 Mandell RB: Methods to measure the peripheral cornea1 curvature. Part two: geometric construction and “romputers.” J Am Optomrlrrr Assoc 33:585-589, 1962 Mandell RB: Methods to measure the peripheral cornea1 curvature. Part three: ophthalmometry. J Am Optomrlric As.W( 33:xtw892. I962 Mandell RB: Corr/arl Lens Pmclzcr. Springfield, Charles C Thomas, 198X. ed 4, pp 928-935 McDonnell PJ. Garbus J, Lopez PF: Topographic analysis and visual acuity after radial keratotomy. Am J Ophthalmol /Oh:692-695. 198X McDonnell PJ, McClusky DJ. Garbus JJ: Cornea1 topography and lluctuating visual acuity after radial keratotomy. Ophlhalmolog?’ Y&665-670. 1989 MrNeill JI. Wessels IF: Adjustment ofsingle continuous suture to control astigmatism after penetrating keratoplasty. R$aclwr Cornea1 Surg 5:2 16-223, 1989 Rabinowitz YS, McDonnell PJ: Computer-assisted cornea1 in keratoconus. Rtfracli~w Cornea1 Slrrg topography 5;400-40X. 19x9 ROMsey JJ: Ten caveats in keratorefractive surgery. Ophthalmn/o,q 90: 14X-155, 1983 RowseT I.J J, Balyeat HD, Monlux, et al: Prospective evaluation of radial keratotomy. Photokeratoscope cornea1 topography. Ophthalmology 95:322-334, 1988 Rowsey JJ, Monlux R, Balyeat HD, et al: Accuracy and reproducibility of kerascanner analysis in PERK cornea1 topography. C:llrt- E~P Rrs 8:661-674. 1989 Rowsrv J.1, Reynolds AE, Brown R: Cornea1 topography.
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43.
44 45
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48. 49. 50.
.5 I
52.
53.
Arch Ophthalmol 99: 1093-I 100, 198 1 Santos VR, Waring GO, Lynn MJ, et al: Relationship between refractive error and visual acuity in the Prospective Evaluation of Radial Keratotomy (PERK) study. Arch Oph/hnlmo/ 105:86-92, 1987 Swinger CA, Barker BA: Prospective evaluation of myopic keratomileusis. Obhthalmolopv 9Zt785-792, 1984 Wang J, Rice DA,‘Klyce SD:“b new reconstruction algorithm for improvement of cornea1 topographic analysis. Refracfivr Cornea1 Surg 5:379-387, 1989 Waring GO, Lynn MJ, Culbertson W, et al: Three-year results of the prospective evaluation of radial keratotomy (PERK) study. Ophthalmolog? 91: 1339- 1354, 1987 Wilson SE, Klyce SD: Topographic analysis and visual acuity after radial keratotomy (correspondence). ,4m ,/ Oph/halmol 107:436-437, 1989 Wilson SE, Klyce SD: Quantitative descriptors 01 cornea1 topography: A clinical study. ,4rch Ophthalmol (in press) Wilson SE, Lin DTC, Klyce SD: The cornea1 topography of keratoconus. Cornea (in press) Wilson SE, Lin DTC, Klyce SD, et al: .I‘opographic changes in contact lens-induced cornea1 warpage. Ophthalmology 9?:734-744, 1990 Wilson SE, Lin DTC, Klyce SD, et al: Rigid contact lens decentration: A risk factor for corncal warpage. CL.40,/ 16: 377-382, 1990 Wilson SE. Lin DTC, Klyce SD, Inslel- MS: The cornea1 topography of Terrien’s marginal degeneration. K@ZC~II~C Cornea1 Surg 6: 15-20, 1990 Wilson SE, Wang JY, Klyce SD: Quantitative and mathematical analysis of photokeratoscopic images, in Schanzlin DJ, Robin J (eds): Corral Topography. Neh York. Springer-Vcrlag (in press)
Neither Dr. Wilson nor his family members have any c’ommercial or proprietary interest in any of the products mentioned in this review. Dr. Klyce is a paid consultant to Compuletl Anatomy, Inc. (New York, NY). Supported in part by US Public Health Service grants EY03311 and EY02377 from the National Eye Institute, National Institutes of Health, Bethesda, Maryland and the Louisiana Lions Eye Foundation. Computed Anatomy, Inc. generously supplied the Cornea1 Modeling System used in this study. Reprint address: Stephen E. Wilson, M.D.. Dept. of Ophthalmology, University ofTexas Southwestern Medical (:enrer, 5323 Harry Hines Blvd., Dallas, TX 7523.5.
CURRENT EDWARD
COTLIER
VOLUME 35. NUMBER 4 * JANUARY-FEBRUARY 1991
RESEARCH AND ROBERT
WEINREB,
EDITORS
Advances in the Analysis of Cornea1 Topography STEVEN
E. WILSON,
M.D.,’ AND STEPHEN
D. KLYCE,
PH.D.*
‘Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas, and ‘LSU Eve Center, Louisiana State University Medical Center School of Medicine, New Orleans, Louisiana
Abstract. Recent advances in topographic
analysis have provided powerful tools for detecting subtle, but clinically significant, alterations of cornea1 contour. This article compares keratometry, keratoscopy, and computer-assisted topographic analysis and provides specific examples of the sensitivity of computer-assisted systems in revealing topographic alterations that were not previously discernable. Quantitative descriptors ofcorneal topography such as the surface asymmetry index, the surface regularity index, and simulated keratometry value augment the information provided by color-coded topographic maps. (Sure Ophthalmol 35:269-277, 1991)
Key words.
keratometry
contact lens-induced cornea1 warpage cornea1 topography keratoscopy keratoconus l- radial keratotomy l
l
l
l
raphy were the keratometer and the keratoscope. Each instrument, however, provides only a limited amount of information about the contour of the anterior cornea1 surface. The keratometer (also called the ophthalmometer) was first described in 1854 by Helmholtz.” It provides illuminated object mires that are reflected from the surface of the cornea which acts as a convex mirror. The virtual image formed by the cornea is minified and upright and appears to be located within the anterior chamber of the eye. The radius of curvature of the anterior cornea1 surface is determined from four reflected points that are evaluated as two pairs, based on the assumption that the cornea1 surface is spherocylindrical.5”4 The distance between the two points of a pair is measured to determine the radius of curvature in that meridian. This distance varies from approximately 2.6 to 3.7 mm depending on the cornea1 curvature.5’J4 Most commercially available instruments convert the radius of curvature to dioptric power using the standard keratometric index of 0.3375. This value
The systematic evaluation of radial keratotomy, astigmatic keratotomy, epikeratophakia, penetrating keratoplasty, and other cornea1 manipulations has been constrained by limitations in our ability to determine accurately and reproducibly the shape of the anterior cornea1 surface. For example, several clinical studies of radial keratotomy have noted a poor correlation between refraction and keratometry data for individual patients.4*6*46 Discrepancies such as these hinder efforts to improve the predictability of radial keratotomy and other refractive Recently, the emergence of surgical procedures. computer-assisted topographic analysis has provided a sensitive tool for assessing normal and pathologic cornea1 contour and for monitoring changes that occur following surgery. This article will describe the development and application of this technology.
I. Keratometry and Keratoscopy Until the mid 198Os, the only modalities that were widely available for monitoring cornea1 topog269
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1991
Fig. 1. Mire image of the Nidek PKS 1000 photokeratoscope. Note that the mires do not cover a large area of the central and peripheral cornea.
Fig. 2. Mire image of the CMS collimated videokeratoscope. Information is provided from rings extending into the central cornea and into the peripheral cornea essentially to the limbus.
represents
image which appears to be located within the anterior chamber of the eye. Information regarding the power of the anterior cornea1 surface is derived from visual inspection of the size of the mire; flatter corneas produce an image with mires of relatively larger diameter than those produced by a steeper surface. Information regarding the radius of curvature of a localized area of the cornea can be obtained by observing the separation between the mires reflected from that area of the cornea. Irregularity of the surface is displayed as distortion of the mires. The major advantage of photokeratoscopes, such as the Nidek PKS 1000 or the Kera Corneoscope, over the keratometer is that they provide
a combined
anterior
and posterior
tometer
determines
steepest
meridian
refractive cornea1
the power
for
The
and location
and the power
90” away. The instrument
index
surfaces.
the
keraof the
of the meridian
has the capacity
to meas-
ure a regular surface with an accuracy ofbetter than 0.25 diopters. The major limitations of the keratometer are that it assumes that the cornea is a spherocylindrical surface, it provides no information regarding the topography central or peripheral to the points of measurement, and mild cornea1 surface irregularity causes mire distortion that precludes meaningful measurement. Mande113’-33 was the first to demonstrate that the normal cornea is aspheric and flattened from the center to the periphery. The assumption of central spherocylindricity is a useful approximation for most normal corneas; this makes the keratometer particularly well suited for tasks such as fitting contact lenses on normal corneas. Diseased and postsurgical corneas, however, rarely approximate a spherocylindrical surface and keratometer measurements that are performed in these cases are, at best, of limited value. Any particular set of keratometer values could be associated with an unlimited number of cornea1 shapes. This follows from the wide variations that may occur in the topography central and peripheral to the keratometer measurement points. This limitation of the keratometer is likely a major determinant of the poor correlation that has been noted between keratometer and refraction measurements in clinical studies of refractive surgical procedures.‘,‘,4v6,44,46 Both Placido22 and Gullstrand” have been credited with the development of the first photokeratoscope. The photokeratoscope projects a series of concentric circular mires that also form a virtual
topographic map of a patient with 3 diopters of with-the-rule astigmatism. The central area without information corresponds to the lack of mires in the center of the Nidek PKS 100 photokeratoscope image. There is also no information provided for the far peripheral cornea.
Fig. 3. LSUCTS color-coded
ANALYSIS OF CORNEAL TOPOGRAPHY information from a larger portion of the cornea1 surface.gg Photokeratoscopes that are commercially available, however, provide limited information regarding the central cornea because that area is not covered by the mires (Fig. 1). The collimated video-
271 keratoscope component of the Cornea1 Modeling System” (Fig. 2) provides information from a larger portion of the cornea1 surface extending to the visual axis, but it is presently not available as an independent instrument providing photokeratoscopy
Fig. 4. CMS absolute and normalized
scale color-coded topographic maps ofa normal cornea with 3 diopters of with-therule astigmatism characterized by the vertically oriented bow-tie pattern. The absolute color scale (left) has fixed dioptric intervals that are 1.5 diopters in the central power range and 5.0 diopters in the high and low power ranges. Each individual color always represents a specific power interval. In the normalized color scale (right), the power intervals represented by individual colors are smaller and vary depending on the power range of the cornea being analyzed.
Fig. 5. Videokeratoscope image (left) and CMS absolute scale color-coded topographic map (right) of the left eye of a patient with advanced keratoconus in the opposite eye, demonstrating that early changes noted with computer-assisted topographic analysis may not be detectable by visual inspection of the videokeratoscope mires.
Fig. 6. CMS absolute scale color-coded topographic maps of central (left) and peripheral (right) cones of keratoconus. The same pattern is always noted in the two eyes ofan individual patient, even when there is a large difference in the stage of progression between the two eyes.
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1991
WILSON,
KLYCE
Fig. 7. Left: CMS normalized
scale color-coded map of a cornea with rapid changes in power within the optical zone following radial keratotomy. This multifocal topography is characteristic of many corneas that have had radial keratotomy and accounts for the clinical finding of the maintenance of best spectacle-corrected visual acuity at a specified is uncertain in distance over a significant dioptric range during cycloplegic refraction. 35 Since the refractive endpoint such cases, multifocality is also a factor in the phenomenon ofbetter uncorrected visual acuity in patients following radial keratotomy compared with unoperated controls with similar refractive errors. Right: Videokeratoscope image that was of the mire pattern does not reveal the extent topographic map. Visual inspection analyzed to generate this color-coded
Fig. 8. Top left: Example of absolute scale color-coded topographic map of a patient with contact lens-induced cornea1 warpage caused by a rigid polymethylmethacrylate lens with a superior resting position during inter-blink intervals. Note that there is relative flattening of the superior cornea1 contour underlying the decentered resting position. The initial topographic pattern is similar to that noted in some patients with keratoconus (Fig. 5). There was no evidence of Top right: One week after stopping contact lens wear. Despite considerkeratoconus, however, on slit lamp examination. able change, the topographic pattern remains abnormal with a high degree of asymmetry. Bottom left: Videokeratoscope image analyzed one week after stopping contact lens wear. Note that visual inspection ofthe mire pattern does not reveal the extent of residual topographic abnormalities. Bottom right: Six weeks after stopping contact lens wear, the topography has returned to a normal, symmetrical, with-the-rule astigmatism.
ANALYSIS OF CORNEAL TOPOGRAPHY alone without computerized analysis. Photokeratoscopes are for the most part qualitative; studies have shown that clinically significant alterations of cornea1 contour are commonly not detectable with these instruments. p5*48,50Photokeratoscopes, however, continue to be useful and inexpensive instruments for the evaluation of cornea1 topographic alterations that are not subtle. For example, recent publications have used the photokeratoscope to monitor changes in cornea1 topography occurring following radial keratotomy,40 after adjustment of a single running suture following penetrating keratoplasty,“’ and as a guide to selective suture removal to decrease astigmatism following penetrating keratoplasty.‘5 However, it is generally accepted that clinically significant amounts of cornea1 cylinder (up to 3D) can be present in a cornea and can escape detection with traditional keratoscopy.
II. Computer-assisted Topographic Analysis A. TOPOGRAPHIC
ANALYSIS SYSTEMS
Rowsey and coworkers developed some of the earliest methods to provide quantitative information from photokeratoscope images by comparing the mire diameters from corneas to those reflected from standard reference spheres4’ and by measuring hemichord lengths from the center of the mire pattern to each keratoscope ring along several hemimeridians.g~4’ These methods have not, however, found widespread acceptance, primarily because of inaccuracy and difficulty in correlating the quantitative information provided by numerical plots of the power at various points on the photokeratoscope mires with clinical patterns of cornea1 topography. These methods were, however, instrumental in the formulation of some of the earliest principles of keratorefractive surgery by Rowsey.“g Computer-assisted analysis of photokeratoscope images was enhanced by Klyce in 1984.” Interpretation of shape anomalies and clinical utility were augmented with the introduction of color-coded topographic maps JOthat allow the clinician to discern subtle variations in power distribution on the anterior cornea1 surface and to recognize specific patterns of pathologic cornea1 topography. A brief description of instruments that provide computer-assisted topographic analysis is provided below and the current commercial instruments known to the authors are listed in Table 1. The details of the computerized algorithms that allow an accurate three dimensional reconstruction of the cornea1 surface from photokeratoscope data obtained by these devices are beyond the scope of this article and are covered elsewhere where avail-
273 TABLE 1 Clinical Automatic Keratoscope Cornea1 Topoprafihy Analyzers
Company Computed Anatomy EyeSys Visioptic
able.
Device Topographic Modeling System Cornea1 Analysis System Computerized Cornea1 Topographer
cost $24,950 -$20,000 $29,950
18.19.45.53
The LSU Corneal Topography System (LSIJCTS) analyzes images produced by the 1 l-ring Nidek PKS 1000 photokeratoscope (Fig. l).53 Manual digitization is used to capture data for computerized analysis. The HIPAD (Houston Instruments, Austin, TX) manual digitizing equipment used with this system is capable of 2000 lines per frame resolution, which translates to a resolution of 0.3 diopters on the cornea1 surface. The major limitations of the LSUCTS are that manual digitization is tedious and no information is obtained from the central and far peripheral cornea because these areas are not covered by the mires of the PKS 1000 photokeratoscope (Fig. 1).30 Topographic information is provided in the form of color-coded topographic maps (Fig. 3) that facilitate clinical and research interpretation of cornea1 topographic alterations. At present, the most widely used instrument is the Cornea1 Modeling System (CMS) manufactured by Computed Anatomy, Inc (New York, NY).‘3.53 The CMS uses a 32-ring collimated videokeratoscope that provides a central fixation point as a reproducible reference for the computerized statistical analysis of the power points on the videokeratoscope rings. On each videokeratoscope ring, 256 points are evaluated; therefore, the analysis includes thousands of individual power points and covers nearly the entire cornea1 surface from the visual axis to the far periphery (Fig. 2). The instrument features operator-monitored automated digitization with approximately 500 lines per frame resolution. Statistical procedures are utilized by the computerized algorithms to provide resolution on the cornea1 surface of less than 0.25 diopters. Topographic information is provided in the form of several different color-coded topographic maps (Fig. 4).‘” The CMS system is unable to analyze severely distorted keratoscope mires, but the diagnosis in such cases, however, is usually obvious. The accuracy and precision of this instrument in measuring the curvature of calibrated spherical surfaces has
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been validated.14 Precision and accuracy have yet to be published for radially asymmetric aspheric reference standards that simulate complex cornea1 shapes. A recent study confirmed that the original LSUCTS algorithm was accurate in modeling the central cornea, but was less accurate in the periphery.45 This study also reported new algorithms for improving the accuracy of the LSUCTS, but it is not certain whether this work will have an impact on the accuracy of the CMS, since that device uses independently developed reconstruction algorithms. CMS technology is now available in an instrument called the Topographic Modeling System (TMS-1). The Computerized Cornea1 Topographer EH270 (Visioptic Inc., Houston, TX), designed by El Hage,” duplicates the provision of cornea1 contour displays based on computer-assisted analysis of photokeratoscope images. This instrument provides several video displays that include a cornea1 contour map, a meridional contour map, and an astigmatism map. ” The accuracy and precision of this instrument, however, have not yet been validated by independent study. Additionally, there have been no studies reported in which this system was used to monitor alterations of cornea1 topography. Another videokeratoscope-based instrument currently available is the Cornea1 Analysis System from EyeSys Laboratories (Houston, TX).*’ This is a 16 ring videokeratoscope-based device with fast image processing time (3 seconds) and color-coded contour map plots in addition to a host of other data presentation schemes. Some data regarding the accuracy and precision of this instrument are available in the literature;2’ however, independent validation is not yet available. Additional instruments that use laser interferometry or rasterstereography’* for reconstructing the cornea1 surface have been reported. These instruments, however, are not commercially available and, again, their accuracy and precision have not been validated. B. PARAMETRIC DESCRIPTORS OF COBNEAL TOPOGRAPHY While the qualitative information obtained from color-coded maps and other visual presentation schemes has been shown to provide a unique understanding of cornea1 topographic alterations, the clinical utility and the application of computer-assisted topographic analysis to research would be augmented by the availability of quantitative descriptors of cornea1 topography. The Simulated Keratometry value (Sim K), Surface Asymmetry Index (SAI), and the Surface Regularity Index (SRI) are discussed below. These parametric descriptors are provided during topographic analysis with the
WILSON,
KLYCE
CMS instrument and could be incorporated into other topographic analysis systems. Additional descriptors are under development. The Sim K value5’ provides the power and location of the steepest and flattest meridian from a reconstructed cornea1 surface analogous to values provided by the keratometer. This descriptor allows the clinician to correlate the topography seen on the color-coded map with another common parameter and obviates the need to obtain separate keratometer measurements. The value is obtained from all power points on photokeratoscope rings 7, 8, and 9. These mires were selected because their position on the cornea1 surface approximates the location at which standard keratometry measurements are obtained.5 Both spherocylindrical and nonspherocylindrical values are provided. The spherocylindrical Sim K value provides the power and the location of the steepest meridian and the meridian 90” away. The nonspherocylindrical Sim K value also provides the power and location of the actual flattest meridian regardless of the angle between the steepest and flattest meridian. Pathologic and postsurgical corneas commonly have shapes that do not approximate a spherocylindrical surface and, therefore, the steepest and flattest meridians are often separated by less than 90”. Prospective studies have demonstrated a high correlation between keratometry and spherocylindrical Sim K values.21~“0 The SAI is a centrally weighted summation of differences in cornea1 power between corresponding points 180” apart on 128 equally spaced meridians that cross the four central photokeratoscope mires.8~‘8~2’~53 For example, if the power on ring 4 at 5” is 44.0 and the power on ring 4 at 185” is 46.0, the difference, 2.0, is entered into the centrally weighted summation. Similar differences are entered for all corresponding pairs of points on the four central mires. SAI approaches zero for a perfectly radially symmetrical surface and increases as the contour becomes more asymmetric. Since the normal cornea usually has a high degree of central radial symmetry,’ the SAI is a useful quantitative parameter for monitoring changes that occur in patients with contact lens-induced cornea1 warpage,50*5’ following penetrating keratoplasty,23 and in other cornea1 disorders that cause an alteration of cornea1 symmetry (e.g., off-center keratoconus apices). The SRI is determined from a summation of local fluctuations in power along 256 equally spaced hemimeridians on the 10 central mires4* This index approaches zero for a normally smooth cornea1 surface and increases directly with increasing irregular astigmatism. In a prospective clinical study,48 there was a high correlation between the SRI and best
ANALYSIS OF CORNEAL
TOPOGRAPHY
spectacle-corrected visual acuity (r = 0.80, p < 0.001). Therefore, the SRI value can be used to predict the optical performance that might be expected in a particular patient based on the cornea1 topography, if other components of the visual system such as the lens and the macula are functioning normally. This index would also be of value for predicting visual acuity following experimental refractive surgical procedures in animals.
275
ate the cornea1 curvature from only four points that are separated by approximately 3 mm, they would not detect the multifocal lens effect. Therefore, there can be a poor correlation between the change in keratometer measurements and the change in refraction following radial keratotomy. Visual inspection of photokeratoscope images is not sensitive enough to detect the multifocal topography in the majority of cases (Fig 7). A recent study by McDonnell et al”” reported a correlation between cornea1 topography and flucIII. Clinical and Research Applications tuation in visual acuity following radial keratotomy. of Computer-assisted Topographic Ninety-one percent of eyes with fluctuating vision Analysis had a topography characterized by the presence of Computer-assisted topographic analysis has a split or dumbbell-shaped optical zone. Eyes withbeen used to study normal cornea1 topography’ out fluctuation of visual acuity had either a round and to demonstrate the effects of disease,25~‘8~98~4g~“2 optical zone (80%) or a band-like zone (20%). Studsurgical procedures,26~‘7~2q~‘5~47post surgical maies such as this should provide valuable insights into nipulations,23*24 and contact lenses on cornea1 tocorrelations between cornea1 topography and visupography.“‘.” The examples below illustrate the al function following refractive surgical procedures. utility of the technique for providing information Computer-assisted topographic analysis has also that was not available prior to the development of provided new insights into the changes in cornea1 this method. topography induced by contact lenses.5”B5’ These Several studies have shown that in early keratostudies have demonstrated that there is frequently a conus topographic changes that are noted with correlation between the resting position of a rigid contact lens on the cornea1 surface and the topocomputer-assisted topographic analysis may not be graphic pattern in contact lens-induced cornea1 detected using the keratometer or the keratoscope (Fig. j!j),P5.9xs49 warpage (Fig. 8). Additionally, the time required in Early detection of the disorder allows the clinician to better advise and treat the patient some eyes for the topographic abnormalities to resolve may be far longer (6 or more months) than and may allow the etiology to be better defined by was previously appreciated. confirming the presence of subtle changes in family Color-coded topographic maps and recently demembers during molecular genetic investigations.“8 It has also been possible to demonstrate that the veloped quantitative descriptors of cornea1 topography have made it possible to better document topographic patterns of the two corneas of a patient changes in cornea1 contour that occur during corwith keratoconus show a high degree of mirror imneal surgical procedures. Examples of the usefulage symmetry, despite the fact that large differences ness of computer-assisted topographic analysis are in apex power are usually noted.4g Topographic provided by recent studies on the effect of running patterns in keratoconus patients can be divided into suture removal following penetrating keratoplasty’” two groups: those with central cones and those with and postsurgical manipulation of a single running 1Operipheral cones (Fig. 6).4g The same pattern is al0 nylon suture to reduce astigmatism following peneways noted in the two eyes of an individual patient, trating keratoplasty.‘” although the extent of progression of keratoconus These examples illustrate the usefulness of comis usually different. puter-assisted topographic analysis. Future studies Previous studies on the effect of radial keratotwill likely provide additional insights into the effects omy on cornea1 topography have noted a lack of of disease and surgery on cornea1 contour that correlation between changes in keratometer meawould not have been noted without advances in surements, refractions, and uncorrected visual acureconstruction of the anterior cornea1 surface. 4~6.4”.46 Computer-assisted topographic analysis ity. has demonstrated that one factor contributing to these findings is a multifocal central cornea1 conIV. The Ideal Cornea1 Topographic tour that frequently is present following radial kerAnalysis Instrument atotomy (Fig. 7).27.‘15,47The progressive change in Computer-assisted topographic analysis is useful power within the optical zone in such cases allows clinically and for research. Clinicians must decide, the patient to maintain focus during a cycloplegic based on the requirements of their practice, whethrefraction over an interval that can exceed 10 er they need the advanced analysis capabilities prodiopters. Since keratometer measurements evalu-
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35 (4) January-February
vided by modern topographic analysis systems or if a keratometer or a keratoscope will be satisfactory. At present, there are three computer-assisted topographic analysis instruments commercially available (Table 1) and it is pointed out that not all of these may be equal in either utility or functionality. It is possible that other computerized analysis systems will become available. Current and future instruments should be evaluated on the basis of tttility, accuracy, precision, and affordability.*’ The clinician should have a working understanding of the operation and data interpretation of the device under consideration prior to committing to purchase. A real-time, hands-on evaluation of the instrument will help the potential buyer to determine whether the technology will be practical for operation by staff in the clinic. Currently, the only unbiased method to verify the accuracy and precision of an instrument under consideration is through the availability of independent studies published in peer-reviewed journals. Only a limited number of reports have been published for commercially available instruments,14 although other studies will likely become available in the future. If inexpensive aspheric reference surfaces were available, these would allow the individual clinician or investigator to directly analyze an instrument under consideration for purchase. It is hoped that surfaces such as these will become generally available. The cost of computer-assisted topographic analysis instrumentation has decreased sharply in the past few years from well over $100,000 for a laser scanning device developed in Europe to amounts shown in Table 1. Obviously, the price for any instrument can only be evaluated after carefully considering utility, accuracy, precision, and the needs and resources of the individual practice.
V. Future Developments in Cornea1 Topographic Analysis In the future, systems will likely be developed that will allow affordable, high-resolution, realtime, quantitative monitoring of cornea1 topography during surgical manipulations as an adjunct to surgical keratometers.‘* This will undoubtedly improve the predictability of all keratorefractive techniques. For example, one may envision a system in which a photoablative laser is combined with a realtime topographic analysis system. With such a system it might be possible for the surgeon to specify the desired cornea1 topography and for parameters to be monitored automatically by the instrument so that the ablation is performed accordingly. Helmholtz, Placido, and Gullstrand would certainly have been impressed!
WILSON, KLYCE
1991
References 1. Arffa RC, Klyce SD, Busin M: Keratometry and epikeratophakia. J Refract Surg 2:61-64, 1986 2. Arffa RC, Marvelli T, Morgan K: Keratometric and refractive results of pediatric epikeratophakia. Arch Ophthalmol 103:1656-1659, 1985 3. Arffa RC, Warnicki JW, Rehkopf PG: Cornea1 topography usingrasterstereography. Refractive Cornea1 Surg5:414-417, 1989 4. Arrowsmith PN, Marks RG: Visual, refractive, and keratometric results of radial keratotomy. Arch Ophthalmol 105: 76-80, 1987 5. Dabezies OH, Holladay JT: Measurement of cornea1 curvature: Keratometer (ophthalmometer), in Dabezies OH, Jr, Cavanagh HD, Farris RL, Lemp MA (eds): Contact Lenses: The CLAO Guide to Basic Science and Clinical Practice. Orlando, Grune and Stratton, 1986, pp I-17, 29 6. Deitz MR. Sanders DR, Marks RG: Radial keratotomy: An overview of the Kansas City study. Ophthalmology 91:467-78, 1984 7. Dingeldein SA, Klyce SD: The topography of normal corneas. Arch Ophthalmol 107:512-518, 1989 8. Dingeldein SA, Klyce SD, Wilson SE: Quantitative descriptors of cornea1 shape from computer-assisted analysis of photokeratographs. Refractive Cornea1 Surg5:372-378, 1989 9. Doss JD, Hutson RL, Rowsey JJ, Brown R: Method for calculation of cornea1 profile and power distribution. Arch Ophthalmol 99:1261-1265, 1981 10. Duke-Elder S: System of Ophthamology, Vol V. St Louis, CV Mosby, 1970, pp 127 11. El Hage SC: Acomputerized cornea1 topographer for use in refractive surgery. Refractive Cornea1 Sure 5:4 181124. 1989 ofsurgi12. Frantz JM, Re\dyJJ, McDonald MB: Actmparison cal keratometers. Refractive Cornea1 Surg 5:409-4 13, 1989 13. Gormley DJ, Gersten M, Koplin RS, Lubkin V: Cornea1 modeling. Cornea 7:30-35, 1988 14. Hannush SB, Crawford SL, Waring GO, et al: Accuracy and precision of keratometry, photokeratoscopy, and cornea1 modeling on calibrated steel balls. Arch Ophthalmol 107:123{-1239, 1989 15. Harris DJ, Waring GO, Burk LL: Keratography as a guide to selective suture removal for the reduction of astigmatism after penetrating keratoplasty. Ophthalmology 96:15971607, 1989 16. Helmholtz H von: Handbuch derphysiologicschen Optik. Hamburg, Germany, Leopold Voss, 1909 17. Klyce SD: Computer-assisted cornea1 topography. Highresolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci 25:1426-1435, 1984 18. Klyce SD, Wilson SE: Methods of analysis of cornea1 topography. Refracttve Cornea1 Surg 5:368-37 1, 1989 19. Klyce SD, Wilson SE: Imaging, reconstruction, and display of cornea1 topography. Proceedings of the SPIE 33rd Annual International Technical Symposium of Optical and Optoelectronic Applied Science and Engineering, New Methods in Microscopy and Low Light Imaging. SPIE 1161:409-416, 1989 20. Klyce SD, Wilson SE, Kaufman HE: Cornea1 topography comes of age. Refractive Cornea1 Surg 5:359-361, 1989 21. Koch DD, Foulks GN, Moran CT, Wakil JS: The cornea1 EyeSys system: Accuracy analysis and reproducibility of first-generation prototype. Refractive Cornea1 Surg 5: 424429, 1989 22. Levene JR: The true inventors of the keratoscope and photokeratoscope. BrJ Hist Sci 2:324, 1965 23. Lin DTC, Wilson SE, Klyce SD, lnsler MS: Topographic changes that occur with 10-O running suture removal following penetrating keratoplasty. Refractive Cornea1 Surg 6.21-25, 1990 24. Lin DTC, Wilson SE, Reidy JJ, et al: An adjustable running suture technique to reduce postkeratoplasty astigmatism A preliminary report. Ophthalmology 97:934-938, 1990 25. Maguire LJ, Bourne WM: Cornea1 topography of early keratoconus. Am J Ophthalmol 108:107-l 12, 1989 26. Maguire LJ, Bourne WM: Cornea1 topography oftransverse
ANALYSIS OF CORNEAL
27.
28.
29.
30.
31
32.
33.
34. 35.
36.
37.
38.
39. 40.
41.
4”.
TOPOGRAPHY
keratotomies for astigmatism after penetrating keratoplasties. Am J Ophthalmol 107:323-330, 1989 Maguire LJ, Bourne WM: A multifocal lens effect as a complication of radial keratotomy. Refractive Cornea1 Surg 5:394-399, 1989 Maguire LJ, Klyce SD, McDonald MB, Kaufman HE: Corneal topography of pellucid marginal degeneration. Ophthnlmolog?l 94:5 19-524, 1987 Maguire LJ, Klyce SD, Sawelson H, et al: Visual distortion after myopic keratomileusis: Computer analysis of keratoscope photographs. Ophthalmic Surg 18:352-356, 1987 Maguire LJ, Singer DE, Klyce SD: Graphic presentation of computer-analyzed keratoscope photographs. Arch Ophthalmol 105:223-230, 1987 Malldell RB: Methods to measure the peripheral cornea1 curvature. Part one: photokeratoscopy. J Am Oplom&c As.wr 33:137-139, 1961 Mandell RB: Methods to measure the peripheral cornea1 curvature. Part two: geometric construction and “romputers.” J Am Optomrlrrr Assoc 33:585-589, 1962 Mandell RB: Methods to measure the peripheral cornea1 curvature. Part three: ophthalmometry. J Am Optomrlric As.W( 33:xtw892. I962 Mandell RB: Corr/arl Lens Pmclzcr. Springfield, Charles C Thomas, 198X. ed 4, pp 928-935 McDonnell PJ. Garbus J, Lopez PF: Topographic analysis and visual acuity after radial keratotomy. Am J Ophthalmol /Oh:692-695. 198X McDonnell PJ, McClusky DJ. Garbus JJ: Cornea1 topography and lluctuating visual acuity after radial keratotomy. Ophlhalmolog?’ Y&665-670. 1989 MrNeill JI. Wessels IF: Adjustment ofsingle continuous suture to control astigmatism after penetrating keratoplasty. R$aclwr Cornea1 Surg 5:2 16-223, 1989 Rabinowitz YS, McDonnell PJ: Computer-assisted cornea1 in keratoconus. Rtfracli~w Cornea1 Slrrg topography 5;400-40X. 19x9 ROMsey JJ: Ten caveats in keratorefractive surgery. Ophthalmn/o,q 90: 14X-155, 1983 RowseT I.J J, Balyeat HD, Monlux, et al: Prospective evaluation of radial keratotomy. Photokeratoscope cornea1 topography. Ophthalmology 95:322-334, 1988 Rowsey JJ, Monlux R, Balyeat HD, et al: Accuracy and reproducibility of kerascanner analysis in PERK cornea1 topography. C:llrt- E~P Rrs 8:661-674. 1989 Rowsrv J.1, Reynolds AE, Brown R: Cornea1 topography.
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44 45
46
47
48. 49. 50.
.5 I
52.
53.
Arch Ophthalmol 99: 1093-I 100, 198 1 Santos VR, Waring GO, Lynn MJ, et al: Relationship between refractive error and visual acuity in the Prospective Evaluation of Radial Keratotomy (PERK) study. Arch Oph/hnlmo/ 105:86-92, 1987 Swinger CA, Barker BA: Prospective evaluation of myopic keratomileusis. Obhthalmolopv 9Zt785-792, 1984 Wang J, Rice DA,‘Klyce SD:“b new reconstruction algorithm for improvement of cornea1 topographic analysis. Refracfivr Cornea1 Surg 5:379-387, 1989 Waring GO, Lynn MJ, Culbertson W, et al: Three-year results of the prospective evaluation of radial keratotomy (PERK) study. Ophthalmolog? 91: 1339- 1354, 1987 Wilson SE, Klyce SD: Topographic analysis and visual acuity after radial keratotomy (correspondence). ,4m ,/ Oph/halmol 107:436-437, 1989 Wilson SE, Klyce SD: Quantitative descriptors 01 cornea1 topography: A clinical study. ,4rch Ophthalmol (in press) Wilson SE, Lin DTC, Klyce SD: The cornea1 topography of keratoconus. Cornea (in press) Wilson SE, Lin DTC, Klyce SD, et al: .I‘opographic changes in contact lens-induced cornea1 warpage. Ophthalmology 9?:734-744, 1990 Wilson SE, Lin DTC, Klyce SD, et al: Rigid contact lens decentration: A risk factor for corncal warpage. CL.40,/ 16: 377-382, 1990 Wilson SE. Lin DTC, Klyce SD, Inslel- MS: The cornea1 topography of Terrien’s marginal degeneration. K@ZC~II~C Cornea1 Surg 6: 15-20, 1990 Wilson SE, Wang JY, Klyce SD: Quantitative and mathematical analysis of photokeratoscopic images, in Schanzlin DJ, Robin J (eds): Corral Topography. Neh York. Springer-Vcrlag (in press)
Neither Dr. Wilson nor his family members have any c’ommercial or proprietary interest in any of the products mentioned in this review. Dr. Klyce is a paid consultant to Compuletl Anatomy, Inc. (New York, NY). Supported in part by US Public Health Service grants EY03311 and EY02377 from the National Eye Institute, National Institutes of Health, Bethesda, Maryland and the Louisiana Lions Eye Foundation. Computed Anatomy, Inc. generously supplied the Cornea1 Modeling System used in this study. Reprint address: Stephen E. Wilson, M.D.. Dept. of Ophthalmology, University ofTexas Southwestern Medical (:enrer, 5323 Harry Hines Blvd., Dallas, TX 7523.5.
