Corneal Topographic Changes after Radial Keratotomy

Corneal Topographic Changes after Radial Keratotomy

Corneal Topographic Changes after Radial Keratotomy PETER J. McDONNELL, MD, JENNY GARBUS, COT Abstract: A high resolution photokeratoscope using digi...

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Corneal Topographic Changes after Radial Keratotomy PETER J. McDONNELL, MD, JENNY GARBUS, COT

Abstract: A high resolution photokeratoscope using digital video image acquisition and computer graphics was used to describe anterior corneal topography before and after radial keratotomy. After radial keratotomy, the entire cornea is flattened, but this effect is more pronounced centrally. The flat central "optical zone" varies in size, shape, and location. Regional variations in corneal power may explain phenomena observed after radial keratotomy, such as visual acuity that is better than would be expected on the basis of refractive error, and the occurrence of ghost images. Ophthalmology 96:45-49, 1989

Quantitative analysis of the results of radial keratotomy have documented the lack of direct correlation between the changes in visual acuity, cycloplegic refraction, and keratometric measurement. At the 3-year time point after surgery in the Prospective Evaluation of Radial Keratotomy (PERK) Study, 1 the change in spherical equivalent among patients with moderate myopia averaged 3.66 diopters (D) versus a reduction of only 3.14 D in keratometric power. In the highly myopic group, the reduction in spherical equivalent averaged 4.69 D, compared with an average reduction in keratometric power of only 3. 70 D. Often, radial keratotomy patients with a residual myopic refractive error may have better visual acuity than do individuals with a similar refractive error who have not had surgery. Santos and co-workers2 documented this disparity in uncorrected visual acuity between unoperated eyes and eyes after radial keratotomy, and suggested that the surgery increased corneal asphericity, in analogy with bifocal contact lenses. Assessment of the corneal topographic changes induced as a result of radial keratotomy has been somewhat limited. The keratometer measures only the average radius of curvature of the cornea between two points approximately 3 mm apart, not the curvature of the entire cornea, and it cannot be assumed that all areas within this 3-mm

Originally received: June 24, 1988. Revision accepted: September 26, 1988. From the Doheny Eye Institute and the University of Southern California School of Medicine, Los Angeles. The authors have 010 proprietary interest in the Corneal Modeling System or in the manufacturer, Computed Anatomy, Inc. Reprint requests to Peter J. McDonnell, MD, Doheny Eye Institute, 1355 San Pablo St, Los Angeles, CA 90033.

diameter area will have a refractive power similar to the overall "K reading." 3 The photokeratoscope, as recently popularized by Rowsey and colleagues, 4 examines both the paracentral and midperipheral cornea, and thus provides a greater amount of information. A recently developed system using a 32-ring photokeratoscope combined with computerized curve fitting and generation of colorcoded corneal topographic maps may allow examination of more subtle differences in the topography of the anterior corneal topography. 5 This instrument has been demonstrated to provide measurements of the curvature of calibrated steel balls as accurate as those obtained with a keratometer. 6 Herein, we describe the use of this instrument in the examination of anterior corneal topography after radial keratotomy.

PATIENTS AND METHODS We examined 26 eyes of 20 patients who were at least 18 months postradial keratotomy, using the Corneal Modeling System (Computerized Anatomy, Inc, New York, NY), a computerized corneal topographic analysis system. 5 We also examined ten eyes before and after radial keratotomy to assess the early changes that occur as a result of surgery. All patients had undergone eight-incision radial keratotomies performed with diamond knives. Blade depth was set equal to 100% of the thinnest of the four paracentral pachymetric readings. There were no intra- or postoperative complications in any of the patients. Postoperative refractions were performed without administration of mydriatic or cycloplegic agents. All preand postoperative refractions were performed by the same examiner in the same examination room using the same visual acuity charts. This device uses a photokeratoscope that projects 32-ring images onto the cornea, spanning 45

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Fig 1. Photograph of rings projected over corneal surface from center to limbus of normal unoperated cornea.

most of the corneal surface from the center to the limbus (Fig 1). The rings are then projected using a digital video system; ring images are digitized and curves fit to describe the shape of the entire corneal surface. This information is then translated into a color-coded topographic map to aid individualization of regional changes in corneal curvature. A cursor, directed by the operator, may be moved over the map to determine the approximate corneal power at any point.

RESULTS All ten eyes examined preoperatively showed a gradual flattening from the central to the peripheral cornea, illustrating corneal asphericity (Fig 2). After surgery, the entire cornea flattens, but with a disproportionate amount of flattening occurring centrally, such that the corneal curvature is steeper peripherally than centrally (Fig 3). All eyes after radial keratotomy had zones of central corneal flattening, typically centered around the patient's point affixation. We observed three basic configurations to these optical zones: round or oval (Fig 4), band-like (Fig 5), and dumbbell or split optical zone (Fig 6). Of the 26 eyes examined at least 18 months after radial keratotomy, 13 (50%) had round optical zones, that averaged 6.2 mm in diameter. Patients with band-like optical zones showed large areas of flattening that extended across almost the entire interpalpebral zone, from limbus to limbus. This type of optical zone was identified in 3 ( 12%) ofthe 26 eyes. Split or "dumbbell" optical zones are basically areas of flattening that were rotationally quite asymmetric. Typically, these showed two areas of flattening, 180° apart and separated by relatively steeper areas. Although the keratometry readings were variable in these corneas, eyes with dumbbell and band optical zones tended to have greater degrees of keratometric astigmatism than did corneas with long, round optical zones. 46



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Patients with large round or bandlike optical zones were unlikely to complain of ghost images, fluctuating visual acuity, or visual blur, unlike patients with dumbbell or split optical zones, who almost always reported monocular diplopia and fluctuating visual acuity. In three patients with round optical zones, we noted smaller flat optical zones within the larger roughly circular optical zone. Typically, these small, flatter optical zones were located inferior to the patient's point of fixation. In these three eyes, this smaller and flatter optical zone appeared to allow both excellent uncorrected far vision and near visual acuity. An illustrative example of this is that of a 42-year-old man who was examined 2 years after radial keratotomy. Preoperative spherical equivalents were -7.25 D in the right eye and -9.00 Din the left. Preoperative average keratometry was 42.12 D in the right eye and 42.50 D in the left. Postoperatively, uncorrected acuity in the right eye was 20/25 at distance and Jaeger 1 at near. The patient could be corrected to 20/20 with -2.25 to -0.75 X 94 D, for a spherical equivalent of -2.63 D. The 20/25 uncorrected visual acuity in the right eye, combined with an unaided near vision of Jaeger 1, was surprising given the spherical equivalent in this eye of -2.63 D. Computed topographic analysis of the right cornea (Fig 7), however, suggested an explanation for this apparent inconsistency. Inferior to fixation was an area of local corneal flattening within a larger circular optical zone. This smaller, flatter optical zone measured 2.3 mm vertically by approximately 3.2 mm horizontally. In this area, the average corneal power was 34.8 D (Fig 7), representing a decrease of about 7 Din an eye with an average K reading of 42.12 D preoperatively. This was very close to the preoperative refractive error of -7.25 D, indicating that the patient was effectively plano through this inferior optical zone. The smaller optical zone was encompassed within a larger, overall roughly spherical, optical zone with a diameter of approximately 6 mm (Fig 7). This larger optical zone was steeper, with an average corneal power of approximately 37 Din the center and surrounding the center of the visual axis. This corresponded to a decrease in corneal curvature of approximately 5 D from the preoperative value, and would appear to explain the patient's acceptance of a spherical equivalent of -2.63 D at the time of cycloplegic refraction. In fact, when retinoscopy was performed after this topographic analysis, there appeared to be two possible end points for retinoscopy, one of approximately 2.50 D just above fixation, and a separate zone with an end point of approximately plano just inferior to fixation. Topographic analysis ofthe left eye of the same patient was strikingly different. The left cornea showed a large, roughly circular, flattened central optical zone with a radius of approximately 3 mm (Fig 4). This optical zone was fairly uniform in power with an average curvature of approximately 35.5 D, in contrast to the right cornea. This average 35.5 D represented a 7-D decrease from the average preoperative keratometry reading of 42.5 D, in rough agreement with the postoperative spherical equivalent of -1.38 D, which was a 7.62-D decrease from the preoperative spherical equivalent of -9.00 D. In this eye,

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Fig 2.Top left, color-coded topographic map of unoperated cornea. The cornea flattens peripherally; central curvature is 46.3 D. Fig 3. Center left, same cornea as in Figure 2, 3 weeks after eight-incision radial keratotomy with 3.5 mm optical zone. Central cornea is disproportionately flattened with increase in corneal curvature peripherally; central curvature is reduced to 42.2 D. Fig 4. Bottom left, cornea 3 years after eight-incision radial keratotomy with round central optical zone measuring 6.0 mm in diameter. Fig S. Top right, band-like zone of flattening in interpalpebral region 2 years after eight-incision radial keratotomy. Fig 6. Center right, two separate zones of corneal flattening connected by narrow central isthmus. Fig 7. Bottom right, map of cornea 3 years after eight-incision radial keratotomy (fellow cornea is shown in Fig 4). Small optical zone measures 2.3 X 3.2 mm and has average power of approximately 34.8 diopters. Larger roughly spherical optical zone (diameter approximately 6 mm) encompasses smaller zone and contains two small flatter islands superiorly; corneal power within this zone is 37.2 D.

the patient had 20/25 uncorrected distance visual acuity, but a near visual acuity of only Jaeger 2. With the right eye, the patient reported a faint ghost image for letters

both at distance and at near. Although he correctly read all letters on the 20/25line at distance, he first noted some blurring and ghost imaging with letters on the 20/50 line

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with the right eye. In contrast, with the left eye, with its single, larger, more homogeneous central optical zone the patient detected no ghost image at either distance or near, and he read to the 20/25 line without noticing blurring or ghost images. The occurrence ofghost images and visual blurring beginning with letters several lines larger than the smallest line correctly read by the patient also was observed in the two other patients with the smaller, flatter optical zones within the larger, round optical zone.

DISCUSSION Using the Corneal Modeling System, we have confirmed the previous observations of Rowsey and coauthors7 regarding the anterior corneal topographic features in unoperated corneas after radial keratotomy. By digitizing and thereby quantifying the photokeratoscope figures using the Kera Scan unit (KERA Corp, Santa Clara, CA), these authors demonstrated that all patients demonstrated an increasing radius of curvature, or flattening, from the center toward the periphery preoperatively. Postoperatively, corneal flattening was greatest at the center of the cornea and decreased at the periphery. Using the Corneal Modeling System, which h~s the advantage of providing a color-coded topographic map of the anterior corneal topography, we noted similar changes. Often, radial keratotomy patients With residual myopic refractive errors may have better visual acuity than individuals with a similar refractive error who have not had surgery. Similarly, changes in refractive error may not be mirrored by changes in keratometric values. Waring and co-workers 1•2 attributed the greater change in spherical equivalent in comparison to change in keratometry as being on the basis of induction of greater asphericity in the cornea. They argued that, because the keratometer measures only the average radius of curvature ofthe cornea between two points approximately 3 mm apart, not the curvature of the entire cornea, and because it cannot be assumed that all areas within this 3-mm diameter area will have a refractive power similar to the overall K reading, that keratometry did not provide a sufficient assessment of anterior corneal curvature. These authors suggested that patients after radial keratotomy may have visual acuity that is better than would be expected on the basis of their refractive error because of the increased spherical aberration of the paracentral cornea, which produced a larger blur circle on the retina. 2 Our explanation of the surprisingly good visual acuity of some patients after radial keratotomy is based on the technology that allows quantitative evaluation ofthe corneal curvature of not only the midperipheral but also the central and peripheral cornea. The Corneal Modeling System has 32 rings that span most of the anterior corneal surface, with small ring separations, so that very little of the corneal surface is not examined. As illustrated in the eyes that we examined after radial keratotomy, effective flat optical zones can be identified after radial keratotomy. The explanation for the variations in size and shape of these optical zones is not readily apparent. We have noted

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no obvious correlation between preoperative and postoperative topography in the same corneas (Figs 2, 3). Variability in depth of incision as well as inter-individual and intra-individual variability in wound healing may partly explain the differences. In most patients, however, fairly large (approximately 6 mm) round optical zones are produced as a result of radial keratotomy. As illustrated in one of three eyes of patients who underwent bilateral radial keratotomy, more than one optical zone might be available to the patient. The ability of a 42-year-old patient with a spherical equivalent of-2.63 D to have excellent uncorrected distance visual acuity (20/25) and near acuity (Jaeger 1) would appear to be explained by the presence of regional variation in corneal curvature within the central overall "optical zone" such that the patient had an effective "bifocal" cornea (Fig 7). In the fellow eye, in which there appears to be a greater concordance between the visual acuity, spherical equivalent, and keratometric readings, a large fairly homogeneous optical zone was noted to encircle the center of the visual axis. Is there a potential disadvantage to the creation of such a multifocal or bifocal cornea, which appears to leave some patients with unexpectedly good uncorrected visual acuity after radial keratotomy? Although the patient described in detail in this report who had the multifocal cornea did not spontaneously complain that the quality of vision in his right eye was less compared with that in his left eye, he did report, when questioned, that he had . a faint ghost image in the right eye (bifocal cornea) and that this ghost image was definitely not present when these letters were examined with the left eye ("unifocal cornea"). In essence, this result of surgery has set the stage for development of monocular diplopia. BinderB did report complaints of monocular double vision, ghost images, and degradation of the visual image as complications of refractive surgery. He thought that these occurred when radial keratotomy incisions were carried within the optical axis or as a result of decentration of the optical clear zone. He also noted these problems to occur as a result of decentration of the visual axis in keratomileusis or decentration of the lenticule in keratophakia. We believe that the ghost images observed in our three patients with "bifocal corneas" probably were a consequence of the separate effective optical zones. Although our patients were not particularly disturbed by this, it is conceivable that other patients might not be so fortunate. In some patients, the development of a "multifocal cornea" may, in fact, be a mixed blessing, and this might explain, in some patients, the decrease in best-corrected spectacle visual acuity noted in the PERK study. 1 Dingeldein et al9 performed computer-assisted analysis of photokeratographs and found that focal dispersion analysis with ray tracing may be an accurate predictor of best-corrected spectacle acuity. The "surface regularity index" described by these authors may be a measure of this degradation of the retinal image that may appear to occur with the "multifocal cornea" after radial keratotomy. Dumbbell or split optical zones, in our experience, are likely to be associated with diurnal fluctuation of visual acuity: patients with large central

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round optical zones are unlikely to experience this complication. 10 With the development of tools for examining corneal topography in more detail, further insights will undoubtedly be gained into the topographic alterations induced in the corneas as a result of refractive surgical procedures. In addition, it may be possible to intervene surgically in patients with undesired effective optical zones, such as a "split or dumbbell" optical zone or a "multifocal cornea," in order to produce a single, relatively large circular optical zone, with corresponding improvement in the quality of the retinal image.

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1. Waring GO Ill, Lynn MJ, Culbertson W, et al. Three-year results of the Prospective Evaluation of Radial Keratotomy (PERK) Study. Ophthalmology 1987; 94:1339-54. 2. Santos VR, Waring GO Ill, Lynn MJ, et al. Relationship between re-

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tractive error and visual acuity in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Arch Ophthalmol1987; 105:86-92. Mohrman R. The keratometer. In: Duane TD, Jaeger EA, eds. Clinical Ophthalmology. Philadelphia, Pa: Harper and Row, 1987; vol. 1, chap. 60. Rowsey JJ, Reynolds AE, Brown R. Corneal topography: Corneascope. Arch Ophthalmol1981; 99:1093-1100. Gormley DJ, Gersten M, Koplin RS, Lubkin V. Corneal modeling. Cornea 1988; 7:30-5. Hannush SB, Waring GO Ill. Computer assisted corneal topography: accuracy and reproducibility with three instruments. ARVO Abstracts. Invest Ophthalmol Vis Sci 1988; 29(Suppl):389. Rowsey JJ, Balyeat HD, Monlux R, et al. Prospective evaluation of radial keratotomy: photokeratoscope corneal topography. Ophthalmology 1988; 95:322-34. Binder PS. Optical problems following refractive surgery. Ophthalmology 1986; 93:739-45. Dingeldein SA, Pittman SD, Wang J, Klyce SD. Analysis of corneal topographic data. ARVO Abstracts. Invest Ophthalmol Vis Sci 1988; 29(Suppl):389. McClusky DJ, Garbus JJ, McDonnell PJ. Optical zone shape and diurnal fluctuation after radial keratotomy. Ophthalmology 1988; 95(Suppl): 162.

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