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Screening of Myopic Photorefractive Keratectomy in Eye Bank Eyes by Computerized Videokeratography
Rueben Lim-Bon-Siong, MD;
Joseph M. Williams, MD, PhD;
Sopit Samapunphong, MD;
Roy S. Chuck, MD, PhD;
Jay S. Pepose, MD, PhD
Arch Ophthalmol. 1998;116:617-623.
ABSTRACT
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Background In contrast to incisional keratotomy, corneas that have undergone photorefractive keratectomy may be difficult to detect by inspection with slitlamp biomicroscopy alone. Eye bank corneas that have undergone high myopic refractive surgical correction could potentially result in substantial postoperative hyperopic correction if used as donor tissue for corneal transplantation. Surface irregularities or displacement of the treated optical zone within the graft in relation to the entrance pupil of the recipient could result in significant induced astigmatism and distortion. This study examines computerized videokeratographic screening of eye bank globes as a strategy for detecting myopic photorefractive keratectomy.
Methods Preoperative and postoperative corneal topographic maps of freshly enucleated human and rabbit eyes that have undergone myopic photorefractive keratectomy with an excimer laser were placed in a globe-fixating device and analyzed using a vertically oriented videokeratoscope. The same system was applied in an actual eye bank setting, and potentially transplantable globes from donors without a history of corneal surgery were analyzed.
Results Computerized videokeratography using a vertically mounted system reliably detected photorefractive keratectomy in 12 of 12 human eye bank corneas treated by excimer photorefractive keratectomy in a range between -1.5 to -6.0 diopters. This method also detected similar changes on lased rabbit corneas enucleated 6 weeks after excimer surgery. Data processed with the tangential mode yielded a "bull's-eye" topography pattern reflecting central corneal flattening that was more sensitive in detecting myopic corrections than the conventional axial formula–based color maps. False-positive results were not detected in 96 cadaver globes sequentially screened in the eye bank.
Conclusions Computerized videokeratography represents a feasible method to screen donor globes for myopic photorefractive keratectomy as shown by the in vitro and rabbit models. However, only whole globes and not corneoscleral sections are amenable to processing with this technique. Tangential maps provided greater sensitivity in detecting low myopic corrections than the axial formula–based color maps.
INTRODUCTION
REFRACTIVE SURGICAL techniques such as radial keratotomy (RK), excimer laser photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK), and automated lamellar keratoplasty (ALK) alter corneal shape and power to achieve the refractive goal. Concerns have been raised regarding the possible outcome of inadvertent transplantation of donor corneas with an undetected history of prior refractive surgery.1 Transplanting corneas with undetected abnormalities can lead to a poor visual outcome.2 Similarly, grafting corneas that have undergone previous myopic refractive surgery could potentially result in substantive hyperopia in the recipient eye. Furthermore, surface irregularities or displacement of the ablation zone within the corneal donor button in relation to the entrance pupil of the recipient could result in irregular astigmatism and decreased final best-corrected visual acuity. To avoid this outcome, it will become increasingly important to identify potential corneal donors with a history of refractive surgery.
The pool of clinically eligible patients for refractive procedures in the United States is large. If only 1% to 2% of myopic patients elect to undergo PRK or LASIK, the potential donor pool could involve up to 3 million corneas. This number could increase further, as laser procedures for farsightedness progress through the evaluation and approval process. Unlike incisional keratotomy (RK and AK), PRK, ALK, and LASIK are difficult to detect by slitlamp biomicroscopic examination alone, because these eyes lack the permanent incision scars left by the former. Computer-assisted videokeratography has become the standard method used to evaluate corneal topography,3-6 including changes induced by refractive surgery.7-11 Our study evaluates the role of computer-assisted videokeratography as a potential method to screen donor globes that have undergone excimer myopic PRK.
MATERIALS AND METHODS
TANGENTIAL VS AXIAL FORMULA–BASED COLOR MAPS
Preliminary studies were performed to compare the usefulness of tangential maps vs conventional axial formula–based color maps in detecting the corneal topographic effect of excimer PRK. Corneal topographies of 3 patients who had undergone PRK for myopia at least 6 months previously, performed with the Summit OmniMed UV200 (Summit Technology, Waltham, Mass) were retrospectively analyzed with tangential maps vs axial formula–based color maps using isodioptric, 0.5-diopter (D) increments, nonabsolute scales. Myopic ablations of 1.5, 2, and 3 D were chosen.
GLOBE-FIXATING DEVICE AND VERTICAL VIDEOKERATOSCOPE
A globe-fixating device was designed and built that allowed adjustment of intraocular pressure and corneal centration. Intraocular pressure is controlled by turning the screw underneath the cone-shaped metal casing, which moves a spring platform where the globe is resting, thus varying the external compression pressure being exerted on the globe when the entire device is mounted (Figure 1, A). Corneal centration is checked by measuring the distance from the limbus to the inner ring of the plastic cap at all 4 quadrants (Figure 1, B). A modified vertically oriented videokeratoscope (EyeSys Corneal Analysis System, EyeSys Technologies, Houston, Tex) was used to assess corneal topography (Figure 1, C). The Placido image consists of 16 alternating black-and-white rings that are digitized to analyze 6000 data points. Balanced salt solution (Alcon Laboratories Inc, Ft Worth, Tex) was used to lubricate the cornea before corneal topography was performed.
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Figure 1. A, Components of the dismantled globe-fixating device. A indicates aluminum block stand; B, donut-shaped plastic cap; C, spring platform; and D, cone-shaped metal casing with adjustable screw. The spring platform is placed at the bottom of the metal casing with the flat piece adjacent to the tip of the screw. B, View from the top showing the human eye bank globe mounted in the globe-fixating device. Centration is determined by measuring the distance from the limbus to the inner edge of the plastic cap with a caliper at all quadrants. C, EyeSys Corneal Analysis System (EyeSys Technologies, Houston, Tex). The modified vertical-oriented videokeratoscope is seen to the right. D, side view of globe-fixating device.
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EXCIMER PRK ON HUMAN EYE BANK GLOBES
Human eye bank globes less than 48 hours from enucleation were provided by the Mid-America Transplant Services, St Louis, Mo. After mounting the globe in the fixating device, the entire corneal epithelium was scraped off with a metal crescent blade prior to any procedure. Excimer PRKs were performed with the Summit OmniMed (Summit Technology) laser on globes with myopic corrections of 1.5, 2, 3, 4, 5, and 6 D at a 6.0-mm ablation zone. Preoperative and postoperative corneal topographic maps were obtained immediately after irrigating the surface with balanced salt solution through a blunt 25-gauge needle. Only a few drops were placed, just enough to obtain good quality mires. A total of 12 human eye bank globes were treated, 2 globes at each myopic correction. Corneal topographic images were analyzed with tangential maps using isodioptric, 0.5 increment, nonabsolute scales and profile maps.
EXCIMER PRK ON RABBIT EYES
Right eyes of 6 New Zealand white rabbits received myopic PRK corrections of 2.4 and 6 D (2 per group). The left eyes were not lased and served as controls. Operated eyes were treated with 0.5% erythromycin ophthalmic ointment (Medical Ophthalmics Inc, Tarpon Springs, Fla), 3 times per day for 5 days. The globes were enucleated 6 weeks after PRK and computerized videokeratographic analysis was immediately performed in a masked fashion. Corneal topographic tangential and profile maps, similar to those used in the enucleated human globes, were generated and analyzed.
VIDEOKERATOGRAPHY OF EYE BANK GLOBES
The vertically mounted computerized videokeratography system and the globe fixator were installed in the eye laboratory of Mid-America Transplant Services to evaluate the practical use of the system in an actual eye bank setting and also to determine the rate of false positives. Computerized videokeratographic analyses were performed by a trained technician on consecutive globes in a sterile manner consistent with eye bank regulations. Globes not suitable for transplantation (more than 48 hours from enucleation or with known history of corneal surgery or detectable corneal abnormalities) were excluded. The epithelium was not removed prior to videokeratography. A total of 100 suitable globes were screened and the corneal topographies using the same tangential mode were screened by an investigator (R.L.) for a bull's-eye pattern typical of central corneal flattening.
RESULTS
SENSITIVITY OF TANGENTIAL VS AXIAL FORMULA–BASED TOPOGRAPHY FOR DETECTION OF EXCIMER PRK
Figure 2, A, represents the composite color maps of a patient who underwent a -1.5-D PRK correction using a 5.0-mm ablation zone. By the third and sixth months after the procedure, the central flattening effect of PRK was no longer easily discernible despite the absence of any myopic regression based on refraction. When the same data were processed by the tangential mode (Figure 2, B), the corneal topography clearly showed the bull's-eye pattern, denoting the presence of central corneal flattening after PRK. Similar findings were seen in patients who underwent -2.0 and -3.0 D of correction. The central flattening effect of PRK remained visible on their color maps even at 1 year after PRK, but central flattening was much easier appreciated with the tangential mode. Based on these findings, tangential maps based on instantaneous radius of curvature were used as a reconstruction algorithm in our studies of eye bank globes, given their greater sensitivity in detecting central flattening.
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Figure 2. A, Composite axial formula–based color maps of a patient who had -1.5-diopter photorefractive keratectomy (PRK) corrections. Top left, Preoperative map; top right, 3 days postoperatively, the central flattening effect (dark blue) of PRK is readily discerned; bottom left and right, 3 and 6 months postoperatively, respectively. Central flattening is no longer discernible. B, Composite maps of the same patient in part A processed with the tangential mode. Top left, preoperatively; top right, 3 days postoperatively showing "bull's-eye" pattern denoting central corneal flattening; bottom left, 3 months postoperatively (central flattening is visible); and bottom right, 6 months postoperatively.
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VIDEOKERATOGRAPHIC ANALYSIS OF EYE BANK GLOBES AFTER EXCIMER PRK
Using the tangential and profile maps, computer-assisted videokeratography was able to detect the effect of PRK on all the eye bank globes throughout the range of myopic correction studied. Figure 3 shows the corneal topography of a globe with the lowest amount of correction studied, at -1.5 D. Figure 3, A, is the preoperative picture depicting normal corneal asphericity while Figure 3, B, is the postoperative map showing the effect of PRK, which was also seen on the profile map. The red line represents the 0° to 180° axis and the blue line is the 90° to 270° axis. Both lines showed central inflection that correspond to the central flattening produced by PRK.
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Figure 3. A, Tangential map generated by computerized videokeratography of a human eye bank globe prior to the performance of myopic photorefractive keratectomy (PRK) with the Summit OmniMed excimer laser (Summit Technology, Waltham, Mass) depicting normal corneal asphericity. B, Corneal topographic maps of the same human eye bank globe in part A after being treated with 1.5 diopters of myopic PRK correction. Left, Tangential map showing central corneal flattening (blue). Right, Profile map showing central inflection corresponding to the central flattening effect of myopic PRK.
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VIDEOKERATOGRAPHIC STUDIES OF RABBIT EYES AFTER EXCIMER PRK
All laser-treated eyes of the rabbits showed the effects of PRK on corneal curvature based on videokeratographic analysis with tangential and profile maps (Figure 4). The observed central flattening on computerized videokeratography after excimer PRK was absent in the untreated eyes (Figure 4, A). The tangential map shown in Figure 4, B, demonstrates the PRK ablation zone. In this example, the ablation zone appears to be inferiorly decentered. This may be due to asymmetric wound healing, misalignment, or surgeon decentration given the absence of fixation during the PRK procedure.
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Figure 4. A, Videokeratographic analysis of the contralateral control eye of the same rabbit in part B, enucleated at the same time as the other eye. This control eye was not treated with excimer photorefractive keratectomy (PRK). The cornea is aspheric and is steeper centrally than peripherally. B, Videokeratographic analysis of an enucleated rabbit eye 6 weeks after -2.00-diopter PRK. Note inferiorly decentered ablation zone (blue) seen on the tangential map to the left. Profile map to the right reflects the findings seen on the tangential map.
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VIDEOKERATOGRAPHIC STUDIES OF EYE BANK GLOBES
Of the 100 eligible eye bank globes, only 4 (4%) did not generate mires of sufficient quality for processing by the EyeSys computerized videokeratography instrument, even after application of balanced salt solution. All these globes had very irregular corneal epithelium with zones of sloughing. We elected not to mechanically remove the epithelium to achieve topographic analysis, since the corneas were suitable for transplantation. Of the remaining 96 eyes, no bull's-eye pattern indicative of central corneal flattening was detected based on analysis of the corneal topographies using the tangential mode.
COMMENT
To our knowledge, there have been no reported cases of corneal transplantation using donor corneas that have had any form of refractive surgery. However, it is possible and perhaps likely that transplanting donor corneas that have undergone refractive surgery will result in less than ideal refractive consequences, such as hyperopia and irregular astigmatism. The medical standards of the Eye Bank Association of America state that corneas that have undergone prior refractive surgery are explicitly contraindicated as donor tissue for transplantation.
Our study establishes the feasibility of using computerized videokeratography as a useful tool for detecting myopic excimer PRK in eye bank globes. By modifying the normally horizontally oriented videokeratoscope into the vertical position and with the aid of a globe holder, we were able to effectively detect the central flattening effects of myopic PRK in eye bank human eyes and in rabbit eyes. We have also shown that the system can be practically introduced in the eye bank setting. In sequential screening of a limited number of globes that were suitable for corneal transplantation, no false positives were detected. However, the probability of finding PRK in these eye bank eyes was admittedly very low.
Based on our study, the most critical factors that need to be controlled in videokeratographic screening of potential donor eyes are centration, alignment, focus, and quality of the Placido image. Corneal centration in the globe holder was achieved by proper orientation of the limbus to the inner edge of the plastic cap. Alignment and focus were adjusted by moving the entire globe holder in the stand or by moving the videokeratoscope in the x, y, and z axes as one would do in the clinical setting where the eye image is displayed on the video monitor. Good-quality Placido images are essential in acquiring accurate corneal topographies. We found balanced salt solution to be useful as a tear substitute. However, this has limited value in corneas with very irregular epithelium, which was the case in 4% of the globes sequentially screened in the eye bank in this series. The epithelium may have to be removed in these instances. Pavlopoulos and associates12 do not recommend using artificial tears in the clinical setting because it altered their corneal topographic results. In our case, tear substitute was needed to acquire good-quality mires and did not mask the effects of PRK on the cornea, despite the hydrating effect of balanced salt solution on the cadaver corneas as noted by Simon and associates.13 In our study, corneal topography was immediately performed after irrigation with a small amount of balanced salt solution. Occasional problems occurred in the acquisition of analyzable images in eyes with light-colored irides. Blue irides gave poor contrast and the videokeratography instrument had difficulty in acquiring the mires. This problem was circumvented by decreasing the threshold setting of the instrument.
Our preliminary studies (data not shown) agree with the findings of Simon and associates regarding the effect of corneal epithelium and intraocular pressure on corneal curvature.13-14 They showed that the cornea flattens with increasing pressure, but only minimally, resulting in less than 1.00 D of change. The same investigators in another study14 reported that removing the corneal epithelium increased the central 2-mm-diameter zone keratometric readings by an average of 1.03 D. In a small sample, we were able to confirm their observations using computerized videokeratography. The cumulative data suggest that the intraocular pressure of the cadaver globe and absence of the corneal epithelium do not appear to markedly alter corneal curvature, and these factors need not be critically controlled when screening globes for previous myopic PRK in the eye bank setting.
A major limitation of the videokeratographic screening technique is that it does not allow the analysis of corneoscleral sections. In cases of in situ excision, topographic analysis of the whole globe may not be feasible. To evaluate corneoscleral sections, a different type of holder or artificial anterior chamber must be designed that will insure against endothelial damage, or a different data acquisition method may be necessary.
Another potential limitation of this technique is the premise that the corneal flattening induced by excimer PRK is permanent. Results from our in vitro model cannot be accurately extrapolated to the clinical situation because there is an absence of wound healing in cadaver globes. Although we tried to address wound healing by using the rabbit model, the short duration of the study and the inherent differences of rabbit and human corneas cannot fully answer the question of long-term stability. There are no data at present on the corneal topographic stability of myopic PRK 10 years after surgery in human subjects. However, there are reports of significant myopic regression in selected cases. This can occur without concomitant subepithelial haze or reticular scarring presumably due to epithelial hyperplasia or other forms of corneal remodeling. Signs of refractive surgery may not be detected in such cases by slitlamp examination. If regression occurs after time, videokeratographic screening may also produce a false-negative result, while the refractive outcome of transplanting such a donor cornea remains unknown.
Another consideration is the accuracy of videokeratography instruments in depicting corneal curvature.15-16 The problem partly lies in the use of algorithms that are based on the "axial solution for radius of curvature" method.17 This assumes that corneal profile can be represented by a set of spheres, where in fact the cornea has an aspheric profile. Although we demonstrated that tangential maps were more sensitive than color maps in detecting the bull's-eye pattern after excimer PRK, this caveat should be kept in mind. However, for screening purposes, the present system appears adequate in detecting myopic PRK corrections.
Finally, computerized videokeratographers are not inexpensive. As long as the cost-benefit ratio of this procedure is not fully established, eye banks may be reluctant to invest in such equipment. Obtaining the full medical and surgical histories of donors is still the most economical way of screening; however, it may be virtually impossible to obtain this information in certain situations.
We have shown that use of the videokeratography system and globe fixator that we have described is feasible for screening purposes, but further field studies are warranted to prove their validity. The screening procedure must be tested to determine its sensitivity in identifying eyes that are known to have undergone prior refractive surgery in their lifetime. The system must also be tested after other forms of refractive surgery such as LASIK, myopic and hyperopic ALK, hyperopic PRK, laser thermokeratoplasty, and other methods that may develop in the future.
The optimal method of screening eye bank globes and corneoscleral sections for refractive corneal surgery has not yet been determined. Other corneal topography systems, including those not dependent on Placido images, should also be evaluated for screening purposes, and could be used alone or in concert. Detection of the presence or absence of the Bowman membrane or LASIK-associated corneal changes by confocal microscopy, elevation topography, and high-frequency ultrasound biomicroscopy may be alternatives to the Placido-based corneal topography system as screening methods. Each may have its own inherent sensitivity and specificity depending on the nature of the refractive procedure, the status of the corneal epithelium, and the presence of a whole-globe vs corneoscleral section.
AUTHOR INFORMATION
Accepted for publication December 11, 1997.
This study was supported in part by grants from EyeSys Technologies, Houston, Tex; Mid-America Transplant Services, St Louis, Mo; National Institutes of Health Core Grant EY02687; and Research to Prevent Blindness Inc, New York, NY. Dr Pepose is the recipient of a Senior Scientist Award from Research to Prevent Blindness Inc.
Reprints: Jay S. Pepose, MD, PhD, Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, Campus Box 8096, 660 S Euclid Ave, St Louis, MO 63110 (e-mail: pepose{at}am.seer.wustl.edu).
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