Corneal Epithelium Affects Anterior Corneal Topography in Myopic Patients
Corneal shape, as evaluated by videokeratography, is one of the essential measurements used in screening patients and planning for conventional and customized corneal refractive surgery. Corneal topographic measurements are taken preoperatively with the epithelium in place, but the epithelium is removed before laser surface ablation procedures such as PRK, LASEK, or epi-LASIK. My colleagues and I performed a study to find out what effect removal of the epithelium has on corneal topographic characteristics and how this might affect refractive surgical outcomes.1
We found that the corneal epithelium makes an important contribution to the topographic features of the anterior cornea. The remodeling of the epithelium on the corneal stromal layer may account for some discrepancy observed between anterior and posterior elevation topography in corneas with early forme fruste keratoconus. Our findings, reviewed in this article, may help to develop better algorithms for refractive surgical ablation of the anterior cornea, taking into account the characteristics of the epithelium.
BACKGROUND
Previous studies have shown that epithelial thickness varies over the entrance pupil, and removing the epithelium alters the topography and the refractive power of the cornea.2-4 It has also been observed that postoperative epithelial remodeling can change the intended outcome of surface ablation procedures.5
Others have measured the effects of epithelial removal in cadaver eyes using keratometry5 and corneal topography3 and have measured the thickness of the central corneal epithelium in vivo using high-frequency ultrasound.2 To the best of our knowledge, ours is the first study to use anterior elevation and Placido disk videokeratography with the Orbscan II (Bausch & Lomb, Rochester, New York) to evaluate the influence of the corneal epithelium on anterior corneal shape.
The Orbscan II measures the elevation of anterior and posterior corneal surfaces by means of 40 scanning slit images. It also incorporates measurement of anterior corneal curvature with Placido disk corneal topography.
Because we were interested only in measurements in the area cleared during epithelial removal, a smaller area than normal—the central 7 mm zone—was used for calculating preoperative and post–epithelial-removal values with the Orbscan. The following values were calculated for the purposes of this study: (1) central 2-mm mean optical pachymetry, (2) anterior best-fit sphere radius, (3) keratometric astigmatism (based on simulated keratometry values), (4) central 3-mm mean curvature, and (5) apical curvature and asphericity.
PATIENTS AND METHODS
The study included 44 eyes (25 patients) undergoing PRK for myopia or compound myopic astigmatism. Mean age was 28.7 years, and mean preoperative spherical equivalent refraction was -3.56 D (range, -9.40–0.00 D). Mean refractive cylinder was -0.49 D (range, -2.75–0.00 D). Patients with systemic disease or with suspected or forme fruste keratoconus and those who had worn rigid gas permeable contact lenses within the past year were excluded. Patients underwent routine screening and preoperative examinations for refractive surgery.
In the surgical suite, Orbscan topography was performed 10 minutes before surgery. All measurements were done with the same instrument and performed by the same operator. After topical anesthesia administration and preparation and draping of the patient, the central 8 mm of the cornea was exposed to alcohol for 20 seconds and then rinsed with balanced salt solution. The epithelium was then removed with blunt forceps, so that Bowman's membrane was completely exposed in the central 8-mm zone.
After the epithelium was removed, the patient was placed in front of the Orbscan device again and measured twice more, then returned to the operating table and draped again. PRK surgery proceeded as normal, using the EC-5000 excimer laser (Nidek, Gamagori, Japan).
RESULTS
No intra- or postoperative complications were seen. The time to epithelialization was within normal range in all eyes. Table 1 summarizes the results, which were in a normal range for all eyes.
The mean difference in Orbscan pachymetry in the central 2-mm zone from preoperative to after epithelial removal was 37.20 µm (range, 19–58 µm).
Best-fit sphere radius changed from a mean of 7.75 mm preoperative (range, 7.25–8.42 mm) to 7.92 mm (range, 7.39–9.16 mm) after epithelial removal (P<.0001). The mean keratometric astigmatism increased from 0.75 D (range, 0.10–4.70 D) preoperative to 1.21 D (range, 0.20–4.70 D) after removal (P<.0001), and the mean keratometry decreased from 43.77 D (range, 40.25–47.00 D) to 42.44 D (range, 37.05–45.50 D).
In the central 3-mm zone, mean power decreased from 44.42 D (range, 40.40–47.20 D) preoperatively to 43.46 D (range, 39.70–46.90 D) after epithelial removal. Mean irregularity index increased from 1.07 D (range, 0.50–2.50 D) preoperatively to 2.03 D (range, 1.30–3.3 D) after epithelial removal (P<.0001), and mean asphericity value (Q) went from -0.44 (range, -0.72 to -0.20) to -0.65 (range, -1.04–0.14) after removal (P=.003).
An example of the difference in Orbscan mapping before and after epithelial removal can be seen in Figure 1.
What is the total effect of these statistically significant changes in parameters on the overall corneal shape? Taken together, the changes in mean keratometry, mean curvature, and apical sphere radius mean that the central cornea was flatter after epithelial removal, on average. However, there was a large scatter in the data related to apical radius variation, and a significant proportion of the corneas became steeper at the apex after epithelial removal.
Additionally, the change in central asphericity value in the central 7-mm zone indicates that the central corneal became more prolate after epithelial removal.
There was a trend in our data for a decrease in central curvature, which is the opposite of previous reports that found increases in central curvature after epithelial removal.2,5 This difference may be the result of several factors, including the use of different methods of measurement and the fact that our measurements were taken in young living eyes instead of cadaver eyes.
CONCLUSIONS
From our analysis, it is clear that the epithelium affects the anterior elevation and corneal topography results on the Orbscan device. Significant increases were seen in astigmatism and topographical irregularity after removal of the epithelium, indicating that the epithelium smoothes out some features of the shape of Bowman's layer. The epithelial layer confers to the anterior surface an ability to remodel its shape, unlike the posterior corneal surface. This suggests that the epithelium may mask anterior surface irregularities such as signs of early stages of keratoconus in some patients who exhibit increased posterior elevation. We have coined the term epithelial anterior smoothing effect (EASE) to describe this phenomenon.
Considering the effects of the epithelium on diagnostic imaging of the cornea, it is hoped that in the future, imaging modalities may be developed with the ability to see through the epithelium, showing the shape of the underlying Bowman's membrane and stroma. Intraindividual variability makes it difficult to know how the epithelium contributes to corneal shape in any patient. A combination of techniques such as very high frequency ultrasound and optical coherence tomography may allow the development of layered corneal maps that will allow us to plan corneal refractive surgery with more diagnostic information at our disposal.
Preoperative examinations such as corneal topography or ocular wavefront mapping are performed with an integral ocular surface. Given that a corneal surface receiving laser ablation may be significantly different after epithelial removal, it is surprising that customized surface ablation techniques such as topography- or wavefront-guided PRK give satisfactory results. It may be beneficial to treat for surface irregularities in some cases, although these may be underestimated due to partial epithelial masking. Moreover, the healed epithelial layer may largely contribute to further smoothing of the ocular surface postoperatively.
Our study has shown that the epithelium makes a significant contribution to the topographical features of the anterior cornea. More research is needed to determine the effects of this contribution on outcomes of refractive surgery.
Damien Gatinel, MD, PhD, is the Scientific Director of the Refractive Surgery Unit and an Assistant Professor at the Rosthschild Ophthalmology Foundation and Bichat-Claude Bernard Hospital, Paris. Dr. Gatinel is a member of the CRST Europe Editorial Board. He states that he has no financial interest in the products or companies mentioned. He may be reached at +33 1 48 03 64 86; gatinel@aol.com.
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