Optics of the Corneal Epithelium

Abstract

BACKGROUND: The refractive effect of the corneal epithelium in relation to the cornea as a whole is relatively unknown, yet it may affect the efficacy of keratorefractive surgical procedures, most notably photorefractive keratectomy and epikeratoplasty. This article investigates the retractive effect of the epithelium on the cornea. METHODS: We measured the corneal keratometry in 10 fresh human eye-bank eyes with and without epithelium. All readings were performed at 2.0- and 3.6-millimeter diameter zones with an automated keratometer. RESULTS: When the epithelium was removed, there was an increase in corneal refractive power in every case. The mean change was + 1.03 D (range, 0.55 to 1.85 D) at the central 2.0-millimeter diameter zone and + 0.85 D (range, 0.29 to 1 .60 D) at the 3.6-millimeter diameter zone. We also found a change in both the power and axis of astigmatism in most cases. CONCLUSIONS: The corneal epithelium plays an active role in determining the final surface power of the cornea. It accounts for an average of 1 .03 D of the power of the eye at the central 2-millimeter diameter zone. The difference in astigmatism suggests that the epithelium does not form a layer of uniform thickness over Bowman's layer, it appears that the epithelium attempts to minimize abrupt changes in surface contour. These findings suggest that in refractive procedures such as photorefractive keratectomy the refractive contribution of the epithelium must be taken info account to improve predictability. Refract Corneal Surg 1993;9:42-50.) RÉSUMÉ INTRODUCTION: L'effet réfractif de !'epithelium corneen relatif à la cornée entière n'est pas précisément compris, bien que !'epithelium ait des conséquences sur le résultat réfractif après la kératectomie photorefractive et l'epikératoplastie. Nous examinons ici l'effet réfractif de l'epithelium vis à vis de la cornée. METHODES: Nous avons mesuré la courbure kératométrique chez 10 yeux frais de cadavres humains, avec et sans !'epithelium corneen. Chaque cornée fut mesurée aux zones optiques de 2.0 et 3.6 mm à l'aide d'un kératomètre automatique. RÉSULTATS: Après avoir enlevé !'epithelium, nous avons observé une augmentation du pouvoir réfractif de chaque cornée. Le changement moyen tut de + 1.03 äioptrie ( + 0.55 à + 1.85 D) dans la zone optique de 2.0 mm et de +0.85 ( + 0.29 à + 1 .6 D) dans la zone optique de 3.6 mm. Nous avons aussi observé un changement de l'axe et de la puissance de l'astigmatisme dans la plupart des yeux. CONCLUSION: L'épithélium corneen a un effet important sur la surface cornéenne opérée. Il retranche 1.03 D de pouvoir réfractif à la zone optique centrale de 2.0 mm. Le changement cylindrique suggère que l'épithélium ne forme pas une couche d'épaisseur uniforme sur la membrane de Bowman. Il apparaît que !'epithelium tend à minimiser les changements abrupts dans la surface cornéenne. Ces résultats suggèrent que la contribution refractive de l'epithelium doit être considérée afin d'améliorer la prévisibilité de la kératectomie photorefractive. (Translated by Rob Mack, MD, Iowa City, Iowa)

Biometric Determinants of Epithelial Thickness Profile Across a Wide Range of Refractive Errors

“…The corneal epithelium is the outer layer of the cornea that provides protection against external factors and increases optical quality by ensuring a smooth front surface [ 1 ]. The corneal epithelium contributes to 1.03 diopters (D) and 0.85 D to refractive power in the central 2 mm and 3.6 mm-diameter zones, respectively [ 2 ]. Its thickness shows a nonuniform distribution across the corneal surface ranging from 48 to 60 μm [ 2 , 3 ].…”

“…The corneal epithelium contributes to 1.03 diopters (D) and 0.85 D to refractive power in the central 2 mm and 3.6 mm-diameter zones, respectively [ 2 ]. Its thickness shows a nonuniform distribution across the corneal surface ranging from 48 to 60 μm [ 2 , 3 ]. Techniques available to measure the corneal epithelium include in vivo confocal microscopy, very-high-frequency ultrasound, optical coherence tomography (OCT), and Scheimpflug imaging [ 1 ].…”

Corneal and corneal epithelial thickness distribution characteristics in healthy North Indian eyes using spectral domain optical coherence tomography

“…[ 1 2 3 ] The epithelium also contributes to the refractive power of the cornea, ranging from an average of 1.03 D over the central 2-mm zone to 0.85 D at the 3.6-mm zone. [ 4 ] Thus, changes in the thickness and distribution of corneal epithelium may, on one hand, be the earliest indicators of various corneal disorders, including ectasia, dystrophy, and contact lens-associated keratopathy, while on the other hand, they may be responsible for refractive surprises after keratorefractive surgery. [ 3 5 6 7 8 9 ] As a consequence, epithelial thickness profiles and their relationship with the underlying stroma are being increasingly utilized in algorithms designed to detect “pre topographic” keratectasia and for customization of refractive procedures to refine postoperative outcomes.…”

Intraoperative Swept-Source OCT–Based Corneal Topography for Measurement and Analysis of Stromal Surface After Epithelial Removal

“…Even if the epithelium had a uniform thickness profile and a refractive index identical to that of the stroma, its sheer removal would increase the refractive power of the cornea due to the shortening of the radius of curvature. 1 In fact, the corneal epithelium has a higher refractive index (1.401) than the anterior stroma (1.380), a difference sufficient to affect overall corneal power. 2 Additionally, the epithelium has a non-uniform thickness profile due to the effect of eyelid blinking mechanics, 3,4 and the inherent behavior of epithelial cells to smooth the underlying stromal surface irregularities.…”

“…9,[13][14][15][16] Reinstein et al 17 used epithelial thickness maps to generate stromal topography and performed the first stromal topography-guided treatment, whereas Vinciguerra et al 18,19 used intraoperative stromal topography to plan sequential stromal topography-guided treatment. Studies using either direct measurement of the corneal stromal surface immediately after epithelial debridement, 1,[20][21][22][23] mathematically modeled stromal/Bowman's layer surface, 24 or Bowman's layer topography constructed by edge detection of the surface of Bowman's layer on 12 radial corneal swept-source optical coherence tomography (SS-OCT) B-scans 25 have also shown variation in epithelial and stromal topographic and optical characteristics.…”