Accuracy of OCT Curvature and Aberrations of Bowman's Layer: A Prospective Comparison With Physical Removal of Epithelium.

PURPOSE: To compare optical coherence tomography (OCT) and Scheimpflug curvature and aberrations of the Bowman's layer before and after removal of the epithelium. METHODS: Bowman's layer was mapped with OCT (Optovue Inc., Irvine, CA) before and after removal of the epithelium in normal eyes undergoing photorefractive keratectomy (n = 14) and keratoconic eyes undergoing corneal cross-linking (n = 25). The anterior corneal surface before removal and the underlying Bowman's layer after removal of the epithelium were also mapped with Pentacam (Oculus Optikgeräte, Wetzlar, Germany), and the surface aberrations with ray tracing were computed. RESULTS: The agreement of OCT curvatures before and after removal of the epithelium was excellent (intraclass correlation coefficient [ICC] = 0.9). A similar trend was seen between OCT and Pentacam after removal of the epithelium. The agreement of surface wavefront aberrations of the Bowman's layer before and after removal of the epithelium was excellent (ICC = 0.9) between the devices for keratoconic eyes. However, this agreement was relatively inferior in normal eyes (ICC < 0.5). CONCLUSIONS: The virtual OCT curvature and aberrations of the Bowman's layer agreed well with its actual magnitudes on removal of the epithelium in the keratoconic eyes. In normal eyes, the agreement was inferior for aberrations but not for curvature. [J Refract Surg. 2020;36(3):193-198.].

Comparison of front-surface corneal topography and Bowman membrane specular topography in keratoconus.

PURPOSE: To define the contribution of the corneal epithelium in corneal topography in keratoconus and discuss the implications regarding combined topography-guided photorefractive keratectomy and corneal collagen crosslinking (CXL). SETTING: French National Reference Center for Keratoconus, Bordeaux, France. DESIGN: Case series. METHODS: Specular topographies were performed before and immediately after epithelial removal during conventional CXL surgery in patients with keratoconus. RESULTS: The study included 1 eye with forme fruste keratoconus, 4 eyes in Krumeich stage I, and 3 eyes in stage II. The mean simulated maximum keratometry (K) increased by 2.87 diopters (D) after epithelial removal. The mean effective refractive power increased by a mean of 4.01 D and the astigmatic refractive power, by a mean of 2.17 D. The difference in mean axis deviation was not significant. The mean inferior-superior ratio irregularity index increased 1.51 units. All K and refractive readings except astigmatic refractive axis were significantly increased after epithelial removal. All regularity indices increased significantly without the epithelium. The amplitude of variations in K readings made it difficult to predict the refractive changes after custom laser photoablation based on epithelial-based topography. CONCLUSIONS: The astigmatism refractive axis did not change significantly; therefore, this axis could be useful for astigmatic corrections, whereas the spherical component would not be useful. Topographic changes after epithelial removal can be important but not intuitive in keratoconus eyes. This should be considered in custom topography-guided photoablation strategies when attempting to correct refractive errors at the time of CXL.