Clinical ScienceCorneal Elevation Topography: Best Fit Sphere, Elevation Distance, Asphericity, Toricity, and Clinical ImplicationsGatinel, Damien MD, PhD; Malet, Jacques PhD; Hoang-Xuan, Thanh MD; Azar, Dimitri T MDAuthor Information From the *Rothschild Foundation, Paris, France; †Bichat Claude-Bernard Hospital, University Paris VII, Paris, France; ‡Institute of Statistics, Pierre et Marie Curie University, Paris, France; §Center of Expertise and Research in Optics for Clinicians, Paris, France; and ¶Department of Ophthalmology and Visual Sciences, University of Illinois Eye and Ear Infirmary, University of Illinois at Chicago, Chicago, IL. Received for publication April 27, 2009; revision received June 9, 2010; accepted June 18, 2010. Supported by grants National Institutes of Health EY10101 (D.T.A.), EY001792 (D.T.A.), and an unrestricted departmental grant from Research to Prevent Blindness (New York, NY). The authors state that they have no proprietary interest in the products named in this article. Reprints: Dimitri T. Azar, Department of Ophthalmology and Visual Sciences, University of Illinois Eye and Ear Infirmary, University of Illinois at Chicago, 1855 West Taylor Street, Chicago, IL 60612 (e-mail: [email protected]). Cornea: May 2011 - Volume 30 - Issue 5 - p 508-515 doi: 10.1097/ICO.0b013e3181fb4fa7 Buy Metrics Abstract Purpose: To describe the effect of the corneal asphericity and toricity on the map patterns and best fit sphere (BFS) characteristics in elevation topography. Methods: The corneal surface was modeled as a biconic surface of principal radii and asphericity values of r1 and r2 and Q1 and Q2, respectively. The apex of the biconic surface corresponded to the origin of a polar coordinates system. Minimization of the squared residuals was used to calculate the values of the radii of the BFSs and apex distance (A-values: z distance between the corneal apex and the BFS) of the modeled corneal surface for various configurations relating to commonly clinically measured values of apical radius, asphericity, and toricity. Results: Increased apical radius of curvature and increased prolateness (negative asphericity) led to an increase in BFS radius but had opposite effects on the A-value. Increased prolateness resulted in increased BFS radius and A-value. Increasing toricity did not alter these findings. Color-plot elevation maps of the modeled corneal surface showed complete ridge patterns when toricity was increased and showed incomplete ridge and island patterns when prolateness was increased. Conclusions: High A-values in patients with corneal astigmatism may result from steep apical curvature and/or high prolateness (negative asphericity). The BFS radius may help in distinguishing between these 2 causes of increased A-values. Increased prolateness and decreased apical radius of curvature (often seen in keratoconus) have opposite effects on the BFS radius but similar effects on the apex distance. Copyright © 2011 Wolters Kluwer Health, Inc. All rights reserved.