title: Three-dimensional digital subtraction modeling of corneal topography.
abstract: BACKGROUND
  The introduction of a system of classification of color-coded computerized topography has proved of great clinical value but may be misinterpreted by some who assume that the color plots represent actual corneal contour.
  
  
  METHODS
  A three-dimensional system of contour modeling is used to plot equivalents of each category of corneal shape. The round, oval, symmetric bowtie, and asymmetric bowtie of the widely used classification schema are each translated into contour equivalents. Digital subtraction of an appropriate sphere from the contour data is central to the transformation.
  
  
  RESULTS
  While round- and oval-type color-coded refractive power plots translate into fairly intuitive contour images, symmetric and asymmetric bowties produce surprising results. We demonstrated that the bowtie appearance is a result of the changing curvatures present between the two principal meridia of the toroidal surface.
  
  
  CONCLUSIONS
  The shape characteristics of the corneal surface dictate the familiar color-coded dioptric plots.
authors.affiliation: Cornea Service, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Young
authors.forename: I M
authors.initials: IM
authors.lastname: Siegel
mesh_terms: D001251:Astigmatism / Q000473:pathology; D003315:Cornea / Q000033:anatomy & histology*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008954:Models, Biological; D013382:Subtraction Technique