topography topics

An Elevated Strategy for Viewing Topography Maps

BY KENNETH A. LEBOW, OD, FAAO
November 1999

Just like axial and tangential maps, elevation maps use different algorithms to provide additional profile information about the corneal shape. Elevation maps represent the measured height (i.e., distance) in microns from which the actual corneal curvature varies from either above or below a computer-generated reference surface.

The computer calculates a best-fit surface for each image that is used and uses that as a fixed reference surface for all measurements made to that specific image. Different images of the same eye or of different eyes will have different reference surfaces. Thus, the reference sphere represents an overall measurement of corneal curvature for each image evaluated. Height data is best presented using a reference sphere because it highlights the underlying surface features of the cornea better than a reference plane.

When the actual corneal curvature is above the best-fit curvature generated by the computer, the values have a positive value that appear red in color. Conversely, when the actual corneal curvature falls below the reference sphere, the values have negative numbers that appear blue.

By adding an elevation map to last month's keratoconus figure, you can see that each map locates the corneal apex in slightly different positions (Fig. 1). When the cursor (plus sign) is placed over the location with the highest elevation over the reference sphere, the corresponding locations on both the axial and tangential maps do not correspond to their steepest curvatures. For keratoconus, the area of maximum elevation over the reference sphere corresponds nicely with the apex of the cone.

Fluorescein Pattern Predictors

Elevation maps are also useful in predicting fluorescein patterns with RGP lenses. The higher elevations displace fluorescein and appear as areas of bearing, while the depressed areas (green), relative to the reference sphere, pool fluorescein and appear steep. Figure 2 simulates a contact lens fit on the keratoconic eye shown in Figure 1. The white plus sign corresponds exactly with the point of highest elevation (the apex of the cone) on the elevation map. Inferiorly, the extreme depression (dark blue) is ultimately responsible for the characteristic appearance of blink-induced inferior edge stand-off typically seen when fitting rigid contact lenses on keratoconic corneas.

Remember, elevation maps are extremely useful for locating the highest and lowest points on the corneal surface and are excellent predictors of RGP fluorescein patterns.

Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.