One of the often overlooked features of scleral contact lenses is that it has forced our contact lens industry to view the ocular surface in terms of height rather than radius of curvature. The truth of the matter is that all contact lens fitting, regardless of the lens design (corneal, soft, or scleral), should be based on the height of the ocular surface over which the lens rests. We believe that much of the “failure” of corneal contact lenses can be traced back to our failure to understand the fundamental principles of corneal height.
Unfortunately, today, corneal topography is most often interpreted as a color coded, axial display map (the default map of the corneal topographer). This map does an excellent job of describing the dioptric power of the cornea and illustrating the shape and location of the corneal astigmatism. Surprisingly, the axial display map provides very little of the information needed for properly fitting a corneal GP lens. Rather, it is the elevation map that provides the greatest amount of information about the true shape of the cornea.
The Height of the Matter
- The corneal topographer selects a “theoretical” spherical surface (the best-fit-sphere) and runs it through each meridian of the cornea (the yellow line).
- Along the flat, horizontal meridian, the theoretical spherical surface penetrates into the cornea nasally and temporally.
- Along the steeper vertical meridian, the theoretical sphere penetrates into the cornea superiorly and lifts away from the cornea inferiorly.
This means that the areas of the elevation map that are red represent areas of the cornea with the greatest elevation (where the contact lens is going to be in closest apposition to the cornea), and the areas in blue represent areas of least elevation or depression (where the contact lens is going to be furthest away from the cornea).
Figure 3 shows how the topographer uses the elevation display map to generate the simulated fluorescein pattern of an aspheric keratoconus lens across the cornea. Along the horizontal meridian, the lens has 40 microns of apical clearance and “lands” at 3 o’clock, 9 o’clock, and 12 o’clock. Figure 4 shows how the actual fluorescein pattern perfectly emulates the elevation display map and the simulated fluorescein pattern. CLS