Elevation maps indicate how a contact lens will move and orientate itself when placed on the cornea, and provide the best indication of whether a toric lens should be chosen over a rotationally symmetric (spherical) lens.

Note that this applies to Lux (day-wear RGP) and Forge (Ortho-K) lenses.

You can use the built-in topography module in EyeSpace, or the software that came with your corneal topographer if you prefer. The details below show how to do this with EyeSpace.

Key Points to remember

  • Choose the “Elevation” map type on the top right.
  • Gridlines are spaced 1 mm apart.
  • The central black crosshair marks the centre of the captured topography (the instrument axis).
  • Always identify the steep and flat axes of the cornea as these indicate the principle meridians of the cornea. In many cases these are horizontal and vertical but not always.

Method

  • Click 4 mm from center on the horizontal (flat axis) and note the elevation value
  • Do the same for the vertical (steep axis)
  • If the difference between these two values is greater than 30 microns, a custom toric lens is most likely needed.

The example below shows +7.7 microns on the flat meridian and -39.6 along the steep meridian – the difference is 47.3 microns, so a toric lens is required.

CAPTION:Click horizontally 4 mm from centre (four grid lines) and the elevation reads +7.7 microns.

CAPTION:Click vertically and the elevation reads -39.6 microns

For more indepth information watch our instructional video

Summary

If the difference in elevation (or sagittal height) between the principal meridians is 30 microns or greater, then a toric design should be used.

Thanks for reading, and keep an eye out for our next EyeSpace tutorial!