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.
- 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
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!