Knowledge Base
2. Linking EyeSpace to your topographer

4. Using Elevation Maps

4. Corneal TopographyCharl Laas

With Forge Orthokeratology and Bespoke alignment lenses, fitting a Rotationally Symmetric (RS) lens instead of a Toric or Quad lens can lead to decentration, poor seal, and overall lack of visual results.

It is a good idea to strongly consider the idea of a toric lens when the corneal cylinder is greater than 1.00D. However, looking at the corneal Elevation Maps is an even better way to check for a toric design than relying on Sim K’s alone. If you have 25-30 microns difference in sag height (or more) between the steep and flat meridian, 4.00 mm from the centre, you most likely need a toric design.

Below is an example of elevation maps that would require a rotationally symmetric lens design such as Forge Myopia. The corneal elevation differences between the flat and steep meridians are only 16 microns.

Below is an example of an elevation map that would require a toric lens design instead of a rotationally symmetric design to achieve a good 360-degree seal. The scales in the Medmont map have been adjusted using the custom tab to range from 50 to -50 microns. Note the corneal elevation is -31 um in the steep meridian and +7 um in the flat meridian, which gives a difference of 38 microns.

If a rotationally symmetric lens were fit to this eye, the alignment curve would not “land” evenly in the opposing meridians. When the alignment curve of an RS lens is fitting well in the flat meridian, fluorescein pooling would be seen underneath the vertical alignment curve

Counting 4.00 mm out from the centre is important as a well-fitted Forge orthokeratology lens' alignment curve is “landing/bearing” in the peripheral cornea 4.00 mm from the centre. Remember, each gridline in the Medmont map is equal to 1.00 mm.

The following four elevation maps show a patient that would require a quadrant-specific lens. Note the elevation differences in each quadrant (-5, -32, +14, +7). Unlike the elevation maps that show a toric lens is required, there is no symmetry between the elevation of the cornea in each meridian (especially vertically in this example).

Video

For more information please watch these videos.

https://vimeo.com/82749785

When to choose a toric using EyeSpace? from EyeSpace on Vimeo.

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,https://vimeo.com/169092652

Toricity Analysis from EyeSpace on Vimeo.