Corneal elevation subtraction maps, more commonly known as elevation maps, was first introduced by Belin as a way to qualitative measure the shape of the cornea using the PAR CS corneal topographer1. The elevation map compares and then graphically displays the elevation or height difference between the measured cornea and a known reference shape. Depending on the corneal topographer used, the reference shape will either be a best fit sphere (BFS), best fit ellipse or a best fit toric elipsoid
With the Oculus elevation maps the corneal elevation above the BFS is measured in positive microns and is displayed as green to blue shading on the map. The corneal surface below the reference sphere is measured in negative microns and is displayed in colours ranging from orange-red to purple-grey.
Elevation maps are helpful to ascertain the outcome of rigid contact lens fitting by predicting areas of bearing (green to blue areas) or pooling (orange to grey). The image below on the left shows the elevation map of a cornea with keratoconus and on the right, the NaFl simulation of a lens fitted on the eye. Notice how the areas of lens bearing in the horizontal meridian correlate with the green-blue areas on the elevation map and the areas of excessive sodium fluorescein pooling inferiorly with the orange-purple areas on the elevation map.
In keratoconic cases when the BFS is fit to a cone, the apex of the cone appears as a circular area (yellow island with the Oculus elevation map). Compared to the axial/tangential curvature maps, the size and location of the 'yellow island' as seen with the elevation map correlates more accurately to the actual area and location of the corneal ectasia.
With the Medmont E300 topographer - after adjusting the scale of the map - the corneal elevation above the BFS, shown in positive microns, appears as red shading on the map. Conversely, blue shading indicates that the corneal surface is below the BFS and is displayed in negative microns. To adjust the scale of the elevation map on the Medmont software, select “Custom” and adjust the “Max Value” to 50 microns for the best display result in most situations. Adjusting the scale is most useful when there is more than a 20 micron difference between the steep and flat meridians around 4.00 mm from the centre.
In the Medmont elevation map below, the scale is not calibrated correctly and displays little useful information regarding the height of the cornea.
In the map below the scale is adjusted correctly and shows a blue area in the superior quadrant of the cornea, indicating the lens will move towards and settle on the superior portion of the eye.
A poorly centred map can create a false representation of the corneal elevation. See below for a comparison of two elevation maps of the same eye. The image on the left is based on a poorly centred topographical map. The image on the right is based on a well centred topographical map.
Elevation maps are helpful to determine the best lens design to fit on a cornea. Setting the BFS to a sphere, the resulting value is a good indicator for the the starting base curve of a diagnostic corneal or scleral rigid contact lens.
The elevation map further helps to determine if a rotationally symmetric (RS) or toric lens design must be used. Normally a toric corneal lens is chosen if the corneal astigmatism is above 1.00D. Rather than relying on Sim-K values, a better way to determine if a toric lens is needed is to measure the elevation difference between the two principal meridians of the cornea over a 8.00 mm chord (4.00 mm out from the centre). If the corneal height difference exceeds 30 microns over the 8.00 mm chord diameter, use a toric design
As described in the video above, the alignment curve of a Bespoke Alignment fit or Forge Ortho-K lens lands on the peripheral cornea, 4.00 mm out from the centre. Fitting an RS lens to a cornea with more than 30 microns difference in elevation, the alignment curve of the lens would not bear down evenly on the cornea circumferentially. The result will be a weekend compression force in the steep meridian resulting in poor lens centration, due to the lens rocking or tilting over the flat meridian and under treatment.
Below is an example of a Medmont elevation map with the scales adjusted to range from 50 to -50 microns. Remember, each gridline in the Medmont map is equal to 1.00 mm. In this example the difference between the flat and steep meridians are only 16 microns. This cornea requires a rotationally symmetric lens design such as the Bespoke RS or Forge Ortho-K RS.
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. 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.
The video below describes how to use the elevation map built into EyeSpace software to decide if the captured topography map requires a toric design.
The elevation maps also help to evaluate the baseline topography to determine if any non-symmetrical high or low elevation points exist on the cornea which can cause the Bespoke Alignment fit or Forge Ortho-K lens to decentre on the eye. Remember, the lens will decentre towards the lowest elevation point (the bluest area on the Medmont elevation map). The image below shows an elevation map of a inferiorly tilted cornea and with-the-rule corneal astigmatism.
Unlike the elevation maps of the previous cornea that show a toric lens is required, there is no symmetry between the elevation of the four quadrants of this cornea (especially in the vertical meridian).Note the elevation differences in each quadrant (-5, +14, -32, +7). For this cornea, a quadrant-specific lens design is required to help centre the lens on the eye.
For irregular corneas, an elevation difference greater than 325µm between the highest and lowest points is a good indicator to rather fit scleral lenses, as the big difference in elevation can cause corneal lens instability.
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