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Contact Lens Spectrum

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ORTHOKERATOLOGY TODAY

ACHIEVING CENTRATION: ELEVATION DATA

Contact Lens Spectrum May 1, 2018 Vol 33, Issue May 2018 Page(s): 40

In March, my column talked about centration relative to horizontal visible iris diameter, lens diameter, and elevation. This month, let’s take a closer look at elevation data and how it effects centration and overall success with orthokeratology (ortho-k).

In your pre-ortho-k evaluation, display the elevation map; it looks a bit different from the axial map. Rather than curvature or refractive power, the elevation map displays the height of the cornea at all mapped points relative to a best-fit sphere. As you may have already discovered, refractive astigmatism is not the determining factor; it’s all about elevation. The elevation map will show you what to expect before a lens is placed on the eye. The significant points to note are the areas where the “alignment zone” lands. For most ortho-k designs, this is an 8mm circle, 4mm from the geometrical center of the cornea in all meridians.

Elevation Differences

In a “with-the-rule” cornea, the highest points are in the horizontal meridian, where the lens will land first. Conversely, the deepest points are in the vertical meridian. To calculate the functional vertical-horizontal elevation difference: 1) Map the vertical elevation 4mm above and below the geometric center of the cornea and calculate the average of those two readings; 2) Map the horizontal elevation 4mm nasal and temporal to the geometric center of the cornea and calculate the average of those two readings; and 3) Combine those two readings to find the difference.

In Figure 1, for example, the average of the vertical elevations (–63 and –21 = –42 microns) and the horizontal (+8 and –18 = –10 microns) results in a difference of –42 and –10 = 32 microns. Figure 2 shows even higher elevation differences. As a general rule, a difference in vertical and horizontal elevation of 25 microns or more will require an ortho-k lens with different sagittal depths 90o apart. This can be accomplished with toric reverse curves and/or toric alignment/landing curves.

Figure 1. Pre-treatment axial map showing 1.75D of central with-the-rule astigmatism (A) and elevation (B). This example shows a 32-micron elevation difference between the vertical and horizontal landing points.
Figure 1. Pre-treatment axial map showing 1.75D of central with-the-rule astigmatism (A) and elevation (B). This example shows a 32-micron elevation difference between the vertical and horizontal landing points.
Figure 2. Pre-treatment axial map showing 3.75D of central with-the-rule astigmatism (A) and elevation map (B). This example shows an 88-micron elevation difference between the vertical and horizontal landing points.
Figure 2. Pre-treatment axial map showing 3.75D of central with-the-rule astigmatism (A) and elevation map (B). This example shows an 88-micron elevation difference between the vertical and horizontal landing points.

Determining the toric sagittal depth difference for your lens will vary depending on the lens design and manufacturer. Contact your lab consultant for help in initial design. You will need to provide the numerical elevation values or the actual topography. Ideal ortho-k results will be achieved with a well-centered lens and well-balanced fluid forces in all meridians. CLS

Dr. Lipson is an assistant professor at the University of Michigan’s Kellogg Eye Center. His clinical practice involves contact lenses with emphasis on specialty contact lenses. He conducts clinical research studies and lectures nationally and internationally on specialty contact lens and research topics. He is a consultant to Bausch + Lomb’s Specialty Vision Products relative to ortho-k education and other specialty vision products. He is on the GPLI Advisory Board and has been involved on the board of the Scleral Lens Education Society.

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