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.
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.
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