ORTHOKERATOLOGY (ortho-k) is a fantastic treatment option for patients. But before diving into designing an ortho-k mold, it is crucial to understand the corneal surface, which means utilizing your corneal topographer.
Most topographers use several different maps to visualize the corneal surface. This article will review some of the most common topographical maps and how each is utilized for ortho-k.
In my experience, axial maps are one of the maps most commonly used by clinicians. They give a good representation of the overall corneal curvature. This is useful in detecting corneal ectasias, characterizing spherical versus toric corneas, and defining the type and amount of cylinder.
These maps are useful in ortho-k to determine whether a patient is a good candidate (e.g., are there any corneal irregularities or ectasias present?) and also for determining whether a toric design is warranted. If the corneal toricity is limbus-to-limbus, a toric design is often warranted. If the corneal cylinder is more apical, then spherical designs may work very well.
Tangential maps are excellent for pointing out subtle changes and overall shape. They can show how well ortho-k designs are centering during overnight wear after treatment, and are also excellent for visualizing the peripheral plus created by the ortho-k design and whether it is properly centered and within the pupil margin to provide good myopia control.
Elevation maps show the height differences across the corneal surface. As mentioned previously, the axial map can show limbus-to-limbus astigmatism versus apical astigmatism, and this can be used to determine whether a toric ortho-k design is needed. The elevation map is an even better map option to utilize for this purpose.
Most topographers allow the practitioner to select a given chord length to determine the elevation difference at that distance. It is best to set the chord length to where the alignment curve of your ortho-k design is; this is typically around 8mm. At that chord, the topographer can tell you the elevation difference between the two principal meridians. If the difference is greater than 25μm, a toric design will typically be needed to have the best success in proper alignment. Manufacturers may have different recommendations, but that is a good rule of thumb.
Difference maps are excellent for post-treatment analysis. These maps take your pretreatment map and subtract the post-treatment map to show the differences that you have created in the corneal surface.
You can use the difference maps for any of the previously mentioned maps. Axial and tangential difference maps are the most commonly utilized for ortho-k. The axial difference map can show the amount of refractive error correction you have achieved post treatment. The tangential difference map is best utilized to show the centration of your design and the peripheral-plus ring around the pupil margin.
Topography is a very useful tool to analyze corneal conditions and provides a plethora of information for ortho-k fitting. Topographers provide different representations of the same cornea depending on the map you view, so it is important to understand the differences and use the appropriate map for each scenario. Get to know your topographer, and it will certainly help you master ortho-k cases. CLS