EDUCATION SPOTLIGHT:
"Topography-Guided Troubleshooting
for Orthokeratology"
=====Anita Gulmiri, OD, FAAO
Dr. Gulmiri had the following clinical pearls:
1) The use of corneal topography is essential for fitting, monitoring, and troubleshooting orthokeratology patients.
2) Likewise, hyper Dk lens materials are necessary due to the overnight retainer wear involved.
3) The lens design typically consists of a base curve, a reverse curve, 1 – 2 alignment curves, and a peripheral curve which contribute to a rapid reduction in myopia and – quite often – meeting the ultimate goal of slight hyperopia with only one pair of lenses.
4) FDA approval is present for up to 6D of myopia and 1.75D of astigmatism for the CRT lenses and -1.00 to -5.00D and up to 1.50D of astigmatism for the Vision Shaping Treatment (VST) lenses.
5) The base curve is selected to be flatter than the flat K reading by the amount of the spherical refractive error plus an additional 0.50 – 1D to account for daytime regression (i.e., if the K values are 43/43.50 @ 090, and refraction is -2.00 – 0.50 x 180, you would select a BCR equal to 2 + 0.75D or 2.75 flatter than 43 or 40.25D).
6) These lenses can typically be fit empirically and some have online calculators to arrive at the recommended lens design parameters.
7) The optical or treatment zone averages around 6mm but in higher amounts of myopia a smaller zone is recommended.
8) Ideal candidates include topography readings between 42 – 46D, low amounts of WTR astigmatism, symmetrical astigmatism, and no topographical corneal irregularities.
9) The difference map (i.e., comparing baseline visit to findings after overnight lens wear) is very important to assess. The ideal map after lens wear consists of a “bulls-eye” with central flattening (i.e., blue or blue-green in appearance) and mid-peripheral steepening (i.e., red appearance).
10) A superior decentered or flat-fitting lens will result in a topography map with a superior flattened region with an inferior arcuate steep pattern (i.e., “Smiley Face”). Increasing the sagittal depth as recommended by the manufacturer will often solve the problem.
11) An inferior decentering lens will often result in the opposite problem (i.e., “frowney face”) which can often be solved by flattening the lens/decreasing sagittal depth as recommended by the manufacturer. Likewise, if the topography pattern shows incomplete flattening/treatment centrally, decreasing sagittal depth is recommended.
12) Toric designs are available for astigmatic patients in which the lens is decentering, poor midperipheral alignment exists, or an incomplete treatment ring is shown via topography. Often this is with patients who have greater than 1.50D of corneal astigmatism, limbus-to-limbus astigmatism, or a greater than 30 micron elevation difference over a 8mm chord of the central-mid-peripheral cornea.
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