Ortho-k Fits: The Good, the Bad And the Ugly, Part 2
BY JOHN MARK JACKSON, OD, MS
Understanding reverse geometry lenses require a bit of "reverse thinking." Effective fitting requires troubleshooting skills that are somewhat different from from fitting spherical RGPs. To review, Figures 1 and 2 show an ideal reverse geometry lens fit. Figure 1 shows that the base curve (in red) is flatter than the central cornea. The secondary curve (in blue) "returns" the lens to the cornea, and the alignment or landing zone of the lens (in green) parallels the peripheral cornea, which aids in centration.
|Figure 1. Diagram of a reverse geometry lens with correct sag depth.||Figure 2. Photo of a reverse geometry lens with correct sag depth.|
Figure 2 shows a lens that centers perfectly over the pupil, has a wide area of central touch, a narrow area of pooling under the secondary curve and alignment in the periphery with a small amount of edge lift. The lens sag depth with this fit is close to the sag depth of the cornea, which is the goal for effective treatment.
A Deep Fit
In my last column, we saw a reverse geometry contact lens with a too-shallow sag depth that caused the lens to decenter. Figures 3 and 4 show a lens sag depth that is instead too deep. Notice in Figure 3 that the secondary curve is too steep, causing the lens to vault centrally. Figure 4 shows a reverse geometry lens with excessive sag depth. The small area of central touch and wider area of pooling in the mid-periphery show that the lens is vaulting. An arc-shaped bubble under the secondary zone would also indicate excessive sag depth. Note that the lens centers perfectly when it is vaulting.
|Figure 3. Diagram of a reverse geometry lens with excess sag depth.||Figure 4. Photo of a reverse geometry lens with excess sag depth.|
Deep lenses can result in undertreatment or, even worse, a "central island" topography pattern that indicates central steepening rather than flattening, increasing the myopia. When you observe either of these effects, even if the lens appears to fit correctly, ask the lab to decrease the sag depth of the lens. They usually do this by flattening the secondary curve.
Note that both of the contact lenses shown have identical peripheral alignment patterns. Changing the alignment curves also affects the sag depth of a reverse geometry lens, as I will show the column for the September issue.
Visit www.clspectrum.com for information on the Global Orthokeratology Symposium in Toronto.
Dr. Jackson is a faculty member at the Southern College of Optometry in Memphis, TN. He is currently studying the effects of overnight orthokeratology.