orthokeratology today
Ortho-K Fits: The Good, the Bad And the Ugly, Part 3A
BY JOHN MARK JACKSON, OD, MS

Figure 1. Diagram of flat alignment zone on RG lens.
Figure 2. Photo of flat alignment zone on RG lens.

As noted in Parts 1 and 2, achieving an acceptable fit with a reverse geometry (RG) lens design can be a little challenging without understanding the fitting characteristics of this type of lens.

Previous columns showed examples of good fits and what can happen when the secondary ("steeper") curve of the lens is too flat or too steep, causing the sag depth to be too shallow or too deep, respectively. In this month's column, we'll focus on how the third zone of the lens, often called the "alignment curve," affects lens performance.

The Ideal Fit

To obtain an ideal fit, the alignment curve must closely match the peripheral cornea. This provides a stable anchor for the lens and facilitates lens centration, which is essential for a successful fit of a reverse geometry lens. A lens with the correct alignment curve will show good centration, 360-degree touch in the mid-periphery of the lens and about 0.25mm of fluorescein under the edge of the lens.

Figures 1 and 2 show the effect of a flat alignment curve. The diagram illustrates how the lens is lifted away from the cornea in the periphery (the green area), rather than aligning with it. This can be seen by the excess amount of edge clearance in the photo. Because the periphery of the reverse geometry lens is lifted away from the cornea, the lens becomes unstable, causing the lens to decenter upward. Although the center of the lens shows a broad area of touch and good pooling under the reverse curve, the incorrect periphery results in a poor fit. To correct this problem, make the alignment curve steeper by 0.50D to 1.00D.

Figures 3 and 4 show the effect of a steep alignment curve. The diagram shows that a too-steep alignment curve causes the lens to "dig in" to the periphery. This creates edge seal-off and lifts the lens away from the cornea centrally, causing a too-deep sag depth. The photo shows the result of this steep periphery. Note that there is almost no edge clearance; all the fluorescein is pooling beyond the edge, not underneath it. Also note the bubble in the secondary zone of the lens; this indicates subtle bridging of the lens from the steep periphery. To correct this problem, flatten the alignment curve by 0.50D to 1.00D.

Figure 3. Diagram of steep alignment zone on RG lens.	Figure 4. Photo of steep alignment zone on RG lens

Dr. Jackson is an assistant professor at the Southern College of Optometry in Memphis, TN. He is currently studying the effects of overnight orthokeratology.