orthokeratology today

Ortho-k Fits: The Good, the Bad And the Ugly, Part I

orthokeratology today
Ortho-k Fits: The Good, the Bad And the Ugly, Part I

Understanding reverse-geometry lenses requires a bit of "reverse thinking." Figure 1 shows a schematic view of a reverse geometry lens on a cornea. This "bottle-cap" or "top hat" diagram is commonly used as an aid to understand the nature of this design. The lens is designed with the base curve (red in the diagram) flatter than the cornea, usually by an amount equal to the myopic error plus an extra 0.50D to 1.00D. This causes the base curve to rapidly "fly away" from the cornea as we approach the edge of the optic zone. The steeper secondary curve (shown in blue) "returns" the lens to the cornea, and if designed correctly, will bring the lens just to the cornea but no further. The alignment or landing zone of the lens (shown in green) is flatter than the secondary zone and parallels the peripheral cornea, which aids in centration.

Figure 1: Diagram of RG lens with correct sag depth.

Figure 2: Photo of RG lens with correct sag depth

The Ideal Fit

Figures 1 and 2 show an ideal fit. Figure1 shows the lens just returning to the cornea at the outer edge of the secondary zone, and the alignment/landing zone of the lens is parallel to the cornea. In Figure 2, note the wide area of central touch, followed by pooling in the secondary zone and alignment in the periphery. When this type of fit is achieved, the sagittal height of the lens is close to the sagittal height of the cornea. This is the goal for effective treatment.

Figure 3: Diagram of RG lens with shallow sag depth

Figure 4: Photo of RG lens with shallow sag depth

A Shallow Fit

Figures 3 and 4 show a poor fit. Note that in the diagram, the secondary zone does not make it to the cornea, leaving a gap. This failure creates a sag depth that is shallower than that of the cornea, which allows the lens to tilt and decenter. Figure 4 shows a high-riding lens with a very broad area of central touch, indicating a too-shallow sag depth and excessive pressure centrally. This will lead to induced astigmatism and "smiley-face" topography with a decentered treatment area. To fix the problem, have the lab increase the sag depth by steepening the secondary curve. It is important to note that the lenses have identical peripheral alignment. Changing the alignment curve will also affect the sag depth and the overall fit.

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Dr. Jackson is a faculty member at the Southern College of Optometry in Memphis, TN. He is currently studying the effects of overnight orthokeratology.