Taking the Mystery Out of Orthokeratology

BY EDWARD S. BENNETT, O.D., M.S. ED.
APR. 1997

A desire for alternatives to refractive surgery for reducing myopia, coupled with the increasing availability of corneal topography instrumentation and the introduction of reverse geometry lenses has sparked renewed interest in orthokeratology.

TYPICAL CANDIDATES

Certainly, motivation is an important factor when identifying candidates for ortho-k. Some people may want to meet certain occupational vision standards, while parents may wish to reduce the amount of myopia their children develop. In all cases, it's important to provide a realistic overview of expectations regarding how much myopia can potentially be reduced (one to two diopters) and the time frame over which this reduction will occur.

CORNEAL TOPOGRAPHY ADDS DIMENSION

Sophisticated corneal topography instrumentation enables us to better understand corneal contour changes in ortho-k, particularly the rate of flattening from the center to the periphery of the cornea (corneal eccentricity).

In ortho-k, the central cornea flattens as the peripheral cornea steepens. This decreases the rate of peripheral corneal flattening so that the corneal eccentricity decreases and approaches zero (a sphere). Current research shows promise for determining the ortho-k effect from corneal eccentricity values. Optometrist John Mountford of Brisbane, Australia, describes the relationship as: y = 0.20x, where y is corneal eccentricity and x is the refractive change in diopters. Average corneal eccentricity is approximately 0.5, thus the potential reduction in refractive error to obtain a spherical cornea would be 2.50D. For high corneal eccentricity, such as 0.6, the potential reduction would be 3.00D, and for low eccentricity such as 0.4, it would be 2.00D.

NEW LENSES FOR FASTER RESULTS

Traditional ortho-k, which can take up to 18 months for maximum myopia reduction, may soon be supplanted by an accelerated version using reverse geometry lenses that can reduce treatment time to as little as three months.

Reverse geometry lenses have aspheric peripheries or peripheral curve systems that are as much as seven diopters steeper than the base curve. These lenses allow for a flatter lens-to-cornea fitting relationship, with an initial base curve equal to 1.50D to 2.00D flatter than K. This design achieves a 4mm to 5mm area of central bearing, a 2mm to 3mm band of mid-peripheral tear pooling, and up to 2mm of lens movement. As the central cornea flattens, the corneal contour shifts into the steeper secondary curve region of the lens, thus enhancing lens centration.

To ensure that the pressure is evenly distributed on the central cornea, reverse geometry lenses typically have small optic zone diameters (i.e., 6.0-7.0mm). Nevertheless, patients typically report exceptionally good vision. This may be attributed to the flatter fitting relationship, or to the fact that the posterior curves of the lenses are computer lathe-cut from center to edge, requiring no polishing and improving optical quality.

With accelerated ortho-k, refractive changes may occur as quickly as a few days. Each subsequent lens should have a base curve a quarter- to a half-diopter flatter than the previous lens. When no further change can be induced or topography shows a uniform, spherical corneal contour, the patient is ready for the final phase, retainer lens wear.

The retainer lens can be the last ortho-k lens or a lens with a slightly flatter base curve, worn either a few hours during the day or overnight. Overnight therapy, for both the ortho-k process and retainer lens wear, eliminates rigid lens problems such as environmental irritants, wind and dust. Use a high Dk lens material for overnight therapy. If you use this same material for the ortho-k process, expect more lens changes.

A NEW UNDERSTANDING

Orthokeratology fitting philosophies have varied over the years and reverse geometry lenses are allowing for a more rapid effect with greater control. The key to successful orthokeratology is recognizing its limitations in refractive error reduction and using corneal topography to understand the changes that occur. CLS

Drs. Harue Marsden and Rodger Kame, experts on contact lens application in orthokeratology, provided information for this column.

Dr. Bennett is an associate professor of optometry at the University of
Missouri-St. Louis. He is executive director of the RGP Lens Institute.