Using CRT to Correct Hyperopia
BY JEROME A. LEGERTON, OD, MS, MBA, FAAO
Corneal refractive therapy (CRT) would have expanded value if we could harness it to treat hyperopia as well as myopia. The segment of consumers who want noninvasive methods of reducing their dependence on spectacle or contact lens use during waking hours includes both patient groups.
Given the desire for a complete solution, you may ask if we can apply CRT to hyperopia. I say the answer is "yes."
Testing a Design
When William Meyers, PhD, and I designed what has become Paragon CRT, we envisioned the flexibility of its Proximity Control Technology to treat myopia and hyperopia with and without astigmatism as well as presbyopia. Like many inventors, we first tried it on ourselves. We treated Bill's distance eye for +1.00D and his near eye for +3.25D and found that we corrected the full amount the first night.
Figure 1. Paragon CRT for hyperopia design with 5mm treatment zone.
Proximity Control Technology vs. Reverse Geometry Paragon CRT is significantly different than reverse geometry and far more flexible. Reverse geometry results in two junctions that may impinge on the cornea. This can help "bite" the cornea for resultant centration, but may result in mechanical epithelial trauma.
That aside, reverse geometry has value in corneal reshaping for myopia. At the same time, it's difficult to comprehend how its steep secondary curve could treat hyperopia. For hyperopia, you could envision a flatter secondary curve with a steeper third curve, but you'd have to manage a sharp first and third junction or somehow blend them. You could then call such a lens "reverse geometry" because the third curve would be steeper than the second.
Designing for Hyperopia
Paragon CRT's Proximity Control Technology uses a junctionless design in which a sigmoid geometry commences at the local slope of the base curve. You can use Proximity Control Technology for hyperopia by making the sigmoid geometry initially convex and then concave as it transitions to the local slope of the landing zone.
For hyperopia, fit the base curve steeper than K, and the initial convex portion of the sigmoidal return zone produces a "knee" to contact the cornea. Outside the knee the lens rises above the cornea and transitions into the landing zone. Select the proper return zone depth (RZD) so the landing zone has clearance just above the cornea to which it is tangent. During treatment the epithelium thins below the knee and becomes thicker under the steeper base curve radius. As the cornea thins under the knee, the landing zone makes full contact with the peripheral cornea.
Initial clinical evidence supports that the amount of thinning under the knee is about equal to the amount of thickening under the apex of the lens. The width of the knee is critical to success. If it's too narrow, then it will become sharp and could cause trauma to the epithelium. If it's too wide, then it may applanate, but the resistance of the wider annulus of cornea may not allow for enough thinning.
Selecting the proper RZD, the primary objective of Proximity Control Technology, is critical. A too-deep RZD will result in under-treatment, as the landing zone will touch before the knee and prevent the cornea from taking the treatment zone's shape. If the RZD is too shallow, then the landing zone won't be in close enough proximity to the cornea and the lens will likely decenter with z-axis tip and tilt.
Determine the landing zone angle (LZA) in the same way as with CRT for myopia to provide the proper edge clearance and resultant optimum tangential bearing width for negative pressure.
Figure 2. Topography before Paragon CRT for
Reducing the Treatment Zone
Early work with laser vision correction indicated the value of a reduced treatment zone diameter for hyperopia compared to myopia. Myopia and hyperopia correction differ in the location of the deepest point of the treatment. For myopia the greatest depth is in the center, and for hyperopia it's at the edge of the treatment. Surgeons selected the deepest portion at 5mm to minimize the total overall width and depth of hyperopia treatment.
Caroline et al (2003) have completed animal studies using the Paragon CRT lenses for hyperopia with the identical design used for myopia, with a treatment zone of 6mm. While the results appear promising for low to moderate hyperopia, a 5mm chord of greatest depth appears optimum (Figure 1). The difference, consistent with the Munnerlyn formula, is a need for a total thickness change of about 8µm/D for the 5mm treatment zone vs. 12µm/D for the 6mm treatment zone.
The early data indicate that thickening of the central cornea accounts for half of the overall change while thinning under the knee accounts for the other half. Before and after topography (Figures 2 and 3) demonstrates the central steepening that provides the correction for hyperopia.
An aspheric base curve may reduce the amount of required thickness while also reducing the natural spherical aberration of the eye. Reducing the treatment zone diameter to 5mm also reduces the apical clearance at dispensing (maximum of 48µm for a lens treating +6.00D of hyperopia).
Fitting CRT for Hyperopia
In CRT for hyperopia, a ring touch occurs at the convex portion of the sigmoid return zone (the knee). The negative pressure under the tangential interface of the landing zone further complements the centration that the ring touch provides.
The clinical methods appear straightforward and similar to those for myopia, with the same three objectives:
1. Select a base curve radius to achieve the treatment
2. Select the proper RZD for proper sagittal proximity of the knee and the landing zone
3. Select the proper LZA for tangency and edge clearance
Determine the base curve by adding the desired treatment to the flat K value plus an additional 0.50D. Don't adjust the base curve to control the fit.
Figure 3. Difference plot after single exposure of Paragon CRT for
Select the RZD by observing the fluorescein pattern. If the lens decenters and has too much clearance under the landing zone, then increase the RZD. If the lens has clearance under the knee of the lens, then decrease the RZD. The same general rule as in CRT for myopia appears to hold -- fit the shallowest RZD that demonstrates centration with a light bearing under the landing zone.
A proper LZA exhibits no less than 0.2mm and no more than 0.6mm of edge clearance as measured inward from the lens edge.
Where Do We Go from Here?
It appears that the current Paragon CRT design with the 6mm treatment zone may correct low to moderate hyperopia with the appropriate steeper base curves and decreased RZD parameters. Ultimately, you may need a separate set of lenses with a 5mm treatment zone diameter.
More clinical evidence will help us to better understand the use of the Paragon CRT design for correcting hyperopia. A protocol controlled multisite clinical trial will prove valuable in profiling CRT for treating hyperopia and will provide the basis for product labeling and FDA marketing clearance.
For references, please visit http://www.clspectrum.com/references.asp and click on document #106.
Dr. Legerton has practiced in San Diego for 26 years. He served as director of Clinical Research for Pilkington Barnes Hind and as a consultant to VISX and Paragon Vision Sciences. He has six patents pending for corneal refractive therapy and high-Dk hybrid lenses.