Contact Lens Spectrum

What Makes CRT Different?
Corneal refractive therapy is not the orthokeratology of old. Advances in technology, materials and science set CRT apart.
By Patrick Caroline, FCLSA, FAAO

Visual scientists have long struggled to provide patients with stable, uncorrected visual acuity throughout the day. Advances in refractive surgery have clearly shown that corneal reshaping is possible. However, the permanency of these procedures (especially in light of an undesirable outcome) has been a major concern for practitioners and patients alike. Therefore, a non-surgical modality which can provide stable and reversible vision correction has been an important goal of the eyecare industry.

Our understanding of corneal reshaping with contact lenses has steadily increased, culminating in the birth of modern corneal refractive therapy or CRT. The Paragon CRT lens is the result of years of ongoing research into the science of overnight corneal reshaping with RGP lenses. This has been made possible by five new technologies:


Figure 1. Zones of the Paragon CRT lens.

The Paragon CRT lens consists of three primary zones (Figure 1). First, a central base curve radius provides the "mold" for the final corneal shape necessary to correct the refractive error. This flatter radius of curvature is instrumental in creating the forces beneath the lens to facilitate the movement of corneal tissue. Second, adjacent to the base curve is a patented sigmoid-shaped curve that controls the amount of lens clearance across the central cornea. A shallower sigmoid in the return zone brings the base curve into closer apposition to the cornea, where a deeper sigmoid in the return zone results in greater apical clearance. Third, a unique tangent (straight line) angle provides an appropriate landing zone in the peripheral cornea. The tangent angle terminates in a controlled edge ellipse designed to maximize patient comfort.


	Figure 2. Basal cells attach to Bowman's layer with the mobile anterior wing and surface cells.

The Paragon CRT Lens consists of a series of curves and angles designed to optimize the controlled redistribution of corneal tissue during overnight lens wear. Recent studies by Swarbrick in 1998 and by Alharbi in 2001 have shown that the topographical changes taking place beneath corneal reshaping lenses are predominately epithelial. Their results can be explained by the fact that the human epithelium is about 50 microns thick and that half the thickness is occupied by the basal cells which remain immobile and firmly attached to Bowman's layer by a series of anchors (hemidesmosomes) (Figure 2).

This indicates that between 20 to 25 microns of mobile wing and surface cells are available for redistribution. The controlled redistribution of epithelium can be indirectly visualized by viewing the difference display map on a topographer (Figure 3). This display allows the practitioner to compare the pre-fitting topography with the post-fitting topography while a third map (lower map) shows the difference, clearly illustrating the flattening of the central cornea and subsequent steepening of the mid-peripheral cornea.


Figure 3. A difference map shows tissue distribution.	Figure 4. Parameters to correct the 4.25D refractive error.

Final Beta testing is nearly completed on the new Paragon CRT software designed exclusively for use with the Humphrey Atlas corneal topographer. The software will use the Atlas elevation data to more precisely select the initial diagnostic lens. Additionally, post-fitting topographic analysis continues to be the best means of identifying the exact position of the lenses in the overnight, closed eye environment.

Paragon CRT lenses, manufactured in Paragon HDS 100, are best fitted from an inventory system which allows lenses to be dispensed to the patient at the initial visit. The inventory significantly reduces chair time and allows the patient to begin his refractive therapy the same day. The actual lens fitting is a simple three-step process.

Step 1 Base Curve Selection Keratometric readings are obtained manually or from the simulated Ks from the corneal topographer. The flat K and the spherical component of the refractive error (vertexed to the plane of the cornea) are cross-referenced with the Paragon CRT Initial Lens Selector. The Initial Lens Selector then determines

If the flat K is 44.12 diopters and the vertexed spherical power to correct is 4.25D sphere, the Initial Lens Selector selects a lens with the base curve of 8.60mm (39.25 diopters) that would be required to correct the refractive error. The Lens Selector has determined the initial diagnostic lens to place on the eye. The parameters are shown in Figure 4.


Figure 5. This lens provides acceptable centration and touch.

Step 2 The Initial Diagnostic Lens The 8.60mm base curve lens with a return zone depth of 0.550 and a landing zone angle of 33 is placed on the eye and evaluated with fluorescein. The initial diagnostic lens is acceptable if the lens provides centration over the pupil, a 4mm area of central treatment and adequate touch in the peripheral cornea (Figure 5).

If the initial diagnostic lens decenters, the overall sagittal height of the lens is inadequate, and the lens is resting too heavily on the apex of the cornea. Apply a lens with the next greater sagittal height (RZD 0.575) and evaluate for centration (Figure 6).

If the initial diagnostic lens exhibits excessive apical clearance, inadequate treatment will result. In this case the return zone depth can be decreased 0.525, bringing the base curve into closer apposition to the cornea (Figure 7).

Step 3 Evaluation of the Peripheral Landing Zone Angle As previously stated, the Paragon CRT lens incorporates a tangent landing zone in the lens periphery. The landing zone provides a gentle landing touch to facilitate lens movement and centration on the eye. The angles within the diagnostic set are provided in varying degrees (Figure 8). Once the appropriate return zone depth has been established, the landing zone angle can be easily evaluated with fluorescein (Figure 9).


Figure 6. The next greater saggital height helps centration.	Figure 7. Decrease the return zone depth to bring the base curve in.

Figure 8. Landing zone angles can vary to facilitate edge lift and centration on the eye.	Figure 9. Landing zone angles and edge lift are easily evaluated with fluorescein.

The Paragon CRT lens is the only lens that is FDA approved for overnight corneal refractive therapy. The ultimate goal of the procedure is to provide the patient with safe, comfortable overnight therapy so that upon awakening, the patient can remove the lenses and enjoy the benefits of uncorrected vision throughout the day.


	Figure 10. Example of an optimum topography map post-treatment.

Most patients do extremely well with overnight lens wear without prior contact lens adaptation experience.

The steps I follow in instructing patients on overnight lens wear are as follows:


Figure 11. Attempted correction vs. achieved correction.

A total of 205 patients from 11 sites participated in the FDA clinical evaluation on the safety and efficacy of Paragon CRT for overnight therapy. Subjects were fitted with Paragon CRT lenses in both Paragon HDS and Paragon HDS 100 materials and monitored for nine months. The pre-treatment manifest refraction sphere powers were as follows:

Figure 11 shows a scattergram of achieved correction (at the end of the day) vs. attempted correction. The middle bar represents the target prescription of plano, the top bar represents an over correction of 1.00D and the lower bar an under correction of 1.00D.

At the conclusion of the study, 67.4 percent of the subjects had visual acuity of 20/20 or better, 93.3 percent had 20/32 or better, and 94.4 percent had 20/40 or better. Additionally, no adverse events were reported during the study. These data established the safety and efficacy of the Paragon CRT system.

Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.