Interactive Software Controls
Corneal Shaping

BY SAMI EL HAGE, O.D., PH.D., D.SC.; NORMAN LEACH, O.D., M.S.; JEAN-PHILIPE COLLIAC, M.D.; & XAVIER DEZARD, M.D.
AUG. 1997

Orthokeratology is enjoying renewed popularity with the advent of reverse geometry contact lens designs and refinement in fitting procedures. Controlled kerato-reformation (CKR) is a noninvasive keratorefractive procedure like orthokeratology, but it incorporates computerized corneal topography and a new interactive software program developed by Dr. El Hage to help design the contact lenses. Simply enter the amount of myopia reduction desired, and the fitting program will design the CKR lens based upon the corneal topography measurements.

The display screen shows the overall diameter, the optic zone diameter, the invagination position and height, the anchorage location (where the lens rests on the corneal periphery), the corneal shape factor and the CKR mold shape factor (Fig. 1). With a single keystroke, you can change any parameter to elicit the desired design. You can then e-mail the data file to the laboratory to manufacture the lens.

CLINICAL ASSESSMENT

We observed the refractive progress of 17 patients to whom we dispensed CKR lenses. Patients included six women and 11 men with a mean age of 31.5 years (range of 12 to 46 years). All had good ocular health, less than 3.00D of myopia and 1.50D or less of astigmatism. We divided the patients into three groups based upon their spherical equivalent refractive error, which ranged from ­1.25D to ­3.75D.

We gathered corneal topography data of each patient using the Alcon EyeMap EH-270/290 and applied the data to the CKR nomograms. We ordered and dispensed the resultant CKR lenses and followed each patient for 60 days.

The CKR lens should center with a fluorescein pattern showing central bearing of approximately three millimeters, followed by gradual pooling that tapers to light bearing, followed by minimum edge lift (Fig. 2).

At each follow-up visit, we assessed lens fit and ocular health, measured aided and unaided visual acuity using a standard Snellen chart, and performed corneal topography and a post-wear refraction.

FIG. 2: FLUORESCEIN PATTERN OF CKR LENS DESIGNED WITH THE CKR FITTING NOMOGRAM.

RESULTS

Figures 3 and 4 illustrate average spherical equivalent refractive error and unaided visual acuity for each group. We combined the results for right and left eyes for each group since we found no significant differences between the two eyes within each group (ANOVA, p=0.8549). Corneal topography illustrates that the corneal shape factor shifted from a prolate surface to an oblate ellipsoidal surface as refractive error decreased (Fig. 5).

In each group, the spherical equivalent refractive error was reduced by 50 percent or more from baseline within the first day of wear. The only ocular health problem encountered was early lens adherence in six patients due to the total aspheric back surface of the mold fitting so closely to the peripheral cornea. A change in the edge lift and the anchorage position solved the problem.

These results suggest that the interactive software for CKR nomograms using data from the Alcon EyeMap computerized corneal topographer can produce aspheric contact lens designs that will rapidly reduce low to moderate amounts of myopia. CLS

References are available upon request to the editors. To receive references via fax, call (800) 239-4684 and request document #27.