Corneal Eccentricity and GP Lens Fitting
BY JOHN MARK JACKSON, OD, MS, FAAO
Although you can fit a corneal GP lens using keratometry values on normal eyes, a corneal topographer can make the job faster and more accurate. There are lots of ways to use a topographer to analyze corneal shape and improve lens fitting. For this article, I want to focus on one topography factor that can affect lens fit: eccentricity.
A normal cornea is a close approximation to a prolate ellipse. That is, it gradually flattens across its surface from center to periphery. One common measure of the rate of flattening is the e-value, or eccentricity. Normal corneas typically have an average e-value of about 0.5, ranging between about 0.3 and 0.7 (a higher value equals more peripheral flattening). There are other ways to mathematically describe this change in corneal curvature, the most common being shape factor, but they all describe this same anatomical feature of the cornea.
The Effect of Eccentricity
How will this affect the way that a corneal lens fits? If two corneas have the same K readings, but one cornea flattens more quickly, that cornea will have a lower sagittal height. If you pick the same GP lens to fit on both corneas, it will fit steeper/tighter on the one that has the higher e-value due to this lower corneal sag value.
You will find that selecting a base curve using a table based only on K values, without taking e-value into account, can lead to errors in base curve selection and, consequently, to more chair time.
As an example, Figures 1 and 2 show two corneas with 1.00DC of with-the-rule astigmatism. Simulated fluorescein and tear thicknesses (horizontal meridian) are shown for lenses fit on K. In Figure 1, the e-value is 0.77, and in Figure 2, the e-value is 0.40. Although they are both “on K,” the two patterns are not alike. Figure 1 shows a central tear thickness of about 20 microns, close to ideal. Figure 2 shows less than 10 microns of central tear thickness. Note that the higher e-value cornea, with the lower overall sag depth, yielded a slightly steeper lens fit.
Figure 1. With an e-value of 0.77, the central tear thickness is 20 microns, or close to ideal.
Figure 2. With an e-value of 0.40, the central tear thickness is only 10 microns.
A Different Fit
We can disagree on which lens fits better, but it should be clear that they don’t fit the same. This difference in fit would not have been apparent from just knowing the K values.
By using the simulated fluorescein pattern and tear film thickness, adjustments can be made to the lens to account for these corneal variations before you ever try on a lens. Less chair time should make both you and your patients more satisfied with the process. CLS
Dr. Jackson is a professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.