contact lens case reports
Elevating Our Knowledge
Of the Corneal Surface
BY PATRICK J. CAROLINE, FAAO,
FCLSA, & MARK P. ANDRÉ, FAAO, FCLSA
Elevation maps describe the height (elevation) of the cornea in microns from a reference surface commonly called the "reference sphere" (Figure 1). It's similar to viewing the fluorescein pattern
of a spherical rigid gas permeable lens in that areas of the cornea that fall below the best fit sphere (fluorescein pooling) are noted in negative microns and correspond to the blue or cooler colors of the map. Areas of the cornea higher than the reference sphere (corneal thinning) are noted in positive microns and correspond to the red or hotter colors of the map.
Figure 1. The reference sphere superimposed through the cornea.
This can create some confusion in that practitioners are used to axial maps in which the hotter colors are steeper and the cooler colors are flatter. Elevation maps measure the difference between the modal (reference sphere) and the actual corneal surface in microns of height, not diopters of curvature.
|.Figure 2. Axial map OS
||Figure 3. Elevation map OS
Patient RS has mild keratoconus OS, Ks 44.25 @ 150 / 46.00 @ 60. Figure 2 shows the axial map while Figure 3 shows the elevation map with a reference sphere of 45.00 diopters. In Figure 4, the plus numbers indicate corneal areas higher than the 45.00 diopter curve, the minus numbers below 45.00 diopters. An actual lens of 45.00 diopters (Figure 5) shows touch on the plus and clearance over the negative values. The fluorescein pattern illustrates that the elevation map is more accurate in predicting the lens-to-cornea fitting relationship.
|Figure 4. Plus numbers indicate areas above the reference sphere, negative numbers below.
||Figure 5. Actual fluorescein pattern base curve, 45.00
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences
Contact Lens Spectrum, Issue: April 2001