contact lens case reports
Fitting Piggyback Lenses on Excessively Flat Corneas
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
In a past column (March 2004), we described how to use piggyback lenses to improve lens comfort and to help position GP lenses on a topographically compromised anterior corneal surface. The article described how it's possible to create a new surface to the eye by manipulating the anterior surface power of the soft lens.
A Matter of Comfort
A case in point is patient JS, who underwent bilateral radial keratotomies in 1992. Today, his uncorrected visual acuities are OD 20/25 and OS 20/200. Keratometric readings are OD 40.00 @ 160/40.87 @ 70, with an apical power of 39.25D, and OS 39.87 @ 109/40.62 @ 19, with an apical power of 36.00D.
We failed with a wide variety of GP contact lens designs and therefore decided to try a piggyback lens system.
The top left map in Figure 1 shows the topography of JS's left eye without contact lenses. The top right map shows the topography over a +0.25D Night & Day contact lens (CIBA Vision). Note the change in apical curvature from 36.00D to 37.50D. A GP lens, fit with alignment to the mid-peripheral cornea (BC 43.50D), resulted in a 3.0mm bubble trapped beneath the center of the lens.
Changing the Fitting Surface
We subsequently fit JS with a +6.00D Night & Day lens. Corneal mapping over the lens revealed a central apical radius of 43.25D (Figure 2). We fit an aspheric GP lens to the anterior surface of the soft lens. JS now enjoys all-day lens comfort and 20/25 vision (Figure 3).
Is the Oxygen Enough?
Historically, we've all been concerned with the physiological performance of thicker, plus-power lenses, especially on compromised corneas. However, Kollias and Mutti (2002) found that plus-power (+6.00D) silicone hydrogel lenses induced corneal swelling comparable to that induced by no lens wear.
|Figure 1. Corneal mapping of the patient's left eye without a contact lens (left) and with a +0.25D Night & Day lens (right).|
|Figure 2. Composite corneal topographies (upper left), without a lens (upper right), with a +0.25D lens and (lower center) with a +6.00D lens.|
|Figure 3. The final piggyback lens combination, with an aspheric GP lens over a +6.00D soft lens.|
Patrick Caroline is an associate professor of optometry at Pacific University and is an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences University and serves as an assistant professor of optometry at Pacific University.