Mapping the Future of Contact Lenses

BY LORETTA SZCZOTKA, O.D., M.S.; KENNETH A. LEBOW, O.D.; PATRICK CAROLINE, C.O.T.; & CRISTINA SCHNIDER, O.D., M.SC.
MAR. 1996

A corneal topographer may be the diagnostic troubleshooter you need to drive your contact lens practice into the next generation.

Corneal topographers: They're not just for research anymore. From a practice management standpoint, a corneal topographer just may be the best new tool a contact lens practice can use. Many practitioners are finding them useful in fitting contact lenses, diagnosing disease and determining causes of decreased vision.

FITTING CONTACT LENSES

Long a standard instrument for the refractive surgeon, the corneal topographer is gaining ground among contact lens practitioners as well.

A corneal map can assist with fitting RGP lenses by:

• locating and determining the curvature of the corneal apex;
• indicating the approximate range of change in curvature of the cornea across the analyzed map area;
• determining the degree and axis of astigmatism;
• measuring the amount of irregularity;
• monitoring changes after contact lens fitting;
• determining the initial lens in a trial lens or empirical fitting; and
• explaining complications such as corneal irregularity to patients.

As a rule, topography is used for fitting RGPs but it can also help determine the cause of reduced visual performance of toric and spherical soft lenses. For example, a seemingly normal slit lamp evaluation may prompt you to prescribe a soft lens, but the hint of a corneal irregularity that may only be picked up by a topographer would rule out that possibility. In other instances, a topographical map taken while a soft lens is on the eye may determine whether the lens is draping the cornea properly. In soft toric applications, a topographer is useful to ensure that the lens adequately neutralizes the astigmatism obtaining a spherical result.

Corneal topographers present some limitations, however. Data can be unreliable and there are limits to the area of the peripheral cornea that can be measured.

DIAGNOSING DISEASE

Any disease that alters the shape of the cornea's curvature, such as keratoconus or lattice dystrophy, can be viewed with a corneal topographer. Typically, there are three methods of diagnosing disease. First, most topographers have built-in screening programs that analyze top and bottom, right and left asymmetry. This helps the practitioner evaluate the likelihood of disease and may even reveal which disease is present. The second method is via potential acuity, which examines local distortions to determine if the cornea is irregular. The third method uses the Placido image. When a disease or corneal irregularity is present, distortions in the rings (which are pictured as bent or wobbly lines) are present. The tangential (or instantaneous) curvature is more widely used here as it gives us a more powerful tool to diagnose corneal distortion and keratoconic readings.

These methods are also useful to determine other causes of decreased vision. Hypoxic conditions, a poorly molding lens, corneal warpage, distortions associated with compression rings caused by lens adhesion or corneal changes caused by jelly bump deposits (Fig. 1) may all be easily viewed on a corneal map.

FIG. 1: DISTORTED CORNEAL TOPOGRAPHY DUE TO MODERATE JELLY BUMP DEPOSITS ON THE SURFACE OF A HYDROGEL CONTACT LENS.

LOCAL VS. AXIAL SCALES

One of the most important features to look for in a corneal topographer is the ability to read both local and axial scales. Both scales have their advantages and it's important to know which scale you're reading so that you can interpret the map accurately.

The local scale is interpreted differently by each manufacturer so it's important to know the various terms and definitions. For example, Alliance's Keratron calls local curvature an instantaneous curvature, EyeSys and Humphrey Instruments refer to it as tangential curvature. Basically, the difference in terminology lies in how each machine calculates the radius of curvature.

The local scale is an ideal measurement for fitting the irregular cornea. It reveals more local irregularities and better estimates asphericity, whereas the axial (or sagittal) scale implies the cornea is more spherical at every point (Figs. 2a & 2b).

FIG. 2A: THIS CORNEAL POWER MAP ILLUSTRATES A KERATOCONIC CORNEA AS SHOWN BY A SAGITTAL OR AXIAL SCALE.

FIG. 2B: THE SAME KERATOCONIC CORNEA AS SHOWN AT LEFT IN TANGENTIAL SCALE. PURCHASING YOUR OWN CORNEAL TOPOGRAPHER

The axial scale works well in the central portion of the cornea to give you a refractive measurement or contact lens power. Since some topographers convert this reading to calculate the tangential curvature, some manufacturers believe the tangential curvature is not a real curvature because of errors in the axial reading.

Differentiating between other features such as differential maps and comparative maps can also be confusing. A differential map calculates the difference between two separate maps to derive a single map. This may be called subtractive analysis. In other words, if you took a map of a patient with early signs of keratoconus at 6 months, and then took another map of the same patient at 12 months, you could obtain a map that plots the increase (or the difference) in keratoconus over that 6-month period.

A comparative map allows you to visualize that increase over two or more individual maps. You could take a map of a keratoconus patient at 6 months, 12 months, 18 months and 22 months and analyze each map on the same screen. Or you could analyze two or more fixation angles from the same patient on the same screen to gain a broader perspective of the patient's cornea.

THE PRACTICE BENEFIT

More and more optometrists are using corneal topographers to evaluate both soft and hard contact lens wearers. This increased usage can often translate to as many as 15 patients per day, making the corneal topographer a standard of care when it comes to measuring the cornea. We expect the future to bring us lower prices and improved models to help the busy practice fit every contact lens patient via computer. With less chair time and discomfort for the patient, we can broaden our patient base and improve turnover for our practices, which will be essential with increased managed care programs. CLS

Dr. Szczotka is an assistant professor at Case Western Reserve Univ. Dept of Ophthalmology, Cleveland, Ohio; Dr. Lebow is in private practice in southern Virginia, and is a clinical research consultant at the New England College of Optometry; Mr. Caroline is director of contact lens research at Oregon Health Sciences Univ., Portland, Ore.; Dr. Schnider is director of professional relations, clinical affairs at Menicon U.S.A., Inc., in Clovis, Calif.