Computer-Aided Contact Lens Fitting With the Corneal Topographer

BY MARJORIE JEANDERVIN, OD, MS
MARCH 1998

While corneal topography software programs can expedite the RGP fitting process, there's still no substitute for a knowledgeable practitioner.

Rigid gas permeable contact lenses deliver more oxygen to the cornea, are more durable, are easier to care for and often provide clearer vision than soft contact lenses. However, efficient RGP contact lens fitting with videokeratoscopy can be a time-consuming task, especially for the inexperienced practitioner. RGPs may also require more frequent and rigorous follow-up care during the initial fitting process. Therefore, manufacturers of computerized videokeratography systems have continued their attempts to perfect contact lens fitting modules to aid in the lens designing and ordering process.

Computerized videokeratography systems are now available on the market, and most are equipped with the option of purchasing a contact lens fitting program. Studies have estimated a 50 percent reduction in chair time with the use of contact lens fitting programs, but a knowledge of current contact lens fitting methods is still required to provide quality patient care.

Become An Educated Buyer

Before purchasing a corneal topographer, research the various systems available to determine which instrument best fits the needs of your office. The best way to learn about the most current instruments on the market is to casually wander around a conference exhibit hall, talking to various company representatives and test-driving each model. Once you've found the model that is most user-friendly and most applicable to the needs of your office, ask the manufacturer for an in-office demonstration.

After you have selected the topographer and you and your staff have become familiar with the basic features of corneal topography and map interpretation, then your office can maximize the utility of this new technology.

Basic Features

The first step of RGP contact lens fitting with the corneal topographer is obtaining a good refraction and a topographic map of the patient's cornea. Most programs allow you to set certain default parameters such as: lens design, diameter, cylinder convention and your fitting method (on flat K, etc.) You can then store these personal nomograms in the instrument and use them each time you fit a lens.

Generally, the most common rigid gas permeable manufacturers and materials are listed in the contact lens module. You may select a specific material for the lens fitting, but in some instruments this may override your custom design parameters. When you select a specific contact lens material, the instrument typically calculates lens parameters using the manufacturer's nomogram. In some cases, this results in the use of data from only the central three millimeters of the cornea, even though computerized corneal topography is capable of using vastly more corneal data points than traditional keratometry to design the lens. Check with the contact lens manufacturer to determine what data is used in the lens design for a specific corneal topography system.

Most contact lens fitting programs have some basic features in common. Once you enter the patient's refractive error into the fitting program, the instrument can calculate the design. The computer will generate a listing of the base curve radius, peripheral curve radii, curve widths, lens diameter, optic zone diameter and lens power. Some systems also display edge clearance and axial edge lift. Edge clearance is a measurement of the perpendicular distance from the lens edge to the cornea, and axial edge lift is the distance from the lens edge to an extension of the base curve. Edge clearance changes from patient to patient and axial edge lift is dependent on the design of the contact lens .

After the lens calculation is complete, you can make changes to any of the parameters. The program will then recalculate the remaining parameters to compensate for the change. Many programs will also provide warnings or guidelines related to the contact lens fit.

Simulated Fluorescein

Based on the contact lens design generated, the topographer will display a fluorescein pattern simulation, which illustrates the expected fitting relationship of the lens to the cornea (Fig. 1).

If you make changes to the contact lens parameters, the program will generate a new fluorescein pattern upon recalculation of the parameters. Be careful to note the diameter of the lens. Most instruments will also provide a predicted level of fluorescein for each point under the lens. In addition to the simulated fluorescein pattern, lid and lens position may also be adjustable on a few of the available programs.

Once you finalize the contact lens specifications, the final fitting parameters can be displayed on the screen in a format similar to an order form. You can then print this form and place it in the patient's file. With some instruments, you can also e-mail the order form directly to the laboratory. Laboratory information can be stored in the computer for maximum efficiency.

Reaffirming Your Role in Fitting Success

In order to demonstrate the differences in contact lens designs computed by a few of the various videokeratographers available, Tables 1 through 3 display rigid gas permeable lenses designed on three different instruments for three patients. The Alcon EyeMap EH-290, the EyeSys System 2000 and the Humphrey MasterVue Corneal Topography Systems are compared to each other and to results based on traditional trial lens fitting. The contact lenses were designed using each system's default settings, rather than customized nomograms. For the EyeSys System 2000, which requires that a lens diameter be specified, I selected the lens diameter used in the trial lens fitting. Keep in mind that the instruments used at the time of these lens design calculations may no longer be the most current software programs available.

Not surprisingly, all of the contact lens designs calculated were different. Of interest are the differences in power of the lenses even when the base curves are similar. There was always at least a range of one diopter or more in power even when the range in base curves reflected only half that amount. For this reason, I recommend that you pay special attention when finalizing the contact lens power prior to ordering. With the possible exclusion of the lens power, all of these lenses appear to provide an adequate lens design for the patient. However, the only way to be certain is to observe the lens on the patient's eye.

Topography can serve as a guideline for choosing an initial trial lens, or it can be used as a tool for ordering lenses empirically. Regardless of the method you choose, keep in mind that the only way to assess the dynamics of the lens on the eye is to actually place the lens on the eye. A computer can attempt to simulate contact lens dynamics and fluorescein patterns, but it cannot completely predict centration and lid dynamics. At least for now, the computer cannot replace the practitioner, but it may make the practitioner more efficient.

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Dr. Jeandervin is a graduate student at The Ohio State University. She is currently studying the effects of near work on myopia