Article Date: 10/1/2008

Map Out Your Lens Fitting
topography

Map Out Your Lens Fitting

Increase the satisfaction of your challenging-to-fit contact lens patients by using a corneal topographer.

DIANNE M. ANDERSON, O.D., F.A.A.O., AURORA, ILL.

A financially thriving practice is comprised of satisfied patients who sing your praises to their friends and family. I've been able to instill greater patient satisfaction than before in my difficult-to-fit contact lens patients by employing a corneal topographer.

Here, I discuss how the device's basic and advanced features have enabled me to enhance the lens fitting process.

Axial/sagittal map

Using the axial/sagittal map is an excellent way to qualify and quantify astigmatism. This is because it shows you the exact amount and position of astigmatism on the cornea. The display of the different forms of astigmatism:

Apical. This appears as more centrally distributed at the pupil area. Because these corneas exhibit central toricity with a spherical periphery, they tend to achieve success in aspheric lens designs, which are steep in the center and flatten toward the periphery.

Limbal-to-limbal. This type of astigmatism extends the entire length of the cornea along the steep meridian. As a result, these patients usually require toric peripheral curved or bitoric lenses to best match their peripheral corneal cylinder.

With-the-rule (WTR). This displays as steep along the vertical meridian. The fitting process of these patients is facilitated by the fact that a contact lens moves vertically with each blink.

Against-the-rule (ATR). This appears as steep along the horizontal meridian. The ATR patient will experience visual fluctuations, especially if the lens' base curve is too flat.

Oblique. This form of astigmatism appears as steep along the oblique meridians of the cornea between 30° to 60° and 120° to 150°. Because a soft toric lens tends to rotate toward the steep meridian of the cornea, achieving lens stability on these corneas is often a challenge.

Irregular. This presents as an asymmetric distribution of corneal cylinder. Common examples are keratoconus, pellucid marginal degeneration (PMD), Terrien's marginal degeneration, post-corneal trauma, post-corneal transplant and post-LASIK ectasia. Providing an acceptable spectacle prescription is often difficult in these cases due to the large-amount of cylinder, which induces a great deal of aberration in ophthalmic lenses. I explain to these patients how the information from the axial map will, however, enable me to customize a contact lens fit.

Without this map, you have no way of differentiating the level of fitting difficulty that lies ahead. This could place you and the patient on an unnecessarily long road.

Tangential/instantant Map

The tangential map is very sensitive to curvature changes on the corneal surface and displays small distinct areas of curvature with great detail. This is helpful in locating the exact location and size of the apex in keratoconus. In addition, it's helpful in defining the location of the treatment or ablation zone in orthokeratology and refractive surgery cases.

Refractive power map

This map enables you to identify the location and dioptric effect of the treatment zone in cases of post-refractive surgery and orthokeratology — information that will allow you to troubleshoot prior to lens fitting.

For instance, if the treatment zone is decentered off the pupil, the patient is likely experiencing haloes and glare. Further, if the dioptric effect is too low or too high, the patient maybe under- or overcorrected.

Elevation map

The elevation map provides you with an idea as to how a spherical RGP will fit the cornea. Specifically, it displays the difference in height of the cornea in microns from a reference sphere or best-fit sphere (based on the average curvature for that particular cornea), in millimeters of radius.

Red areas (positive micro-meter values) indicate that the cornea is higher than the reference sphere, and blue areas (negative micrometer values) indicate that the cornea is below the reference sphere. Red represents areas of contact lens bearing or touch, and blue represents areas of bubbles or pooling beneath the lens.

I find this map especially helpful for designing RGPs in cases of high corneal toricity. This is because looking at the extent of the peripheral elevation enables me to determine whether I should fit a bitoric or toric peripheral lens (see figure 1).

Figure 1: Here's an elevation map and Simulated NaFl pattern. Notice how the elevation map corresponds with the simulated NaFL pattern to indicate the need for a bitoric RGP design.

Shape factor

The corneal topographer's shape factor quantifies the asphericity of the cornea via a number, helping you determine whether a keratoconic or reverse geometry lens is best. The larger the number, the greater the asphericity. Prolate corneas provide a positive shape factor value, and oblate corneas provide a negative shape factor value.

Two clinical pearls to keep in mind:

• Because keratoconic corneas display high-positive (greater than 0.7) shape factor values, you should fit these patients in a highly aspheric back surface keratoconic lens design.

• Patients who have negative shape factor values — characteristic of post-refractive surgery and cases of post-corneal transplant — achieve success in reverse geometry lenses.

Sagittal height value

Corneal sagittal height (z-value), which isn't available on all topographers, is a measurement in millimeters or microns of the distance between the geometric center of the cornea and the intersection of a specified chord length (y-value). Because reverse geometry and scleral lenses — fits based on corneal vault rather than base curve — have a specified chord length and sagittal value that varies with the base curve and reverse curve values, this measurement is very helpful when fitting either lens on post-surgical corneas (see figure 2).


IMAGE COURTESY OF PRECISION TECHNOLOGIES SERVICES

Figure 2: You can match the corneal sagittal height (z-value) with the sagittal value (s-value) of a contact lens to achieve an optimum fit.

To effectively fit these difficult corneas, match the sagittal height of the reverse geometry or scleral lens with the sagittal height of the cornea, and add 15 microns, as doing so allows for a sufficient tear layer — something needed for adequate tear exchange. A caveat: Be sure to check with your GP lab to obtain information on the sagittal height of a specific lens design. If you send the lab a copy of the topography map, many times they will be able to recommend the initial lens parameters.

Simulated fluorescein patterns

The simulated fluorescein (NaFl) pattern enables you to visualize the effects of the base curve, diameter and edge lift changes on the lens fit (see figure 3). Specifically, it appears over a given topography map, allowing you to evaluate the tear layer clearance beneath a specified RGP on that cornea. As a result, when you use the simulated NaFl pattern as a guide to achieving the optimal tear layer clearance, you have an excellent chance of designing the best RGP for any given cornea.

Figure 3: This GP lens illustrates the ideal central tear film clearance, horizontal mid-peripheral bearing and vertical peripheral edge clerance.

Keep in mind, however, that the simulated NaFl pattern has limitations, as it doesn't take into consideration the effects of lid tension, corneal tilt and the tendency of a lens to gravitate toward the cornel apex (which may not be at the geometric center of the cornea). Each of these factors can cause a lens to decenter away from the geometric center of the cornea. The bottomline: The simulated fluorescein pattern allows you to more accurately pick the initial trial lens, which you then evaluate on the eye. The trial lens then enables you to consider the effects of lid tension, etc.

Topography and lens designs

The following designs rely on corneal topography to determine the final custom lens:

Orthokeratology lenses. When fitting these designs, corneal topography enables you to monitor the centration and dioptric effect of the treatment zone regardless of the fitting method and lens design (see figure 4).

Figure 4: This difference map shows the excellent centration and refractive change resulting from a corneal reshaping lens.

Wave (EyeQuip). This proprietary topography-based lens design is specific to the Scout and Keratron topographers. (The software is also compatible with the Medmont E300). The topography map displays a simulated NaFl pattern, and you design the best fitting lens. Options include Radially Symmetric (spherical/aspheric), Geometrically Symmetric (toric/bitoric) and Freeform (quadrant specfic/wavefront) custom soft and RGP designs.

Refractive surgery specific (RSS) (Blanchard Contact Lens). You can order this reverse geometry lens empirically by sending a copy of the topography map along with the spectacle prescription to Blanchard labs. The lab assesses the color differences — as mentioned earlier, an indication of curvature changes — to determine the base curve, paracentral fitting curve and peripheral edge configuration.

Keratoconus Bi-aspheric (KBA) (Essilor Contact Lens). This is a 10.2mm bi-aspheric lens with variable eccentricity. It works well for central and oval cones because the highly aspheric anterior surfaces of these specific cone types exhibit a high eccentricity, as they are steep in the center and very flat toward the periphery. The KBA lens set comes with software to guide you through the fitting process by calculating the effects of eccentricity changes on base curve and sphere power.

SynergEyes (SynergEyes, Inc.) You fit the SynergEyes KC (keratoconus) and PS (post-surgical) lenses diagnostically using keratometry or topography as a guide. Having a topography map helps you to streamline the fitting process on these irregular corneas by providing you with curvature readings over the entire cornea. In cases of moderate keratoconus and PMD, the SynergEyes A (ametropia) lens works well when fit diagnostically with the guidance of a topography map.

As you can see (pun intended), employing a corneal topographer on challenging-to-fit patients enables you to fine-tune your lens fitting, which increases your chances of providing the best fit the first time.

This, in turn, increases patient satisfaction, ensuring patient loyalty to your practice and referrals. Two components very important to a financially thriving practice. OM

Dr. Anderson practices in suburban Chicago, specializing in orthokeratology, keratoconus, post-surgical lens fits and anterior segment disease. E-mail her at dianne.anderson@comcast.net


Optometric Management, Issue: October 2008