DIAGNOSTICS
Build Your Practice with Today's Corneal Topographers
by Jeff Schafer O.D., M.S., F.A.A.O. and David A. Berntsen O.D., M.S., F.A.A.O.

For most state-of-the-art optometric practices, corneal topographers have replaced the keratometer. While keratometry may be quicker, cheaper and easier to use for measuring corneal curvature on routine patients, corneal topography has far more advantages. Topography can measure up to 10,000 data points across the entire cornea; keratometry only measures four within the central 3mm of the cornea. A keratometer will miss any distortion of the cornea that occurs within or outside those four data points. A corneal topographer will allow you to map the curvature across the majority of the corneal surface, as well as document any changes in the cornea over time.

You can utilize a variety of output calculated from the data a corneal topographer gathers which is presented graphically in various displays or "maps." Your ability to interpret these outputs and statistical indices will allow you to maximize your knowledge of each patient's corneal shape. 

Figure 1: Topography from a patient with keratoconus measured by the Keratron Scout (Optikon). An axial map, tangential map and spherical offset map are displayed. Note that the location of the cone can be more easily identified on the tangential map (lower left) than on the axial map (upper right). Images courtesy of Dr. Michael Twa.

Start with maps

The most common topography maps are axial, or sagittal, maps. An axial map simply displays the curvature of the cornea in diopters, often using a color image. They do not display the power of the cornea, and are subjected to an overall smoothing effect of the data as they represent a running average of curvatures across the cornea. Axial maps are limited by the assumption that all light rays are refracted to a focal point along the optical axis.

Tangential, or instantaneous, maps take into consideration that all light rays are not refracted perfectly along the optical axis. The topographer calculates curvature based on the tangent to the normal for a particular point on the cornea. This allows tangential maps to be more sensitive to abrupt changes in curvature.

Both axial and tangential maps are valuable in the treatment of corneal disease and fitting of contact lenses. Axial maps generally provide more accurate central detail, but are smoothed out, often showing flatter steep curvatures and steeper flat curvatures. Tangential maps display more accurate corneal shape with smaller, more detailed patterns. The increased sensitivity to changes in curvature is ideal for locating corneal defects or evaluating the apex of a cone (figure 1).

Bringing it into practice

Topography is also essential in any specialty contact lens practice, or even in a practice hoping to expand its contact lens patient base. It will improve your ability to design lenses to correct for astigmatism by demonstrating where the cylinder is located (central vs. limbus to limbus) and whether it is regular or irregular. It will also enable you to fit and troubleshoot orthokeratology (figure 2). And fitting irregular corneas is made simpler with the data topography provides. If you co-manage refractive surgery patients, topography allows you to track the corneal shape over time as it heals, as well as detect complications such as central islands or decentered ablations.

How do you choose?

Figure 2: Difference map from a patient wearing overnight orthokeratology contact lenses. The upper left map is the patient's baseline map and the lower left map is the map taken three weeks later. The difference map on the right is the three-week map minus the baseline map. The elevation scale at the bottom of the screen shows the change in elevation between the two maps and the location of the treatment zone.

If you've decided to add a topographer to your practice or to trade in your old topographer for one with all of the latest technology, how do you decide which to purchase? There are many factors to consider. Al-though we'll present examples, we do not suggest one specific system. (Also see the chart "Corneal Topographer Manufacturers" on page 30. A complete list of manufacturers and models appears in the "Diagnostic Instrument Buying Guide," July 2006 issue of OM.)

The most fundamental difference between topography systems is the method by which they measure the eye. The most widely used method is the Placido disk, which acquires data in less than one second. Other methods include scanning slits or a rotating Scheimpflug camera. Each takes more than a second to acquire data and therefore they are more sensitive to eye movements.

■ The Placido disk consists of a series of concentric rings that are reflected off the anterior surface of the cornea. A digital camera captures the image and software detects the location of the rings and uses this information to calculate the shape of the cornea. One basic difference among topographers with a Placido disk is the size of the Placido cone. Manufactures use both large and small cone sizes.

► Large cones focus further away from your patient, which some patients may appreciate. Because the Placido disk is further away from the eye, they're less sensitive to magnification errors. However, if a patient has a prominent brow or nose, large cone designs can lose data because these features can block the reflection of the Placido disk off the cornea. Examples of large-cone designs include the Atlas (Carl Zeiss Meditec) and the ReSeeVit Focus (Veatch Instruments).

► Small-cone Placido disk topographers project the Placido mires further on the peripheral cornea and also help eliminate data loss due to the brow or nose obstructing the reflection of the Placido disk. Because small-cone topographers come very close to the eye, a precise working distance is necessary; however, many highly accurate topographers on the market feature alignment methods to tackle this. Examples of small-cone topographers include the E300 Corneal Topographer (Medmont), the Keratron (EyeQuip), the TMS-4 (Tomey), and the Magellan Mapper (Nidek).

Figure 3: Topography and pachymetry of a patient with mild Pellucid Marginal Degeneration. The keratometric map utilizes the data from the partial Placido disk, which improves the accuracy of the topographical data but does not provide results for the superior and inferior cornea. The thinnest point can be observed to be inferior on the cornea on the pachymetry map.

■ Scanning slits provide another method of measuring the cornea. Utilized by the Orbscan IIz (Bausch & Lomb), this method measures both anterior and posterior surface elevation and curvature. Because both corneal surfaces are measured, the topographer obtains full corneal pachymetry in addition to other features such as anterior chamber depth (figure 3). The Orbscan IIz uses a partial Placido disk to improve the accuracy of the topographical data and therefore does not provide data for the most superior and inferior parts of the cornea. Although not as accurate as the partial Placido disk, you can use the slit data to generate a full corneal keratometric map. The acquisition time for the Orbscan IIz is 1.5 seconds, which can make it more difficult to use if a patient can't hold still.

You can also use a rotating Scheimpflug camera to measure the anterior segment. This allows you to view individual, cross-sectional slices of the anterior segment and 3-D anterior segment images. This method also allows for analysis of both the anterior and posterior surfaces of the cornea. An example of a Scheimpflug camera topographer is the Pentacam (Oculus). The Pentacam provides full corneal pachymetry, anterior chamber depth. It requires up to two seconds for data acquisition.

Additional features

System portability is another important factor for many practitioners. In addition to a table-mounted system, some topographers are handheld, can be run by a laptop or mounted on a slit lamp. Examples of portable and/or slit lamp mountable topographers include the Scout (Eye Quip) and the Easygraph (Oculus).

Also consider whether the system has an integrated computer, or can be operated by a stand-alone computer. Stand-alone computer systems may allow for easier upgrade than systems that integrate the computer and topographer into a single unit. Check if the system price includes the computer and a table.

Finally, consider software features, which may include contact lens fitting aids, orthokeratology fitting modules and keratoconus-detecting algorithms. Some calculate anterior surface wavefront aberrometry. Others combine a topographer with a total wavefront analyzer, such as the 3-D Wave (Marco).

Billing and coding

You may find that your bottom line actually benefits from incorporating topography into examinations. You may bill insurance when the procedure is medically justified, or charge private-pay individuals for the service. We bill corneal topography as a 92499 general, unlisted ophthalmic procedure, but recently the American Medical Association approved a new code for topography that will be released in January 2007.

Many practitioners bill a la carte for this service, but it's becoming increasingly more common to include corneal topog- raphy with the comprehensive exam fee, or better yet, to bundle it with your global contact lens evaluation fee. Assuming you see a reasonable number of contact lens patients, even a nominal increase in professional fees would likely cover a lease or pay for the unit in a short amount of time. For instance, if you increase fees $10 and see five contact lens patients per day, six days a week, that's $1,200 per month additional income. The device may help retain revenue because you will no longer need to refer patients and risk losing them to a more sophisticated practice.

It's well worth it

Corneal topography will help improve your patient's perception of your practice. The sophisticated nature of corneal topography instrumentation really impresses patients, and shows them that you provide modern, high-quality, comprehensive eye care. You can network topographers and feed information into computers in your exam rooms, creating a powerful patient-education tool.

Additionally, newer software allows you to integrate topography seamlessly into your electronic medical records. These features translate into great benefits for your practice.