instrumental strategies

ANDREW S. MORGENSTERN, O.D.

Elevate Yourself Above other O.D.s

Accurately measure corneal elevation and curvature in your patients.

Because corneal ectasia has become the common denominator for post-refractive surgery corneal transplants and the instigator for some of the largest lawsuits in refractive surgery today, we must properly screen refractive-surgery candidates for this and other corneal conditions that can preclude successful visual outcomes.

One way you can do this: by using the Pentacam corneal mapping device, from Oculus, Inc. The device provides elevation images of the anterior and posterior cornea.

The Pentacam difference

Common corneal topographers (placido disc imaging) are excellent devices, but you should not solely rely on these instruments to determine refractive-surgery candidacy, for two reasons:

1. They only measure the curvature of the anterior surface, which is only half of what determines candidacy.^1,2 Many conditions, such as keratoconus, originally develop or provide clues from the posterior surface before we can see them on the anterior surface.
2. Imaging with these devices is based on one arbitrary reference point. Curvature, when imaged by any device, is an element of elevation of the cornea. The curvature is measured based off of a specific reference point. If this point is not exactly in the center of the map, the relative image of curvature is skewed.

For example, when looking at a basketball from the top down, if your line of sight lines up directly with the center of the ball, you'll have symmetrical curvatures on either side of the reference point at which you're looking. But, if you move the basketball three inches to the left, still looking straight down on to the ball, your reference point has shifted over, and the curvatures on either side of the reference point will be asymmetric.

The Pentacam delivers a different perception of the cornea than common corneal mapping devices because it provides an image of the elevation and curvature with respect to both anterior and posterior surfaces. The device does this by taking a rotating image (Scheimpflug) of both the anterior and posterior surface. Then, it makes specific and accurate calculations to determine the best-fit sphere (BFS) of both surfaces independently.

Once the Pentacam calculates a surface BFS, it shows (in microns) how much elevation or depression is relative to the BFS. So, you get a much better understanding of the elevation and depression of the cornea with respect to curvature. As we now know from elevation mapping devices, such as Pentacam, the apex of a keratocone, which is generally the thinnest and weakest area of the diseased cornea, is not in the area that is "in the red" on a placido disc system. The area "in red" is just the steepest part of the cone, which generally doesn't correlate to the area of the cone where the apex actually lies. Understanding the proper anatomy of the cornea in cases such as keratoconus makes irregular lens fits much easier.

Beyond corneal calculations

Via the Holladay formula, the device can also aid you and the comanaging ophthalmologist in selecting the best IOL for post-refractive surgery cataract patients, as it provides accurate information to help you choose an IOL.

The device can estimate actual Intraocular pressure (IOP) based on changes in corneal thickness in glaucoma patients, and it enables you to view a 360° image of the irido-corneal angle, so you can check for angle closure or recession. Finally, you can use the device to monitor patency of peripheral iridotomies (PI) and iridectomies in post PI and trabeculectomy patients.

Pentacam also helps cataract and glaucoma patients.

The Pentcam's many practical applications enable me to identify bad refractive-surgery candidates, recommend the best IOL for post-refractive surgery cataract patients and accurately monitor my glaucoma patients — a lot of benefits in one instrument. OM

References furnished upon request.