the contact lens exam
Who's Who in Corneal Topography
BY COLLEEN RILEY, OD, MS, FAAO

Corneal topography is an invaluable tool in the contact lens practice. Routine uses include: differential diagnosis and monitoring of conditions such as keratoconus, pellucid marginal degeneration and corneal warpage; pre- and post-op evaluation of the corneal surgery patient; and managing challenging contact lens fits. Additionally, with advanced ortho-k or corneal refractive therapy, the topographer is essential in monitoring contact lens position and the corneal response to refractive treatment efforts. Furthermore, many topographers have software available that will aid in designing appropriate GP contact lens parameters.

Here's a discussion of three types of corneal topographers.

1 The Orbscan II/IIz Corneal Topography/Pachymetry System (Bausch & Lomb Surgical, Orbtek) uses two scanning slit lamp beams projected at 45 degrees to measure more than 9,000 points in 1.5 seconds on the anterior and posterior surfaces of the cornea. The instrument determines pachymetry by calculating the difference in elevation between the anterior and posterior surfaces of the cornea.

The Orbscan II/IIz can detect posterior corneal abnormalities as they first begin to appear. It can also measure anterior chamber depth, Angle Kappa and corneal and pupil diameter and perform simulated keratometry.

By calculating curvature in all directions, the Orbscan II/IIz can give an accurate assessment of corneal size, shape and power, making it a standard of care in refractive surgery. However, in the measuring process, the Orbscan requires a longer acquisition time and may be hindered by excessive eye movement. A quality control feature written in the software will prevent the measurement from completing if it doesn't achieve sufficient accuracy and consistency.

2 The E300 Corneal Topographer (Medmont) uses a series of 32 placido rings to take video images of the reflections on the corneal surface, measuring 15 to 120 points over an 11mm diameter area. This topographer effectively measure curvatures between 10 diopters and 100 diopters. It uses an "arc-step" algorithm to derive curvature from variations in the reflection of the rings and has software that corrects for off-center or out-of-focus images.

The E300 takes the images and displays the four best results for review along with the percent of accuracy that the photo indicates. Eye and lid movement present less of an obstacle to image quality. The software is particularly useful with ortho-k by creating multiple and overlay maps, along with measuring corneal eccentricity. The E300 is available with a contact lens fitting software package.

3 The Keratron (EyeQuip) employs a "placido cone" design that takes measurements over a 10.7mm diameter with 15 percent or less extrapolation of data. It measures 7,168 points and has an automatic image capture that is triggered at the correct corneal vertex. The Keratron features mires tracking even with irregular corneas and has the ability to measure below 10 diopters and above 120 diopters of curvature. Additionally, software offering contact lens fitting simulation is available. This program makes the Keratron useful for difficult GP fits.

The Keratron's portable, handheld version (the Scout) performs all the same functions at the same level of accuracy as its more fixed counterpart, yet has become completely portable. Use it in your hand or fit it onto any slit lamp.

Dr. Riley is an assistant clinical professor at Indiana University School of Optometry and is program coordinator for the school's Cornea and Contact Lens Residency.