Wang Vision Institute - Dr. Ming Wang Publications

EACH of the three leading corneal topographers has its own strengths and weaknesses, an interim report from the first three-way clinical comparison trial of its kind indicates.

Ming Wang MD, Nashville Tennessee, presented the results of a study comparing the checkerboard 3-D AstraMax, the scanning slit Orbscan and the placido disc Humphrey Atlas topography systems at the annual meeting of the ASCRS.

"3-D topographic systems are becoming increasingly important for generating multi-dimensional images to improve visual quality after refractive surgery," noted Dr Wang.

Dr Wang and colleague Dr Tracy Swartz imaged the eye of 324 eyes of 321 patients with each of the systems. Patients included those with dry eyes, Fuch's dystrophy, myopia, hyperopi, astigmatism and irregular astigmatism. Some patients had been treated with LASIK for hyperopia or myopia. Diseases were diagnosed based on clinical exam or topographic consensus-or two out of three results from the corneal topographers. The study used ultrasound corneal pachymetry as the reference standard for corneal thickness.

The interim analysis revealed that the AstraMax showed the highest specificity for anterior curvature, followed by the Humphrey and Orbscan systems. However, the learning curve for technicians operating the AstraMax was much higher than with the other machines, he noted.

Image failure tended to occur more frequently when using the AstraMax (33/324 eyes) versus the Orbscan (6/324). But image failure occurred even less often in the Humphrey (2/324 eyes).

"The Humphrey system is the most reliable in terms of being able to capture images. The image is more difficult to capture with the AstraMax, but when it is captured correctly, it is more sensitive, and clinically useful" Dr Wang reported.

The Orbscan system most readily captured images in the study, and showed larger areas of data points with more reliable resolutions than AstraMax, but the data was prone to artificial distortion caused by dry eyes or very flat or steep corneas. Dr. Wang said.

The AstraMax system, for its part, tended to show thicker pachymetry than ultrasound pachymetry, and Orbscan tended to show thinner pachymetry even with calibration factor built-in. The AstraMax reveals more dynamic features in anterior elevation and curvature, which is helpful in identifying vision loss after refractive surgery, Wang commented

"The AstraMax is like a pretty, but particular girlfriend. It's a bit more difficult to capture image, but more reliable and clinically relevant. It adds to our armamentarium of weapons and provides added sensitivity in mapping topographic features to improve post-refractive surgery outcomes," Dr. Wang said.

Brian Boxer Wachler, MD, of the Boxer Wachler Vision Institute in Beverly Hills, California commented that Dr Wang's study was thought-provoking and well done, but said he would reserve judgment until he had seen the results from other investigators.

The latest 2002 ASCRS survey of cataract and refractive practices found that the Humphrey corneal topographer was the most popular and the Orbscan the least popular among its members.

"Orbscan may be the least popular for a number of reasons-including cost, the fact that it is slightly more difficult to acquire data, and the reliability of its posterior curvature analysis. Recent studies by Dimitri Azar MD show that there are limitations to the test-retest reliability of the Orbscan posterior curvature analysis," commented Dr Boxer Wachler.

The AstraMax 3-D topographical system uses three cameras, and generates 35,000 data points in 0.2 seconds. With 3-D systems like the AstraMax, images are captured at multiple angles and 2D checkerboard technology provides improved accuracy in the central and paracentral corneal measurements, Dr. Wang said.

Orbscan topography is derived from a series of slit-beam images. Two scanning slit-lamps project beams at 45 degrees to the right or left of the instrument axis. During an exam, 40 images-20 with slit beams projected from the left and 20 from the right-are acquired in two intervals, each 0.7 seconds in duration.

Orbscan also uses specular refractive technology, which increases anterior surface accuracy, by using the specularly reflected image of a placido or other reflective target to directly measure surface slope.

The ATLAS Eclipse Model 995 offers ultra-low illumination and increased peripheral coverage. The Model 995 is ideal for high volume corneal and contact lens specialists who require comprehensive and detailed peripheral corneal and pupil assessments.

The ATLAS' 22-ring conical placido provides a large limbus-to-limbus field of view. The patented cone-of-focus gives the topographer superior electronic alignment, resulting in greater repeatability and accuracy.

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Comparative trial provides tips to the top topographers

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