Corneal topography shows its colors

Take-home

Point source color LED topography has the potential to offer faster and more accurate corneal measurements than existing Placido disc or Scheimpflug imaging technology.

Dr. Weikert

By Fred Gebhart; Reviewed by Mitchell P. Weikert, MD

Houston-Point source color LED topography (CLT)-though a relatively new technology-has theoretical advantages to provide more accurate, more repeatable measurements of corneal topography compared with Placido disc and Scheimpflug imaging devices.

However, there are few head-to-head comparisons of the three technologies.

“We wanted to take this new point source color LED device and compare it [with] what is already available,” said Mitchell P. Weikert, MD, associate professor, Cullen Eye Institute, Baylor College of Medicine, Houston.

“We wanted to see where it agrees with existing devices and where it disagrees,” Dr. Weikert added. “No two devices are going to agree in all cases, but this will help us understand how this new technology works and the potential clinical utility.”

Comparison study

He presented the initial results of a study comparing three devices: corneal topography based on CLT technology (Cassini, i-optics); Placido disc (Atlas 9000, Carl Zeiss Meditec); and Placido/Scheimpflug (Galilei, Ziemer Ophthalmic Systems).

The study enrolled consecutive patients: 64 normal eyes and 15 post-LASIK/PRK eyes. Similar measurements were obtained from all three devices.

Measurements included keratometry (average and astigmatism) and wavefront aberrations with a 6-mm pupil (total higher-order aberrations [HOAs], third to eighth order; third-order coma; third-order trefoil and fourth-order spherical aberration).

Placido-based topography offers several advantages, including a large number of data points, accuracy to within 0.25 D, and surface quality information.

However, the mires reflect off tear film and provide data on the anterior surface only. Because Placido technology uses concentric rings, it more easily measures changes in the radial direction.

Point source CLT technology projects about 700 red, yellow, and green LED point sources onto the cornea to measure its shape. Each LED is a discrete data point.

The combination of point sources and distinct colors helps ensure that there is no source-image mismatch. The image processing software automatically locates feature points in the reflected images and accounts for smearing and deformation in irregular corneas.

Given the short time required for image capture, results are not affected by motion artifacts caused by eye movement.

“We found good agreement [among results with] the three devices when measuring corneal curvature in normal eyes, which is what we hoped to see,” Dr. Weikert said. “They should all be able to measure normal eyes accurately.

Discovering differences

“When we looked at eyes that had had prior refractive surgery, we found differences,” he said. “As the corneal curvature increased in these post-LASIK/PRK eyes, the differences [among] the devices increased as well.”

The increasing differences in measurement as curvature increases may support the theoretical advantages of point source CLT in the real world. Measuring corneal shape point by point may have greater advantages in more irregular corneal surfaces.

Potential advantages in measuring irregular surfaces were seen again in HOAs. There was good agreement among comparisons of the three devices in normal eyes.

However, in post-LASIK/PRK eyes, the point source CLT device measured higher levels of aberration, except spherical aberration, which was similar across devices.

This makes sense, since spherical aberration and Placido rings are both rotationally symmetric, Dr. Weikert said.

Astigmatic measurement crucial

In measuring astigmatism, Cassini showed increasing differences compared with the Atlas and Galilei devices as astigmatism increased in both normal and post-LASIK/PRK eyes.

Measuring astigmatism has always been important, but it is becoming more important all the time, he said.

“Toric IOL use is steadily increasing and as femtosecond laser use in cataract surgery increases, we will be performing even more astigmatism correction using corneal incisions,” Dr. Weikert said.

“To treat astigmatism accurately, we must first be able to measure it accurately,” he said. “To know where to make our incisions, we have to be able to locate corneal astigmatism accurately.”

Increased precision in measuring and locating astigmatism may be a real benefit of this technology, he said.

Clinical potential

More testing is needed to evaluate the true clinical utility of point source CLT, however.

Though early evidence is positive, just because a new technology produces different results compared with older methods does not automatically mean the newer technique is better or more accurate, Dr. Weikert said.

“We definitely see the potential in this technology,” he said. “We like what we see with point source CLT, but we can’t say at this point this one is better.

“The grid-like point source technology has the potential to improve measurement in corneal curvature and aberrations, but we still need to expand this evaluation, measure more eyes, and more differently shaped eyes,” he said.

It is already known that point source technology can measure manufactured surfaces very accurately compared [with] existing technologies. The next step is to see if it can measure corneas with the same degree of accuracy, Dr. Weikert said.

Mitchell P. Weikert, MD

E: mweikert@bcm.edu

Dr. Weikert receives research support from Optovue.

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