Topography's role in wavefront systems

by Michelle Stephenson Correspondent

While some researchers say topography has a role in wavefront systems, what that role is and whether topography will be needed in the future are still being debated.
Raymond Applegate OD, PhD, of the University of Houston, said that it is useful to do topography before and after surgery when using wavefront systems, so that the change in corneal elevations that occurs after surgery can be seen.

"Ideally, that should be identical to the ablation pattern chosen. If it's not, it teaches us the error or provides an error signal. If the error signal is constant across patients, then it can be folded into the ablation design for future surgeries. Topography is very useful in diagnosing the problems of unpredictable errors and defining them, which will be useful in the future in designing other surgeries," he said.
However, he noted that topography ignores some major optical components of the eye, because it measures the corneal surface only.

"On the other hand, the corneal front surface makes up the majority of the optical system," he said.
The wavefront system measures all the optics together.
"Measuring the wavefront of the eye will tell you how to design an ideal correction if the eye was a piece of plastic, but it's not.

"In my opinion, as corneal refractive surgery progresses, the importance of corneal topography may diminish, but I believe that it's really important right now as we're developing the surgeries and trying to perfect techniques," he said.
Stephen D. Klyce PhD, of Louisiana State University Health Sciences Center, New Orleans, agrees. He said that corneal topography can be ignored if one can assume that the cornea is some standard shape.

"That's a safe assumption when you're treating a fairly normal myopic eye. Early trials have been done on low to moderate myopes. The problems come in patients who really need customization of their correction. If corneal topography is ignored in these cases, the errors grow significantly. That's a big worry," he said.
Jack T. Holladay MD, professor at Baylor College of Medicine and in private practice at the Holladay LASIK Institute in Houston, also believes that topography is necessary. In fact, he thinks wavefront systems may not be needed with the current precision of the excimer lasers.

Topography
"No one who understands wavefront and topography thinks that you can do this without topography. Wavefront tells you there's a problem, but it doesn't tell you the change that you need to make to the surface of the cornea to fix that problem," he said.
He noted that two people can have the exact same wavefront measurements, but the shape of their corneas can be different. He said that 99% of the aberrations in the optical system to be corrected are in the cornea.
"We use wavefront analyzers to prove our results," he said.
Holladay is the Medical Director for LaserSight, which recently launched the Astramax, a stereotopographer, at the annual meeting of the American Society of Cataract and Refractive Surgery.

"This will move topography to the next generation in terms of providing accurate height data of the surface of the cornea all the way out to the periphery," he said.
On the other hand, John F. Doane MD, on the clinical faculty of the University of Kansas, Kansas City, in private practice at the Discover Vision Centers, Kansas City, Mo., said he thinks topography is helpful, but not necessary.

"Several systems have been developed where you do not need the surface topography to direct the treatment. In a specific system, topography may help guide you in decision making, but it is certainly not an absolute necessity to have it," he said.
Klyce said that while it's important to know a patient's corneal topography, topographers can't measure aberrations of the rest of the eye.

"For example, lenticular aberrations can sometimes be very significant aberrations, arising from the lens itself. With wavefront measurements, one is able to measure not only the aberration of the cornea, but the aberrations of the lens and the rest of the eye," he said.
Scott M. MacRae MD, from the University of Rochester Medical Center, Rochester, N.Y., agreed.

He said that systems that rely on topography only to do customized ablation for normal individuals fail to take into account all the aberrations of the eye. "In younger individuals, corneal aberrations are compensated by the lenticular aberrations, so they end up neutralizing each other. If you treated based solely on corneal aberrations, you may actually induce aberrations unnecessarily," he said.

He pointed out that wavefront analyzers are making refractive surgery safer. "The wavefront systems are driving us to much more sophisticated decision making and are making the procedures much safer. You know when someone has some higher-order aberration that something's not right, and you would want to treat that patient with customized ablation, not conventional treatment," he said.

While wavefront systems have many advantages, they also have some disadvantages.

"The current generation of wavefront sensors doesn't have much dynamic range. They don't have much capacity to measure a lot of irregularity, whereas corneal topographers do have a more dynamic range. The wavefront sensors can measure only the size of the pupil, so that if you can only dilate the pupil to 5 mm, then you can't get data beyond that. With a topographer, you can usually get data out to 10 to 12 mm," MacRae said.
He noted that the two systems could complement each other. For this reason, the combination systems may be the best of both worlds.

Combination systems
The Nidek OPD Scan and the Bausch & Lomb Zywave incorporate topography, and both Applegate and MacRae said that the future will see more combination systems.
"I'm an advocate of the systems that integrate both and I think they'll become more popular in the future. One reason is that it's more cost-efficient. You can use a lot of the systems for both purposes, so you don't have to duplicate equipment," Applegate said.

MacRae believes that the next generation of wavefront testing will probably be done using combinations of topographers and wavefront sensors or a system that actually allows physicians to separate the corneal contribution from the lenticular contribution.

"I've been working very intensely with several wavefront sensors for a year and a half now, and it's really helpful to be able to separate the lenticular higher-order aberrations caused by the lens versus higher-order aberrations caused by the cornea," he said.
Klyce said that while some companies use wavefront only and assume that the cornea is some known average shape, it is necessary to do corneal topography and align the results of the corneal topographer with the measurements taken with wavefront.
"You need to use both of those data sets in good alignment to achieve optimum results with refractive surgery," he said.

He said that putting the wavefront measurement and the corneal topography measurement in the same machine is a distinct advantage, because it makes the alignment of those two measurements a lot more straightforward.

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