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6 tactics to measure astigmatism


Repeatability of corneal astigmatism measurement was compared using six different devices.


Repeatability of corneal astigmatism measurement was compared using six different devices.

Dr. Kohnen

By Fred Gebhart; Reviewed by Thomas Kohnen, MD, PhD

Frankfurt, Germany-Given that toric IOLs are driving the need for more precise, more repeatable measurements of corneal astigmatism, the question remains: Which of the competing measurement technologies provide the most effective results?

“What [surgeons] really need is consistent measurement of the astigmatic component of the cornea,” said Thomas Kohnen, MD, PhD, professor and chairman of ophthalmology, Goethe-University, Frankfurt, Germany. “That was why we conducted [a] study of six different devices and four different methods.”

The prospective, randomized study was designed to assess the repeatability of corneal astigmatism measurements using the following technologies:

  • Automated keratometry (IOLMaster 500, Carl Zeiss Meditec; Lenstar LS 900, Haag-Streit)

  • Manual keratometry (Model G, Carl Zeiss Meditec)

  • Placido disc (Keratron Scout, Optikon; Atlas 9000, Carl Zeiss Meditec)

  • Scheimpflug imaging (Pentacam HR, Oculus)

Researchers measured 1 eye in each of 45 patients who were aged 18 or more years and who did not have any type of corneal trauma, pathologies, or prior surgery.

The mean age was 53 years; 33 patients were female; and the study included 23 left eyes

Two full standard measurements were made with each of the six devices to test for visual acuity and manifest refraction. The choice of eye to be measured-left or right-and the order in which the six devices were used to measure each eye were both randomized.

Analysis included anterior surface astigmatism and total corneal astigmatism using ray-tracing calculations from the Pentacam measurements of anterior and posterior corneal curvature and pachymetry.

Analysis diameters from 1 to 8 mm were used in 1-mm steps. Measurements were subjected to Bland and Altman analyses of repeatability.

Measure of device performance

The good news was all six instruments delivered very similar results, Dr. Kohnen said.

Manual keratometry had the lowest absolute and relative coefficient of repeatability, 0.619 and 64%, respectively. Scheimpflug imaging delivered the best scores, 0.321 and 39%, respectively.

“Manual keratometry performed the worst, but with very minor differences,” he said. “These differences are very small and we don’t want to make too much of them.

At the same time, clinicians want to get the most precise measurement from a device, Dr. Kohnen noted.

“Repeatability is key-we got the best repeatability from the [Scheimpflug imaging] device,” he said.

The absolute and relative coefficient of repeatability for the other devices-from more repeatable to less repeatable-were:

  • Atlas (0.332 and 41%)

  • Lenstar (0.424 and 47%)

  • Scout (0.448 and 51%)

  • IOLMaster (0.512 and 56%)

The study found significant differences between the most repeatable of the automated devices (Pentacam) and the least repeatable (IOLMaster), according to Dr. Kohnen.

The correction for the IOLMaster was 0.51 D compared with 0.32 D for the Pentacam, the relative coefficient of repeatability was 58.06% and 40.16%, and the limits of agreement range was 1.02 D and 0.64 D.

Differences in outcomes to be seen

The study looked only at the repeatability of measurements and did not investigate any possible differences in clinical outcomes using the different devices, Dr. Kohnen said.

Such an outcomes study would require at least 200 eyes, possibly more, with long-term follow-up, he added.

The best correlation was not seen with the IOLMaster, which is what is seen in clinical practice, Dr. Kohnen said.

“We use the IOLMaster in our clinical practice,” he continued. “But experience tells me that when I am using toric IOLs, I like to see a topographer-or, better still, a Scheimpflug device-to get the best measurements possible to correct astigmatism. That is the most important take-home message for me.”

Thomas Kohnen, MD, PhD

E: Kohnen@em.uni-frankfurt.de

Dr. Kohnen has financial relationships with Alcon Laboratories, Oculus, and Carl Zeiss Meditec.


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