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Toric IOL nomogram accounts for posterior corneal astigmatism
Refractive measurement should include pre-existing, surgically induced and posterior corneal astigmatism.
All testing devices overestimated with-the-rule astigmatism, and all but one device underestimated against-the-rule astigmatism, a study found.
The study authors proposed a new nomogram for toric IOL selection that factors posterior corneal astigmatism into power calculations.
The study confirmed previous reports on the presence of posterior corneal astigmatism, Douglas D. Koch, MD, the corresponding author, said in an interview with Ocular Surgery News.
“First of all, the power almost always is against-the-rule in terms of its refractive effects. Secondly, there is a different amount of it depending upon whether the anterior corneal surface has with-the-rule or against-the-rule,” Koch said. “That very much confirmed what we had seen before in just studying normal eyes from ages 20 to 89.”
The Baylor toric IOL nomogram accounts for mean posterior corneal astigmatism and aims to leave eyes with small degrees of with-the-rule refractive astigmatism, the authors said.
Douglas D. Koch
“The bottom line is that, in all of our toric lens patients, we need to take into account posterior corneal astigmatism,” Koch said. “We need to take into account many factors. We need to take into account the surgically induced astigmatism. Of course, the pre-existing astigmatism should be No. 1. The effect of the astigmatism of the IOL may vary according to IOL power and its depth in the eye. And, finally, we need to take the posterior corneal astigmatism into account.”
The study was published in the Journal of Cataract and Refractive Surgery.
Patients and methods
The prospective study included 41 eyes of 41 patients. Preoperatively, 17 patients had with-the-rule astigmatism with a corneal steep meridian at 60° to 120°, and 24 patients had against-the-rule astigmatism with a corneal steep meridian at 0° to 30° or 150° to 180°.
Mean IOL power was 18 D.
Investigators used five devices to measure corneal astigmatism: the IOLMaster (Carl Zeiss Meditec), the Lenstar (Haag-Streit), the Atlas corneal topographer (Carl Zeiss Meditec), a manual keratometer (Bausch + Lomb) and the Galilei combined Placido-dual Scheimpflug analyzer (Ziemer).
The Holladay II formula was used to calculate effective toric power. Prediction error was the difference between astigmatism measured with each device and actual corneal astigmatism. Vector analysis was used for all calculations.
“We can measure the anterior cornea pretty well. Surgically induced [astigmatism] will vary from surgeon to surgeon, and for any given surgeon, it will vary from eye to eye. So, there’s a little variability there. The effective toricity of the IOL can be calculated right now using the AMO calculator or the Holladay II formula,” Koch said.
Results and conclusions
Mean prediction errors for with-the-rule astigmatism were 0.59 D at 89.7 with the IOLMaster, 0.48 D at 91.2 with the Lenstar, 0.51 D at 78.7 with the Atlas, 0.62 D at 97.2 with the manual keratometer and 0.57 D at 93.9 with the Galilei.
Mean prediction errors for against-the-rule astigmatism were 0.17 D at 86.2 with the IOLMaster, 0.23 D at 77.7 with the Lenstar, 0.23 D at 91.4 with the Atlas, 0.41 D at 58.4 with the manual keratometer and 0.12 D at 7.3 with the Galilei.
With-the-rule prediction errors were significant, ranging from 0.5 D to 0.6 D, with all devices. In against-the-rule eyes, with-the-rule prediction errors were significant, ranging from 0.2 D to 0.3 D with all devices except the Galilei.
“Overall, the Galilei tended to be more accurate than devices that only measure the anterior corneal surface,” Koch said. “However, it needs to be even more accurate, which is likely to occur with updated software. The obvious goal is to have devices that accurately measure all corneal astigmatism so that regression tools like the Baylor nomogram won’t be needed.” – by Matt Hasson