Eliminating astigmatic defocus
Treating low corneal astigmatism brings benefits, toric IOLs recommended as the preferred tool.
Toric IOLs are not merely suitable for eyes with low corneal astigmatism (0.5 to 0.75 D). Rather they are an indispensable tool for optimising refractive outcomes, visual performance and patient satisfaction after cataract surgery. And considering the relative predictability and risks associated with incisional techniques for treating astigmatism, toric IOLs are a far superior option, said Graham Barrett MD, at the 37th ESCRS Congress in Paris, France.
Dr Barrett told attendees that he performs a toric calculation targeting 0D residual astigmatism in all cataract surgery patients. An analysis evaluating outcomes using that approach showed that in a series of 290 eyes, 85% presented with ≥0.5D keratometric cylinder and 81% underwent toric IOL implantation. Postoperatively, 90% of eyes had ≤0.5D residual astigmatism, only 1.4% of eyes had 1.0D residual astigmatism and no eye had >1.0D.
“Astigmatic defocus is just as bad as spherical defocus. There is no benefit for leaving anyone with residual astigmatism,” said Dr Barrett, Professor, Lions Eye Institute, Perth, Australia.
Parameters for predictable outcomes
Obtaining accurate measurements is the first step to success when correcting low levels of astigmatism. Dr Barrett recommended measuring keratometry (K) with at least two devices – a biometer and topographer or tomographer – and ideally a third instrument such as a second biometer, autokeratomter or even a manual keratometer, for a third measurement. Likely the values will differ, but when put into the current Barrett Toric Calculator, an integrated K value is generated using vector analysis. The integrated K, which represents the mean if two data points are input and the median for three, is then used for the toric IOL calculation.
“The integrated K is significantly more accurate than a value from a single device, and its use enhances surgeon workflow because it eliminates time spent cogitating over which K value to use,” Dr Barrett said.
For the toric IOL calculation, Dr Barrett’s calculator uses a theoretical model to predict posterior cornea astigmatism or accepts a measured value. His calculator also incorporates an algorithm accounting for IOL tilt.
Incisions are another critical consideration. Dr Barrett said they should be made small (≤2.4mm) and consistently on the same axis. Regarding surgically induced astigmatism, he said the centroid value, not the mean, should be used in the toric IOL calculation.
Although an alignment error will result in increased residual astigmatism, the lower the toric power, the less the effect. To optimise alignment accuracy, Dr Barrett said he uses both a free app that he developed (toriCAM) as a manual marking aid and an intraoperative image guidance system.
“Using both methods helps because each checks the other,” he said.