Centroid value, posterior cornea input adds game for toric calculators

Reviewed by Warren E. Hill, MD

Cataract surgeons using toric IOL calculators are more likely to have better outcomes when using a centroid value for surgically induced astigmatism (SIA), rather than a mean or a median value.

Warren E. Hill, MD, in private practice, Mesa, AZ, provided a comparison of toric IOL calculators and also how they differ.

He addressed the value that Douglas Koch, MD, of Houston, added to toric IOL calculations when he recommended the inclusion of posterior corneal astigmatism.

In addition, Dr. Hill noted that Graham Barrett, MD, of Perth, Australia, took this insight a step further by adding an algorithm to automatically calculate both the magnitude and the orientation of posterior corneal astigmatism as part of his popular Barrett toric calculator. Subsequently, other toric calculators are adopting this approach.

Dr. Hill provided an example of a series of toric IOL outcomes with the original Alcon Laboratories toric calculator and the Holladay toric calculator that did not include the posterior cornea.

“These calculations typically produced outcomes that were skewed in the same way,” he said. “The moment we added posterior cornea, toric IOL outcomes began to normalize.”

For SIA, it was previously popular to calculate the mean, or a median value, for a series of patients.

“Contrary to conventional wisdom, both the magnitude and the orientation of SIA is not always predictable from one patient to the next,” Dr. Hill explained. “For individual patients, the amount of SIA might be as low as 0.00 D to as high as 1.50 D. In addition, the orientation of SIA produced by the cataract incision is not always orthogonal. That was a surprise.”

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