Toric IOL calculations
Improve outcomes by using exact vergence and total SIA to calculate toric power
For the median cataract surgeon implanting toric IOLs, 75% of cases end up within ±0.5 dioptres of the residual astigmatism target. But wherever you fall on the toric outcomes bell curve, Jack T Holladay MD, MSEE, believes you can improve your toric lens performance 10% by making two changes in IOL power calculations.
First, switch from an IOL power calculator using a fixed constant for determining vergence to one that uses exact vergence for the spherical equivalent (SEQ) power and residual astigmatism of the toric lens to be implanted, Dr Holladay, of Baylor College of Medicine, Houston, USA, told the 2019 ASCRS ASOA Annual Meeting in San Diego, USA.
Constant vergence calculators use the same ratio (1.46) for the toricity of the IOL to the keratometric astigmatism regardless of lens SEQ, while the actual IOL toricity required drops as SEQ increases, throwing off power calculations for most lenses, Dr Holladay explained. For example, a constant vergence ratio of 1.46 calls for 3.0D toricity for 2D of corneal astigmatism for lenses ranging from 10 to 34D SEQ – 3D of toricity is only correct for a 22D SEQ lens, but undercorrects a 10D SEQ lens by 0.5D, and overcorrects a 34D SEQ10 lens by 0.6D.
An exact vergence calculator changes the ratio based on IOL SEQ, reducing one significant source of toric lens power error, Dr Holladay said. Exact vergence calculators include the AMO Express, the Alcon Holladay Toric Calculator and the Holladay IOL Consultant (http://www.hicsoap.com/).
THE ROLE OF TOTAL SIA
In 2012, research by Douglas Koch MD and colleagues on posterior corneal astigmatism (PCA) found that, on average, using anterior Ks alone underestimates total corneal astigmatism by 0.22D at 180° meridian, and exceeded 0.5D in 5% of eyes. This led to more accurate toric IOL calculations, but still misses some factors influencing total surgically induced astigmatism (SIA), Dr Holladay said.
So, Dr Holladay set out to develop a new approach based on optics that determines directly total SIA by subtracting pre-surgery Ks from post-surgery measured refractive astigmatism using non-toric IOLs. This accounts for all factors, including differences in measured keratometric versus actual corneal refractive astigmatism, actual versus aggregate PCA values, and even lens tilt and decentration.
Data from clinical trials of non-toric IOLs implanted using a 2.5mm temporal incision showed that post-op total SIA vectors correlated consistently with the axis and magnitude of the pre-op steepest K, Dr Holladay said. Applying a formula based on this relationship retrospectively significantly increased toric IOL power prediction accuracy, bringing residual astigmatism below 0.25D for most cases (Holladay J, Pettit G. JCRS 2019;45:272-283).
“Since the goal is minimum long-term residual astigmatism, remember one more thing,” Dr Holladay concluded. Astigmatism drifts about 0.25D against-the-rule each decade, so a small amount of with-the-rule is desirable immediately after surgery.
Jack Holladay: firstname.lastname@example.org