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EW San Diego 2015 4 Dr. Vukich: Posterior astigmatism doesn't have to be part of your routine calculation, but it's another influence that can make a difference in some patients. Dr. Hovanesian: Dr. Koch's guide- lines for 0.5 D or more against- the-rule astigmatism work best for patients who have with-the-rule or against-the-rule astigmatism. It doesn't apply as nicely for oblique astigmatism. In addition, many patients have about 0.5 D or more drift toward against-the-rule during a period of 10 years. Dr. Donnenfeld: I have a simple way to incorporate posterior corneal cylinder into my calculation. When operating temporally, you flatten and reduce against-the-rule cylinder. With posterior cylinder you have more against-the-rule cylinder, so I enter zero for my surgically induced cylinder on my Abbott Medical Optics toric IOL calculator, which, when you operate temporally, takes into account posterior corneal cylin- der effectively. Dr. Hovanesian: That's exactly right. If your usual SIA is 0.5 D of flatten- ing at 180 degrees, then the average patient's posterior corneal astigma- tism essentially negates your SIA. Dr. Donnenfeld: What would you do if you are operating superiorly? Dr. Hovanesian: When operating superiorly, you're adding your SIA at 12 o'clock to that 0.5 D against-the- rule, so add the two together rather than considering them canceled out. Dr. Donnenfeld: Does it matter if you leave the patient with a little with-the-rule or against-the-rule cyl- inder? Do you worry about flipping axes when implanting toric IOLs? Dr. Hovanesian: My goal is to obtain the lowest amount of cylinder, gen- erally at the same axis. You have to factor in posterior corneal astigma- tism and SIA, and all calculators do not do that for you. Sometimes you have to adjust for what you expect the posterior corneal astigmatism to be. Dr. Garg: If astigmatism is against- the-rule, I have less problem flip- ping the axis. I think these patients tolerate a little with-the-rule residual astigmatism, depending on their age. Younger patients probably will drift against-the-rule with time. If they're with-the rule, I try to leave them with-the-rule for the same reason. I use calculators as a guide to where I want the lens orientation to be and to help determine the toric power. I also employ intraoperative aberrometry to orient the lens and toricity. The effective toric power of an IOL is influenced by the spher- ical power. The classic 3:2 ratio of IOL power to refractive power does not hold true for IOLs outside the normal range. To the best of my knowledge, the only free commer- cial calculator that takes this into account is the Abbott Medical Optics toric IOL calculator (www.amoeasy.com/toric). Dr. Vukich: Minimizing the amount of residual astigmatism provides the best visual quality. You can reach a point where you are technically flipping the axis, but this small amount of residual refractive error is negligible. Dr. Donnenfeld: Because of posterior cylinder and latent wound drift, I leave a little with-the-rule cylinder. " I use calculators as a guide to where I want the lens orientation to be and to help determine the toric power. I also employ intraoperative aberrometry to orient the lens and toricity. " Sumit "Sam" Garg, MD Figure 3. Tecnis Toric calculator with data input. All data including Ks were from IOLMaster, axis from the topographer.

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