EyeWorld India December 2018 Issue

ods on each patient. We’ll look for agreement. Graham Barrett, MD’s toric calculator has a feature that takes the median of three different Ks computed from anterior surface measurements. This essentially throws out the outliers, and he’s found it to provide better correc- tion targets overall. But you have to make sure you use Ks based on anterior surface measurement and not the total corneal power ray traced through the front and back of the cornea. YES connect co-editors: How do you deal with discrepan- cies between different measure- ments when planning to deal with astigmatism? Dr. Miller: Here we rely on our technicians. They’re very good at determining whether a map is lousy or good. If we’re repeating a map, we’re repeating it on the spot. It’s not that often that the cor- nea is so messed up, such as with punctate keratopathy, that we have to bring them back on a different date. We’ll do the measurements until it either plateaus and we get consistent measurements or their irregularities go away. Dr. Weikert: If it’s a quality of measurement issue, we’ll repeat it. If it’s a weird cornea, we might look at that region of the cornea where they’re not getting a good measurement. Sometimes the pa- tient might have had several meas- urements by that point, which can change the ocular surface. We did a study where we back calculated the ideal axis of astigmatism using the Berdahl-Hardten Toric Results Analyzer, and none of devices we looked at stood out from the group. In other words, we did not identify a “go-to” device. I think looking at the steep meridians from your an- terior surface devices and compar- ing them to measured total corneal powers can be interesting and help show the contribution of the back of the cornea. YES connect co-editors: When planning for an LRI, what is your preferred nomogram? Dr. Miller: My personal nomogram is easy to remember, so you don’t have to go to a calcula- tor or plug in any data. If we’re looking at a symmetric bowtie on corneal topography, I will look at the Sim K value of astigmatism. I make paired peripheral corneal incisions that are as long in clock hours as the cornea is steep in di- opters, assuming this is done at the time of cataract surgery and that the phaco incision will be placed through one of the LRI incisions. I don’t mark the lengths of the inci- sions on the cornea when I do my relaxing incisions; I approximate. I can envision clock hours better than I can degrees and I’m pretty accurate. That nomogram is super simple, and it works. Dr. Weikert: We have an in- house nomogram. There are also several femto nomograms that are available. Eric Donnenfeld, MD, has a femto nomogram, and Julian Stevens, MD, has a femto nomo- gram for intrastromal incisions. Nichamin and Donnenfeld have manual LRI nomograms; that have been around for awhile and are very useful. YES connect co-editors: If you only had one option for to- pography, what would you prefer for routine cataract planning— Scheimpflug or Placido disc? Dr. Miller: I use Scheimpflug because you’re seeing the whole cornea with it. You’re only seeing the anterior cornea with a Placido disc device. You have to make assumptions about the posterior cornea unless you actually meas- ure it, and those assumptions may be wrong. It’s better to measure it. The problem is that tomography devices are more expensive than topography devices. You have to make sure you plug the right data into the right formula. You don’t want to take Scheimpflug data and plug it into the Barrett formula be- cause it is going to double compen- sate for the posterior surface. Dr. Weikert: We think there’s a role for both, but right now for cataract surgery, I’d want a Placido. You get a lot of surface information and it’s easier to measure curvature with reflection technology than it is with Scheimpflug or elevation- based measurements. When we’re planning toric correction, we primarily rely on methods that use population-based levels for posteri- or corneal astigmatism, such as the Baylor nomogram and the Barrett toric calculator. EWAP Editors’ note: Dr. Miller and Dr. Weik- ert have no financial interests related to their comments. Contact information Miller: kmiller@ucla.edu Weikert: mweikert@bcm.edu Notice the irregularity in the patient’s mires in the left image compared to the right image after a phototherapeutic keratectomy. Source: ASCRS Preoperative diagnostics – from page 49 50 EWAP CATARACT/IOL December 2018