Eyeworld

EW CATARACT 26 August 2018 plan our astigmatism correction, we look at the total corneal power, but when selecting the magnitude of correction, we still rely on popula- tion norms to assess the posterior contribution to the patient's astig- matism. Current devices still have a lot of variability in their assessment of posterior corneal astigmatism. Attendee: Do you use manual Ks ever? Dr. Miller: I do not, but I do think there's a value. There is something to be gained experience-wise in learning how to obtain manual Ks. For instance, one of the things you find when you use a manual ker- atometer is that it's hard to nail the axis when the amount of astigma- tism is low. You also learn that cen- tration is important. I wouldn't use manual Ks anymore because there's just not enough information. There's much more information if you use a topographer or tomographer. Dr. Weikert: [I do not.] They can be challenging to measure, and they have more of a learning curve as compared to auto-Ks. Even though our calculation formulas were developed using manual Ks, once we optimize our lens constants, auto-Ks work well. When you look at manual Ks compared to auto-Ks, they measure different zones of the cornea, so you would expect some disagreement between them. Attendee: What are your experienc- es with surgeon-specific biometry like the Hill-RBF? Any tips for transi- tioning from your go-to calculations to the program? Dr. Miller: The Hill-RBF is my pri- mary formula now. What attracted me was Warren Hill, MD's data showing that 90% of his patients are within 0.5 D of emmetropia after surgery. That's pretty compelling and a reasonable reason to use it. Like anything, I don't immediately transition. I go through a phased transition, and I'm still doing that. I advise surgeons to use their comfort zone formulas for awhile and be looking at the other one; then they can eventually flip over and plan off the new one, while looking back at the old formulas to make sure they don't have things that are complete- ly nonsensical. Dr. Weikert: Any time you have something new, you want to con- tinue to do what you're comfortable with, what you have experience with, then you can compare those results to the new method. When we do our printouts, we have three formulas. We use the Holladay 1, Barrett, and Hill-RBF. I get those for every patient and I compare them. You often find that in a certain type of eye you might lean toward one formula over another. YES connect co-editors: When planning for a toric IOL, do you rely more on the topographer (Placido disc) or the biometer for the axis of astigmatism? What about for the amount of astigmatism? Dr. Miller: I use tomography devices, primarily the Pentacam, although we have a Galilei and oc- casionally I'll look at that. I look at the Pentacam axial map for both the amount of astigmatism, which I get from the Sim K values, and the axis, which I get from the Sim K axis. I do look at the total cornea. If the Sim K axis is 45 but the peripheral cornea is clearly against the rule, more like 180, I bias my toric axis toward the 180. For weird corneas, I'll split the difference. Often there's a difference between the anterior cornea and the total cornea, and a purist would say you should go with the total cornea, but my comfort level isn't totally there yet. I take the total, but I swing the axis a little bit toward the anterior. For the power, I tend to bias toward the total cornea. Dr. Weikert: I would say I don't defer to one device all the time. We bring in multiple methods on each patient. We'll look for agreement. Graham Barrett, MD's toric calcula- tor has a feature that takes the me- dian of three different Ks computed from anterior surface measurements. This essentially throws out the outliers, and he's found it to provide better correction targets overall. But you have to make sure you use Ks based on anterior surface mea- surement 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 discrepancies between different measurements when plan- ning 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 cornea 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 measure- ment. Sometimes the patient might have had several measurements 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-Hard- ten 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 anterior surface devices and comparing them to measured total corneal powers can be interest- ing 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 nomo- gram is easy to remember, so you don't have to go to a calculator 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 diopters, 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 incisions on the cornea when I do my relaxing incisions; I approx- imate. 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 avail- able. Eric Donnenfeld, MD, has a femto nomogram, and Julian Ste- vens, MD, has a femto nomogram 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 topography, 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 Placi- do disc device. You have to make assumptions about the posterior cornea unless you actually measure it, and those assumptions may be wrong. It's better to measure it. The problem is that tomography devices are more expensive than topogra- phy 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 because it is going to double compensate 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 mea- surements. When we're planning to- ric correction, we primarily rely on methods that use population-based levels for posterior corneal astigma- tism, such as the Baylor nomogram and the Barrett toric calculator. EW Editors' note: Dr. Miller and Dr. Weikert have no financial interests related to their comments. Contact information Miller: kmiller@ucla.edu Weikert: mweikert@bcm.edu Preoperative continued from page 24 The webinar "Know Your Tools/Toy Box: Preoperative Diagnostics" is available to ASCRS members on the ASCRS Center for Learning at www.ascrs.org/cen- ter-for-learning/ video/yes-know-your-toolstoy-box- preoperative-diagnostics.

• Cover