August 2018


YES connect
Preoperative diagnostics for cataract surgery

by Liz Hillman EyeWorld Senior Staff Writer

The stakes are higher than ever to achieve an optimal refractive result after cataract surgery. It is imperative that we know what preoperative tools we have and how to interpret them to come up with an optimal plan for our patients.
In this month’s “YES connect” column, we highlight some important questions that came up during the recent ASCRS webinar “Know Your Tools/Toy Box: Preoperative Diagnostics.”
Although you may have different tools available in your office than the ones reviewed in the webinar, there is a common theme that can be applied to your individual practice. Understanding the ins and outs of your topographers and biometers can give you confidence that your lens selection and surgical plan are providing you every advantage possible for an optimal outcome.
For those of you in training, I recommend that you expose yourself to and familiarize yourself with all the different tools you have access to now in your program, as there is a good chance that you will have a similar device in your practice after training. Alternatively, if you are already in practice, perhaps at some point you may want to purchase a new topographer or biometer, and this webinar may help guide your decision on what device to purchase.
This column launches a new pair
of “YES connect” co-editors, and David Crandall, MD, and I are excited to be involved.

Samuel Lee, MD,
YES connect co-editor


Notice the irregularity in the patient’s mires in the left image compared to the right image after a phototherapeutic keratectomy
Source: ASCRS

An ASCRS Young Eye Surgeons (YES) Clinical Committee-sponsored webinar featured an in-depth discussion about preoperative diagnostics for cataract surgery

Mitchell Weikert, MD, associate professor and residency program director, Cullen Eye Institute, Baylor College of Medicine, Houston, and Kevin M. Miller, MD, Kolokotrones Chair in Ophthalmology, chief of cataract and refractive surgery, Stein Eye Institute, David Geffen School of Medicine at UCLA, Los Angeles, provided expert insights for “Know Your Tools/Toy Box: Preoperative Diagnostics.” The webinar was hosted by YES Clinical Committee members Zaina Al-Mohtaseb, MD, Houston, and Samuel Lee, MD, Sacramento, California. It included specific case examples in which Dr. Weikert and Dr. Miller shared their thoughts on topography, tomography, biometry and specific differences among preoperative diagnostic devices; their experience with different calculators and formulas; and their perspective on handling special circumstances like how long to wait before taking measurements after a patient stops rigid gas permeable lens use or has a Salzmann’s nodule removed to ensure reliability.
EyeWorld caught up with Dr. Weikert and Dr. Miller after the webinar so they could answer a few questions from attendees of the webinar, as well as follow-up questions that expound upon the topics that were discussed during the hour-long educational event.

Attendee: What is the difference between axial curvature and instantaneous curvature?

Dr. Miller: On some machines, axial is called sagittal and instantaneous is called tangential. With axial maps, the software assumes that light rays, as they travel through the cornea and entrance pupil, all intersect somewhere along the optical axis of the eye. Instantaneous maps don’t make that assumption; they allow rays to intersect wherever they actually do, and often times that’s off the optical axis. What does that mean when you display those maps? You get a greater smoothing function with axial maps. The cornea will look smoother than perhaps it actually is. If you want to see the hills and valleys, the little bumps, the little dry spots, then you look at the instantaneous or tangential map. For astigmatism planning, I use axial maps, and if I’m trying to trouble shoot an eye that’s not seeing all that well and doesn’t refract very well, I will often use a tangential map to highlight subtle pathology.

Dr. Weikert: It’s the same data, just analyzed two different ways. Axial curvature looks at a point on the corneal surface and references the radius of curvature at that point to the visual axis. By tying the surface curvature at every point to the visual axis, the devices end up averaging the curvature. You may lose a little detail, especially in the peripheral cornea, but this method can reduce a lot of noise. Instantaneous maps determine curvature by looking at small areas adjacent to the point of interest and don’t tie their analysis to the visual axis. By doing it this way you get more detail, but you can also get more noise. We look at both maps, since they each can provide useful information. I like to use axial maps to assess astigmatism, but I think instantaneous maps will often give you the cornea’s true shape and highlight irregularities and subtle sources of vision loss.

Attendee: With the Pentacam (Oculus, Wetzlar, Germany), which Ks are used for astigmatism management in cataract surgery (toric or LRI)?

Dr. Miller: I have a Galilei G6 (Ziemer, Port, Switzerland) and a Pentacam HR, but I tend to use the Pentacam more. The Pentacam produces simulated keratometry values, so it will give you the Sim Ks of the anterior cornea that a corneal topographer would produce. But the Pentacam also measures the posterior surface, calculating total corneal power and total corneal astigmatism. So that’s what I use now; I use the total corneal astigmatism and put those Sim Ks into the appropriate formulas.

Dr. Weikert: I use the Galilei, which combines Placido and Scheimpflug imaging, and I don’t have access to a Pentacam. Since the Galilei is a tomographer, it can also provide the total corneal power by ray tracing light through the anterior and posterior corneal surfaces. When we plan our astigmatism correction, we look at the total corneal power, but when selecting the magnitude of correction, we still rely on population norms to assess the posterior contribution to the patient’s astigmatism. 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 keratometer is that it’s hard to nail the axis when the amount of astigmatism is low. You also learn that centration 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 experiences with surgeon-specific biometry like the Hill-RBF? Any tips for transitioning from your go-to calculations to the program?

Dr. Miller: The Hill-RBF is my primary 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 completely nonsensical.

Dr. Weikert: Any time you have something new, you want to continue 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 occasionally 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 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 correction 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 discrepancies between different measurements 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 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 measurement. 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-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 anterior surface devices and comparing 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 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 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 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 Placido 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 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 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 measurements. When we’re planning toric correction, we primarily rely on methods that use population-based levels for posterior corneal astigmatism, such as the Baylor nomogram and the Barrett toric calculator.

Editors’ note: Dr. Miller and Dr. Weikert have no financial interests related to their comments.

Contact information


The webinar “Know Your Tools/Toy Box: Preoperative Diagnostics” is available to ASCRS members on the ASCRS Center for Learning at video/yes-know-your-toolstoy-box-

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