Nomogram personalization necessary for femtosecond astigmatic correction
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I developed my personal nomogram for femtosecond astigmatic incisions in cataract surgery by starting with the Nichamin-Woodcock nomogram and programming that into the femtosecond laser software.
I then made specific changes to manipulate the nomogram with the Lensar software. I changed my radius of treatment to 4.3 mm, so instead of having a 9 mm diameter of treatment, I have a diameter of 8.6 mm. I also set my incisions at 90% depth instead of 85% to 90%. I determined these specific changes by starting out with the suggested nomogram settings and then checking my results after roughly 30 to 40 eyes to see whether the patients were overtreated or undertreated. I then adjusted one parameter and repeated the process, continuing to tweak my settings until I was satisfied. When I achieved the best possible results, I settled on my current parameters.
Keep in mind that every laser is different, so each surgeon will need to see what kind of results they get and then tweak them as they see fit by adjusting the incision radius, depth or length. You also want to make sure that everything you do surgically is technically and consistently the same. Eliminating variables in your technique enables you to titrate the nomogram more accurately. After you look at the results of a couple dozen patients, make adjustments, rinse and repeat.
When adjusting the nomogram, we use a spreadsheet. We write down the topography, astigmatism and how much we decide to treat. We look at several different topographies to determine what we are treating and where, and then we look at the refractive result. Even if someone is 20/20 uncorrected, we attempt refraction to see if the vision can be improved. We do not want to assume that the patient is plano. Next, I consider the depth of my incision. Ninety percent is as deep as I prefer to go, and I look for a trend. If I operate on 50 people, and only one person misses the mark, I will not change my whole nomogram for that one result. But if I were consistently 0.5 D undercorrected, I would strengthen my treatment by making the incisional cord length longer, the incision deeper or the arcuate incisional radius smaller (my personal limit is 4.3 mm).
Additionally, incorporating a nomogram is important when performing astigmatic incisions. The nomogram requires a lot of work on the front end to find your laser’s sweet spot, but it is essential to ensure that your corrections are adequate. Equally as important is getting the treatment on the correct axis of astigmatism. Because the Lensar system performs an iris registration maneuver, our incisions are perfectly aligned. Although iris registration is the most accurate method of axis alignment, systems using limbal blood vessels or other modalities also have value. We work hard to know what and where to treat, so it is critically important to have a system to mark as accurately as possible and treat as accurately as possible.
- For more information:
- Denise M. Visco, MD, MBA, can be reached at Eyes of York, 1880 Kenneth Road, Suite 1, York, PA 17408; email: deniseviscoeyes@gmail.com.
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