Toric IOLs have been hailed for enabling improved surgical accuracy. But with that gain, refractive cataract surgeons have also become more aware of corneal astigmatism and its management. Compared to what we once may have thought, astigmatism management requires more surgical planning than just picking a toric power based on topography values alone. Surgeons must factor in various other considerations, including different nomograms, online calculators, the patient’s age, target for astigmatic correction and the relaxing effects of the surgical wounds. Here are six tips to help guide your thought process for surgical planning.
1. Measure anterior corneal astigmatism accurately and completely. You should obtain a minimum of two consistent measurements from two reliable devices (e.g., manual keratometry, optical biometry, topographer). Be sure to get both axis and magnitude measurements on each. Device notes:
- The Lenstar (Haag-Streit) may be more accurate than the IOL Master (Carl Zeiss Meditec) because more data points are acquired; the Lenstar captures 32 data points within the central 2.30 and 1.65 mm.
- The IOL Master captures data from only six data points within a central 2.50-mm ring.
- Placedo-based topographers are excellent for both instantaneous capture of anterior corneal measurements and identifying corneal irregularities or dryness. The latter is usually depicted by areas of dropout or poor placedo disc images.
- The Cassini topographer (i-Optics) is based on color LED technology with 700 projected data points rather than placedo rings. It may be less dependent on the precorneal tear film to capture accurate data points.
2. Factor in the posterior cornea when calculating total refractive astigmatism. In recent research, Douglas Koch, MD et al, found that 84 percent of all patients have with-the-rule (WTR) posterior corneal astigmatism. Posterior corneal astigmatism optically functions like a negative lens. As a result, WTR posterior corneal astigmatism functionally adds to the total ocular against-the-rule (ATR) astigmatism.
- On average, corneas with anterior WTR astigmatism have about 0.5 D of WTR posterior corneal astigmatism.
- Corneas with anterior ATR astigmatism have about 0.3 D of WTR posterior corneal astigmatism.
- Unlike anterior corneal astigmatism, the posterior corneal astigmatism does not drift over time.
Presently, no devices capture the magnitude and axis contribution of the posterior corneal curvature well, but some promising software upgrades to current topographers and tomographers may change this in the near future. Intraoperative aberrometry devices can capture the total corneal astigmatism during cataract surgery and thus can also be helpful in refractive cataract surgery.
Because no devices can yet capture the posterior corneal curvature accurately, the Baylor astigmatism nomogram can be very helpful during the surgical planning process. It adjusts limbal-relaxing incisions and toric IOL selections according to the actual anterior corneal curvature measurements as well as the average posterior corneal curvature measurements based on Dr. Koch’s aforementioned findings.
3. Account for the astigmatic effects of your corneal incisions. In order to perform refractive cataract surgery, it is very important to refine your surgically induced astigmatism separately for your femtosecond laser–assisted cataract and manual cataract surgeries. This can be done by reviewing and comparing preoperative and postoperative topographies, ideally three to five weeks after surgery.
4. Plan for anterior corneal astigmatism changes over time. Ken Hayashi, MD, PhD, et al demonstrated that corneas drift approximately 0.34 D from WTR to ATR over a 10-year period. You may want to target your patients to be slightly WTR in order to account for this natural drift. This would mean overcorrecting your ATR patients and undercorrecting your WTR patients. You should also consider the patient’s age when accounting for this potential drift. Because a younger patient is theoretically likely to experience more astigmatic drift, this may affect your plan to neutralize the astigmatism completely versus targeting for some WTR astigmatism.
5. Don’t be confused by oblique astigmatism — it is likely WTR that is marching ATR over time. Determining how much oblique astigmatism to correct can be confusing, but you may consider treating “spot-on” based on the anterior corneal curvature values. Comparing the magnitude and axis of topographic astigmatism to the refractive astigmatism can sometimes provide some insight. For example, if the topography demonstrates that the oblique cylinder is favoring more ATR, and there is a greater magnitude of astigmatism found refractively, consider treating the patient more like one who is an ATR astigmat, and adjust your astigmatism correction accordingly.
6. Keep an eye on the lens position for the most effective toricity correction. Jack Holladay, MD, provides the best explanation of this phenomenon. In short, the average axial length and IOL power will accurately correct the predicted cylinder correction at the corneal plane.
This relationship is based on the average effective lens position. Just as spherical equivalent outcomes can result in a refractive surprise in eyes that are less than 22 mm and greater than 26 mm, the actual toricity correction is also less predictable for similar reasons. In shorter eyes, you may consider decreasing the toric power beyond nomogram recommendation. Conversely, in longer eyes, consider increasing the toric power, especially if the desired astigmatism correction is between two toric power choices.
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About the author: Elizabeth Yeu, MD, is an assistant professor at the Eastern Virginia Medical School and in private practice with Virginia Eye Consultants in Norfolk, Va. After completing fellowship training in cornea, anterior segment and refractive surgery at the Cullen Eye Institute, Baylor College of Medicine, Dr. Yeu stayed on faculty for several years before moving to Virginia.