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Posterior corneal astigmatism can be incorporated into toric IOL selection
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Since before the turn of the 20th century, we recognized that there was a mismatch between anterior corneal astigmatism and refraction, although we were unable to pinpoint the cause. However, a new look with the aid of modern technology has shown us that posterior corneal astigmatism, or PCA, is also a significant player in the overall picture.
Why PCA?
In 2009, Ho and colleagues published a study of 493 subjects comparing the keratometric corneal astigmatism with the total corneal astigmatism derived by double-angled vector analysis of both corneal surfaces with the Pentacam (Oculus). They reported that in 28.8% of eyes, total corneal astigmatism differed from anterior corneal astigmatism by more than 0.5 D and that, overall, PCA reduced total corneal astigmatism by an average of 13.4%.
Furthering our understanding of the impact of PCA, my colleagues and I published data on 715 corneas of 435 consecutive patients in 2012 with several useful observations. First, the mean magnitude of PCA was –0.3 D. Second, the posterior cornea is steeper along the vertical meridian in more than 80% of eyes, creating against-the-rule (ATR) refractive astigmatism. Third, we demonstrated that in eyes with increasing amounts of anterior with-the-rule (WTR) astigmatism, there is a corresponding increase in PCA that tends to partially compensate for the WTR astigmatism of the anterior cornea. Fourth, mean PCA stays relatively constant in eyes with increasing amount of ATR anterior astigmatism.
Overall, we understood that not measuring PCA would lead to overcorrection of WTR astigmatism and undercorrection of ATR astigmatism. We confirmed this in a follow-up case series that measured corneal astigmatism using five different devices before and after cataract surgery in which a toric IOL was placed. WTR prediction errors of 0.5 D to 0.6 D were caused by all devices in WTR eyes. In ATR eyes, the WTR prediction errors varied from 0.2 D to 0.3 D when using all devices except the Placido dual Scheimpflug analyzer (Ziemer Ophthalmic Systems). Now our attention turned to how to measure PCA.
Image: Koch DD
Determining PCA
Direct measurement of each eye is the most accurate way to determine PCA; however, this is not always feasible due to a variety of factors such as equipment limitations and logistical barriers. Currently, there are a variety of ways to measure posterior corneal curvature: Scheimpflug technology, OCT and, with increasing popularity, the Cassini multicolor LED shape analyzer (i-Optics). Intraoperative aberrometers such as ORA (Alcon) and Holos (Clarity Medical Systems) also incorporate PCA into their calculations by measuring the refractive power of the entire eye in an aphakic state. All of these devices calculate refraction and astigmatism of an aphakic eye from both the anterior and posterior corneas, which is not a direct measurement of PCA.
All of these factors are precursors to the idealistic state in which we can measure everyone individually without reliance on population-based means. But until we can actually measure on a patient-by-patient basis in an accurate way, we need to have formulas that can predict and closely approximate PCA.
Starting from what we discovered from the 715 eyes in our 2012 study and the 57 eyes in our case series, we sought to develop a formula that would take into account PCA. Scientists at Abbott Medical Optics in conjunction with Joseph Ma and myself have developed a hybrid algorithm, which is based on the Baylor nomogram and clinical outcomes of trials of Abbott lenses. Now integrated into the Abbott toric calculator with PCA (Abbott Medical Optics), we can easily factor in the estimated PCA for each patient to maximize the accuracy of astigmatic correction with toric IOLs. Optimized for use with the Tecnis family of IOLs (Abbott Medical Optics), the calculator offers an elegant method to select toric IOLs by taking into account PCA, IOL power and pseudophakic anterior chamber depth — the key factors to understanding the effective toricity of the IOL (Figure 1).
Including PCA as a standard of care
Cataract surgery is the most frequently performed and successful surgical procedure in the U.S., but we still have room to improve by increasing predictability of residual astigmatism and consistently achieving zero residual astigmatism. Better astigmatism management starts by incorporating PCA into our preferred method, which reduces the systematic ATR error observed when only anterior corneal data are used for toric IOL power calculations. Another important consideration is the tendency for corneal astigmatism to drift in an against-the-rule direction. When PCA is ignored, the result is an overcorrection for WTR eyes or undercorrection for ATR eyes, leaving the patient with ATR astigmatism that worsens over time and erodes long-term vision quality. When we understand how PCA affects the eye, theoretically, we can leave patients slightly with-the-rule (0.25 D, if possible). When we use this comprehensive strategy, we give our patients many years to enjoy excellent uncorrected vision with little or no astigmatism.
- References:
- Canovas C, et al. Impact of posterior corneal astigmatism in toric IOL power calculations. American Society of Cataract and Refractive Surgery meeting; May 6-10, 2016; New Orleans.
- Ho JD, et al. Am J Ophthalmol. 2009;doi:10.1016/j.ajo.2008.12.020.
- Koch DD, et al. J Cataract Refract Surg. 2012;doi:10.1016/j.jcrs.2012.08.036.
- Koch DD, et al. J Cataract Refract Surg. 2013;doi:10.1016/j.jcrs.2013.06.027.
- For more information:
- Douglas D. Koch, MD, can be reached at Baylor College of Medicine, Department of Ophthalmology, 6565 Fannin, NC205, Houston, TX 77030; email: dkoch@bcm.edu.
Disclosure: Koch reports he is a consultant for Abbott Medical Optics, Alcon and Bausch + Lomb and receives research support from i-Optics and Ziemer.