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Advanced IOL platform provides benefits for surgeons and patients EW San Francisco 2013 7 When should you use a toric IOL for visually significant corneal astigmatism? by James A. Davison, MD, FACS Patients with low cylinder— as well as higher levels of astigmatism—can benefit from toric lenses W arren Hill, MD, evaluated 6,000 cataract patients and found 18% had at least 1.5 D of keratometric astigmatism.1 He also found that 52.6% had at least 0.75 D. So if we confine ourselves to thinking about toric IOL correction only for patients with at least 1.5 D astigmatism, we may be missing more than half the patients who could benefit from the technology. It has been shown that refractive cylinder becomes substantially visually significant somewhere around 1.0 D.2 But when prescribing spectacles or performing LASIK, 0.75 D is normally considered significant. In those cases, correction is fairly easily achieved because those technologies are direct, extremely accurate, and additional individual variables are few. Implanting a lens to do the same job is similar but has a couple of additional variables that need to be considered to produce good results. In order to predict the best result, the equation variables that need to be included are: preoperative measurements of corneal astigmatism and surgically induced astigmatism. Further, to achieve good uncorrected vision, we must minimize the residual spherical equivalent in addition to minimizing refractive cylinder. Without the use of intraoperative aberrometry, my own data, using Wolfe Eye Clinic's best machines and practices, show 7% of my patients have residual spherical equivalent of 0.75 D or more. Toric lenses are quite appropriate for patients with keratometric astigmatism between 0.75 and 1.38 D. Even when using an average SIA of 0.25 D and anterior keratometric astigmatism measurement, the mean postoperative refractive astigmatism was statistically significantly lower with toric IOLs than with spherical IOLs (0.31 D versus 1.06 D; P<0.001).3 Hill reported SIA becomes a more significant issue for patients with lower levels of cylinder.4 Intraoperative aberrometry on each case may be an ideal option, but those of us without the technology use an average SIA derived from our own experience. I've found 2.2 to 2.4 mm incisions will induce about 0.25 D of astigmatism on average; that's what I use in toric calculations. As Ernest and Potvin found: "the lower the degree of astigmatism induced at the time of surgery, the more precise the postoperative correction is likely to be because the variability in that SIA will be lower and lower."3 Flipping axes If we ignore the SIA, we can say that postoperative refractive cylinder magnitude is independent of preop corneal astigmatism axis orientation.3 In essence, that means that a patient with 0.75 D of preop astigmatism may be corrected with a toric IOL value of 1.0 D—leaving him with 0.25 D in the other direction. Some may argue that "flipping" the axis creates more of an issue or that astigmatism provides increased depth of focus. I believe that it's more the magnitude of astigmatism that affects uncorrected vision rather than the axis of cylinder. Corneal vs. lens-based correction Pentacam reading at 3.0 mm ring shows total corneal refractive power of 1.0 D at 7.0 degrees vs. anterior sagittal of 0.8 D at 9.7 degrees Source: James Davison, MD, FACS Compared with limbal relaxing incisions (LRIs), toric IOLs are more reliable, which translates to better postop vision for our patients.5 Toric lenses are manufactured to tight laboratory specifications that are extremely consistent and thus predictable. LRIs, however, are notoriously unpredictable because they are so variably performed, because of the various amounts the diamond knife will penetrate, and because the locations of the arcs are not consistent. In addition, LRIs have a less direct effect than toric IOLs, i.e., operating on the cornea's periphery while trying to effect a change of shape at the visual axis. James A. Davison, MD, FACS Conclusions The key to toric lenses and astigmatic correction is to discuss with patients that we're trying to predict their results. Surgeons are happier with higher toric powers because we know we'll be able to help someone improve his/her vision (presuming we properly insert and align, of course). But with proper measurements including accurate preoperative measurements and equipment and accurately determined variables, we should be able to reduce even smaller amounts of astigmatism as well. References 1. Hill WE. Surgeon recommends toric IOL for patients with low corneal astigmatism. EyeWorld Show Daily Supplement. 2010: Boston. 2. Wolffsohn JS, Bhogal G, Shah S. Effect of uncorrected astigmatism on vision. J Cataract Refract Surg. 2011;37:454-460. 3. Ernest P and Potvin R. Effects of preoperative corneal astigmatism orientation on results with a low-cylinder-power toric intraocular lens. J Cataract Refract Surg. 2011;37:72732. 4. Hill W. Expected effects of surgically induced astigmatism on AcrySof toric intraocular lens results. J Cataract Refract Surg. 2008; 34:364–367. 5. Mingo-Botin D, Munoz-Negrete FJ, Kim HRW, Morcillo-Laiz R, Rebolleda G, Oblanca N. Comparison of toric intraocular lenses and peripheral corneal relaxing incisions to treat astigmatism during cataract surgery. J Cataract Refract Surg. 2010;36:1700-1708. Dr. Davison is in private practice, Wolfe Eye Clinic, Marshalltown and West Des Moines, Iowa. He can be contacted at 641-754-6200 or jdavison@wolfeclinic.com. Even when using "an average SIA of 0.25 D and anterior keratometric astigmatism measurement, the mean postoperative refractive astigmatism was statistically significantly lower with toric IOLs than with spherical IOLs (0.31 D versus 1.06 D; P<0.001). "