Toric IOLs provide an excellent means of correcting pre-existing corneal astigmatism with cataract surgery. Current studies have shown that the combined residual astigmatism and the spheroequivalent error must be less than 0.5 D for the patient to be happy with his uncorrected vision. To achieve this goal, there are three fundamental rules that must be observed.
First, you must use an “exact” toric calculator that does not use a constant ratio between the pre-existing corneal astigmatism and the recommended toricity of the IOL. You can easily check this by changing the spheroequivalent power to 10 D and 34 D, and if the same toricity is recommended, it is an “approximation calculator.” The currently available exact toric calculators are the Abbott Medical Optics online calculator, the Alcon Verion System and the Holladay IOL Consultant Program.
Second, you must add 0.5 D of against-the-rule (ATR) astigmatism to the pre-existing corneal astigmatism and use zero for your surgically induced astigmatism to have a near-zero long-term residual refractive astigmatism after surgery. The Baylor nomogram by Doug Koch and Li Wang does this by recommending a reduction of with-the-rule (WTR) correction by one toric step size and an increase of ATR by one toric step size and no adjustment for oblique astigmatism. The toricity step size for most manufacturers is 0.75 D, which is approximately 0.5 D at the spectacle plane. A recent article by Adi Abulafia and Graham Barrett reaffirms the Baylor nomogram by demonstrating that “exact” toric calculators need to have 0.33 D to 0.5 D ATR added to achieve the best result. The Barrett toric calculator is the only one that includes this adjustment in the calculator. You may see a little WTR (0.25 D to 0.37 D) in the first few months, but it will fade away by 3 to 6 months postoperatively.
Third, the goal must be to achieve the minimum long-term residual astigmatism irrespective of the axis, whether “flipped” or not at 3 to 6 months postoperatively. The myth of choosing the maximum toric IOL that does not flip the axis even if the one above results in smaller residual astigmatism comes from prescribing spectacles. It is true that you should never flip the axis of astigmatism in a pair of glasses in an adult because it produces asthenopic symptoms from meridional aniseikonia. It often causes dizziness with head movement and an inability to walk down stairs because the steps are not where they used to be. Optically, this is called distortion. Following cataract surgery, the goal is to be independent of spectacles, and there is no distortion with uncorrected residual astigmatism. The only factor relating to quality of vision is the magnitude of the residual astigmatism, irrespective of the axis. One should always choose the IOL toricity that achieves the minimum long-term residual astigmatism.
- Abulafia A, et al. J Cataract Refract Surg. 2015;doi:10.1016/j.jcrs.2014.08.036.
- Koch DD, et al. J Cataract Refract Surg. 2013;doi:10.1016/j.jcrs.2013.06.027.
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- Jack T. Holladay, MD, MSEE, FACS, is a clinical professor of ophthalmology at Baylor College of Medicine and the OSN Optics Section Editor. If you have any clinical optics questions, please send them to firstname.lastname@example.org.
Disclosure: Holladay reports he is a consultant to Abbott Medical Optics, AcuFocus, Alcon Laboratories, ArcScan, Calhoun Vision, Carl Zeiss, Elenza, Oculus, Visiometrics and WaveTec.