We congratulate Felipe et al.1 on their excellent study. The authors did a tremendous job quantifying the effects of toric intraocular lens (IOL) rotation using both a matrix definition and vector analysis. They found that the approaches provided equivalent results and that a toric IOL rotation of 10 degrees results in a 30% reduction in effectivity of a toric IOL.
Since a malposition of only 10 degrees can result in a 30% decrease in effectivity, proper alignment is critical, especially with higher-powered toric IOLs and the future of toric multifocal IOLs. It is worth noting that the toric IOL may have been placed in the “intended” axis (based on preoperative keratometry), but the intended axis may not be the “ideal” axis because of inaccurate or fluctuating keratometry, posterior corneal curvature, surprising surgically induced astigmatism (SIA), or variance between the axes of astigmatism at different optical zones, as often seen with topographic analysis. We have recently created the toric results analyzer,A which uses a vector analysis similar to that of Felipe et al. to determine the ideal position of a toric IOL. The analyzer is applicable when a patient has residual astigmatism after implantation of a toric IOL. By entering the patient’s current manifest refraction (post-toric IOL) and the power and current axis of the toric IOL, the toric results analyzer will calculate the ideal axis of the toric IOL compared with the current axis. It will also predict a post-rotation refraction. The postoperative manifest refraction incorporates the necessary information (keratometry, posterior corneal curvature, and SIA) into a final resultant state of the eye, allowing a calculation to determine whether a rotation would be helpful.
One example is a patient with 4.0 diopters of corneal cylinder who had implantation of an Alcon SN6AT9 IOL. Postoperatively, the patient had a manifest refraction of −1.00 +1.75 × 150 and an uncorrected visual acuity of 20/60. The axis of the IOL was positioned at 108 degrees, which was the intended axis based on the preoperative keratometry. However, the preoperative topographic axis with the Humphrey atlas was slightly different than the keratometry, which was also slightly different than the Oculus Pentacam topography. The toric results analyzer predicted that rotating the toric IOL 12 degrees counterclockwise would result in a refraction of −0.40 +0.54 × 112. The patient was taken back to the operating room, and the IOL was rotated 12 degrees counterclockwise (actually rotated 168 degrees clockwise since it is easier to rotate clockwise). The resultant refraction at 1 month was −0.50 sphere with an uncorrected acuity of 20/20−1.
One practical tip is that it is actually more accurate to figure out how much to rotate the IOL than to which axis to rotate the IOL as this can be directly marked in the operating room, leading to greater accuracy.
We hope the toric results analyzer, which is freely available, will be useful to our colleagues.
1. Felipe A, Artigas JM, Díez-Ajenjo A, García-Domene C, Pablo Alcocer P. Residual astigmatism produced by toric intraocular lens rotation. J Cataract Refract Surg. 2011;37:1895-1901.
OTHER CITED MATERIAL
A. Berdahl J, Hardten D. Toric results analyzer. Available at: www.astigmatismfix.com
. Accessed December 9, 2011