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Astigmatic overcorrection and axis flip for targeting minimal remaining refractive astigmatism with toric intraocular lenses

Beheregaray, Simone MD, PhD; Goggin, Michael FRCSI (Ophth), FRCOphth, FRANZCO, MS; LaHood, Ben FRANZCO, MB ChB

Journal of Cataract & Refractive Surgery: January 2018 - Volume 44 - Issue 1 - p 109-110
doi: 10.1016/j.jcrs.2017.11.004
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Toric intraocular lenses (IOLs) correct the likely refractive effect of corneal astigmatism in pseudophakic eyes. With some exceptions, the manufacturers provide the IOLs in astigmatic correcting steps, and surgeons are faced with a choice of power that seldom is predicted to provide exact emmetropia. For toric correction in some eyes, a small overcorrection might yield a predicted remaining refractive astigmatism that is smaller than an undercorrecting toric IOL. An optical consequence of astigmatic overcorrection is that the remaining refractive astigmatism will be at 90 degrees to the axis of the corneal astigmatism; that is, an axis flip occurs. It is an optometric principle that overcorrection of astigmatism in a spectacle prescription causes discomfort by altering meridional magnification and spatial distortion. These are related to the spectacle vertex distance and do not apply for IOL implantation. Despite this, the advice not to “flip the axis” with toric IOLs appears in non-peer-reviewed publications and is the subject of warnings on manufacturers’ online toric calculators.A–C

We reviewed 206 eyes that had microincision phacoemulsification and toric IOL implantation. Keratometry and biometry were performed using the IOLMaster 500 and IOLMaster 700 (both Carl Zeiss Meditec AG). Keratometry values were adjusted using the Goggin nomogram when required.1 The Carl Zeiss Meditec online calculator was used.D All eyes were targeted with the smallest residual refractive astigmatism, whether undercorrected (no axis flip) or overcorrected (axis flip). The preoperative anterior keratometric cylinder, target refractive cylinder, toric IOL sphere and cylinder, and postoperative refractive cylinder were calculated. In the eyes in which astigmatic overcorrection was targeted, the residual postoperative astigmatic outcome was compared with the vector-derived likely astigmatism of a toric IOL with a cylinder power reduced by 0.50 diopter (D). This is the amount by which 1 cylinder power step differs from the next for the manufacturer whose IOLs we used. The paired-sample 2-tailed student t test was used to compare groups.

Forty-eight eyes (23.3%) had toric IOLs inserted with a targeted overcorrection of corneal astigmatism, and 31 effectively achieved overcorrection. Of these eyes, 38 had implantation of an AT Torbi 709MP IOL and 10 had implantation of the AT LISA tri toric 939MP IOL (both Carl Zeiss Meditec AG). Table 1 shows the preoperative keratometric cylinder, target refractive cylinder, and spherical and cylinder powers of toric IOLs implanted. The mean postoperative refractive astigmatic power was 0.52 D ± 0.46 (SD), and the mean vector-derived likely refractive astigmatic power outcome was 0.82 ± 0.54 D (P = .0001) if the IOL cylinder were reduced by 0.50.

Table 1
Table 1:
Demographics of eyes that had astigmatic overcorrection by toric IOLs.

This indicates that should the choice be made not to flip the axis in eyes in which overcorrection of the astigmatism would predictably lead to lower remaining refractive astigmatism, the remaining astigmatism in these eyes would be higher. Hoffmann et al.2 reported astigmatic overcorrection in 42.5% of eyes, and the overcorrection was well tolerated by the patients.

Some online toric calculators, such as those posted by Carl Zeiss MeditecD and the Bausch & Lomb,E will by default suggest that the best choice is the IOL with the highest cylinder power that will not flip the residual astigmatic axis. The Alcon calculatorB and the Johnson & Johnson Vision (formerly Abbott Medical Optics) calculatorC highlight when the residual astigmatism meridian is being flipped. The Barrett toric calculatorF suggests as a choice the one with the smallest residual astigmatism and does not highlight axial flip. The Hoya calculatorG displays IOL options for under and overcorrections. Surgeons should be aware of IOL ranges and calculator characteristics to decide which is the best option for their patients.

Astigmatic overcorrection and axial flip by toric IOLs is beneficial for the astigmatic outcome and is more likely to yield spectacle independence, in particular with toric IOL designs with larger cylinder power steps. It has been argued that with the tendency for anterior corneal astigmatism to change from with-the-rule (WTR) to against-the-rule (ATR) with age, targeting some remaining WTR astigmatism might be beneficial for patients over the long term.3,4 Our patient base is largely elderly and in the age group in which this change has already occurred and ATR anterior corneal astigmatism predominates. We suggest that this sort of overcorrection would therefore be necessary to yield the suggested WTR target. However, we have a different view on the ideal target. The argument for targeting WTR astigmatism is based on an application of the population norm of ATR anterior corneal astigmatism drift with age to individuals who might not end up ATR or who might not survive to have this change. In contrast, our philosophy of targeting the lowest possible astigmatism achievable today for the patient has an immediate benefit for all patients, at least in the medium term. Thus, no matter which technique one uses, overcorrection and axis flip are necessary on occasion.

References

1. Goggin M, van Zyl L, Caputo S, Esterman A. Outcome of adjustment for posterior corneal curvature in toric intraocular lens calculation and selection. J Cataract Refract Surg. 2016;42:1441-1448.
2. Hoffmann PC, Auel S, Hütz WW. Results of higher power toric intraocular lens implantation. J Cataract Refract Surg. 2011;37:1411-1418.
3. Hayashi K, Manabe S-i, Hirata A, Yoshimura K. Changes in corneal astigmatism during 20 years after cataract surgery. J Cataract Refract Surg. 2017;43:615-621.
4. Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toric intraocular lenses: effect of posterior corneal astigmatism. J Cataract Refract Surg. 2013;39:1803-1809.

Other Cited Material

A. Kent C. 37 ways to get great outcome with torics. Rev Ophthalmol January 2012. Available at: https://www.reviewofophthalmology.com/article/37-ways-to-get-great-outcomes-with-torics. Accessed November 21, 2017
B. Alcon Laboratories, Inc. The new ALCON® online toric IOL calculator incorporating the Barrett toric algorithm. Available at: https://www.acrysoftoriccalculator.com/features.htm. Accessed November 21, 2017
C. Johnson & Johnson Vision. TECNIS® IOL Calculator Platform. Available at: https://www.amoeasy.com/calc(bD1lbiZjPTA1MA==)/Default.htm?P_COUNTRYCODE=US. Accessed November 21, 2017
D. Carl Zeiss Meditec AG. [Online calculator] Available at: https://zcalc.meditec.zeiss.com/zcalc/. Accessed November 21, 2017
E. Baush+Lomb. enVista® toric calculator. Available at: https://envista.toriccalculator.com/. Accessed November 21, 2017
F. American Society of Cataract and Refractive Surgery. Barrett toric calculator. Available at: http://www.ascrs.org/barrett-toric-calculator. Accessed November 21, 2017
G. Hoya Surgical Optics. Hoya toric calculation. Available at: http://us.hoyatoric.com. Accessed November 21, 2017
© 2018 by Lippincott Williams & Wilkins, Inc.