Toric IOLs are designed to correct regular corneal astigmatism. In the United States, toric IOLs are available in optical powers that can correct from approximately 1.00 D to 4.00 D of corneal astigmatism. Accommodative IOLs, MFIOLs, and EDOF IOLs all also have toric platforms.
As discussed in the previous sections, careful preoperative measurements and calculation of total corneal astigmatism using a modern toric calculator are essential in order to account for the effect of the posterior cornea. Topography or tomography can help to rule out patients who may not be good candidates for corneal relaxing incisions or toric IOLs (eg, irregular astigmatism, corneal ectatic disease).
Preoperatively, the surgeon makes horizontal and/or vertical reference marks on the cornea while the patient is in the upright position. This positioning avoids misalignment errors from potential cyclotorsion of the eye when the patient is supine. The surgeon uses these reference marks to mark the cornea at the calculated axis for toric IOL alignment. Various marking devices, smartphone apps, and intraoperative alignment systems (which use preoperative reference images) are available to assist in proper alignment of the IOL. The toric IOL is then inserted into the capsular bag and rotated so that the IOL axis markings align with the calculated steep corneal meridian (Video 9-1; Fig 9-9).
Toric IOL implantation and positioning. Courtesy of Cynthia S. Chiu, MD, FACS.
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Unintentional postoperative rotation of the toric IOL can lead to suboptimal correction or even worsening of astigmatism. Each degree of toric IOL rotation away from the optimal meridian reduces the effect of astigmatism correction by 3.3%. Misalignments greater than 30° will therefore increase the astigmatic refractive error. If necessary, a second procedure to rotate the toric IOL to the correct meridian can be considered relatively early in the postoperative period, before capsular fibrosis and tenacious capsular adherence to the IOL occur.
Figure 9-9 A toric IOL in vivo. The axis markings on the IOL are aligned with the calculated steep corneal meridian.
(Reprinted from Journal of Cataract & Refractive Surgery, Volume 40, Issue 12, Joshua C. Teichman, et al. Simple technique to measure toric intraocular lens alignment and stability using a smartphone, pp 1949–1952, Copyright 2014, with permission from Elsevier.)
Table 9-5 Possible Risk Factors for Postoperative Toric IOL Rotation
Several factors have been implicated in case reports of postoperative toric IOL rotation (Table 9-5). Anatomically, longer axial length and larger capsular bag size likely increase the risk of postoperative rotation. Additionally, the lower spherical power IOLs needed for these myopic patients have thinner optics, which may take up less space in the capsular bag. Alignment of the toric IOL to correct with-the-rule corneal astigmatism may also increase the risk of postoperative rotation.
Intraoperatively, complete overlap of the anterior capsular rim with the toric IOL optic is desirable; some surgeons avoid polishing the anterior capsule in toric IOL cases to promote adherence between the anterior capsule and the IOL. Complete removal of the ophthalmic viscosurgical device (OVD), including from behind the IOL, may also help minimize the risk of postoperative toric IOL rotation.
Postoperative incision leakage, trauma, or vigorous physical activity may also contribute to IOL rotation. Insertion of a capsular tension ring (CTR) into the capsular bag may decrease the risk of postoperative IOL rotation, especially in high-risk cases. Despite any of these anecdotal prophylactic measures, postoperative toric IOL rotation may still occur.
Tilting of both toric and nontoric IOLs can contribute to postoperative astigmatic error. For example, horizontal tilting of an IOL around the vertical meridian (with the nasal IOL border tilted anteriorly, and the temporal IOL border tilted posteriorly) will induce against-the-rule astigmatism. Larger amounts of tilt and higher IOL powers will lead to greater induced astigmatism.
Intraoperative wavefront aberrometry may assist in more accurate IOL selection and placement, particularly in eyes that have undergone prior keratorefractive surgery. This technology uses infrared (IR) light and interferometry, both to obtain an aphakic refraction as soon as the cataract has been removed (while the patient is on the operating table) and to confirm proper alignment of the toric IOL. This process allows the surgeon either to rotate the lens to minimize astigmatism or to exchange it immediately if the power is not accurate.
Fram NR, Masket S, Wang L. Comparison of intraoperative aberrometry, OCT-based IOL formula, Haigis-L, and Masket formulae for IOL power calculation after laser vision correction. Ophthalmology. 2015;122(6):1096–1101.
Miyake T, Kamiya K, Amano R, Iida Y, Tsunehiro S, Shimizu K. Long-term clinical outcomes of toric intraocular lens implantation in cataract cases with preexisting astigmatism. J Cataract Refract Surg. 2014;40(10):1654–1660.
Wang L, Guimaraes de Souza R, Weikert MP, Koch DD. Evaluation of crystalline lens and intraocular lens tilt using a swept-source optical coherence tomography biometer. J Cataract Refract Surg. 2019;45(1):35–40.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.