Accurately Aligning Toric IOLs

When aligning toric intraocular lenses (IOL), just how accurate do surgeons need to be? It depends in part on the type of lens, said Douglas D Koch MD.

For monofocal IOLs, it’s generally acceptable to get within 0.75 D of residual cylinder, depending on individual patient needs. But for trifocals, the ideal goal is 0.5 D or less, which Professor Koch noted is more difficult, as the magnitude of astigmatic correction increases.

For 1.0 D of correction, alignment can be off by up to 14 degrees and still leave 0.5 D or less residual astigmatism. That drops to 7 degrees for 2.0 D correction and 3 degrees for 4.0 D correction.

“If we all set targets for ourselves to be well under 10 degrees, hopefully within 5 degrees of target, we [will] meet the needs of almost all our patients,” Prof Koch said.

Still, challenges persist that cannot be entirely overcome. Therefore, surgeons should be ready to deal with residual astigmatism after surgery.

Measurement and prediction formulas

An essential first step, Prof Koch said, should be to address corneal surface issues to obtain accurate corneal measurements.

Device variability is another source of potential measurement error, Prof Koch said. In a snapshot of 129 right eyes measured with the IOLMaster (Zeiss) and Lenstar (Haag-Streit), agreement between the two devices on the location of the steep meridian was within 5 degrees in only 60.5% of eyes and 82.9% within 10 degrees. He recommends taking multiple measurements to reduce the chance of error.

Axis prediction formula performance is another source of variability, Prof Koch said. “You can run three different toric formulas and get three different recommendations.” Still, running multiple formulas may help target an accurate recommendation.

Alignment issues

Manual versus digital alignment is debated, with some studies suggesting digital solutions are more accurate, and others finding no significant difference. In a study Prof Koch and colleagues conducted, there were no significant differences between manual and automated markings, with all deviations less than 10 degrees.¹

“You don’t need to feel compelled to use a digital system, but it offers the advantages of speed and efficiency in the operating room,” he said.

The flaws of manually “eyeballing it” at the slit lamp include marks that are too wide and run before use or are simply misplaced, Prof Koch said. He recommends observing and accounting for shifts in markings measured upright at the slit lamp to lying down on the operating table.

Several digital solutions are available, and Prof Koch’s team uses a digital marking system with manual marks as a backup. If the patient’s head is not in the same position for each, errors can be introduced by recording the axis on one device and the landmarks for finding it on another.

“Ideally, you should measure and mark using the same device.”

So how accurate are surgeons? An American Academy of Ophthalmology technology assessment reviewing 21 toric IOL studies rated level I or II found that less than half showed 80% of cases with 0.5 D or less residual astigmatism.²

“In the end, about 80% within 0.5 D might be the best you can expect,” Prof Koch concluded. “Therefore, be ready to deal with residual astigmatism at the outset.”

Prof Koch spoke at the 2025 ASCRS Refractive Day in Los Angeles.

Douglas D Koch MD is professor and Allen, Mosbacher, and Law Chair in Ophthalmology at the Baylor College of Medicine, Houston, Texas, US. dkoch@bcm.edu

1. Koch D, et al. J Cataract Refract Surg, 2016; 42: 550–555.

2. Al-Mohtaseb Z, et al. Ophthalmology, 2024; 131: 383–392.