On target: Accurate alignment of toric IOLs

San Diego-A digital markerless system can be considered more accurate than manual methods when attempting to align toric IOLs, said Daniel Black, MBBS, FRANZCO.

In his non-randomized, single-surgeon study with the digital markerless system (Callisto, Carl Zeiss Meditec), Dr. Black used a single toric IOL.

Patients who presented before August 2014 had traditional ink marks used to align their toric lenses, and those who presented after August 2014 had the digital system used for lens alignment. He used the ASICO Nuijts-Lane and the Mastel Gimbel Mendez analogue ink markers (for the manual marking group).

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Careful marking of the eye to ensure accurate axis alignment of the IOL is essential for toric lens success, he said, noting that for every 1° a toric lens is off-axis, its effect for reducing astigmatism is decreased by 3.3%-so if the lens is off by even 10°, its efficacy is deceased by a third. Some studies have found if a lens is 30° off target the entire effect of the lens is lost.

“The hypothesis is that the digital system is potentially more accurate because it can eliminate a lot of sources of error,” Dr. Black said, adding he personally uses the Holladay toric formula to determine IOL power.

“When patients first present in our office, we get biometry on virgin tear film, and our data is imported electronically,” said Dr. Black, who practices in Queensland, Australia. “This helps to avoid transcription error.”

All surgeries had temporal bimanual phaco-chop, with the IOL implanted through a 2.2-mm incision; the IOL was aligned with the marked steep axis.

“Ideally, you want the astigmatic correction in the IOL plane,” he said.

Next: Results

There were a total of 507 cases from the ink marking group and 161 cases from the Callisto markerless group, Dr. Black said.

There was no difference in the preoperative metrics between the two group, or in postoperative mean or standard deviation of residual astigmatic refractive error.

Preoperatively, the mean delta K was 1.30 in the manual marking group, and 1.27 in the Callisto group, he said, with K values of 37.21-48.91 in the manual group and 40.18-47.94 in the Callisto group. Postoperatively, the mean refraction was 0.18D in the manual group and 0.17 D in the Callisto group.

There was, however, a statistically significant difference in the outliers, Dr. Black said.

In the manual group, 94% were within 0.5D of target, and 98% were within 0.75D. In the Callisto group, 99.4% were within 0.5D, and 100% were within 0.75D. The differences between the two groups in the percentage of patients who were within 0.5D was statistically significant (p = 0.028).

 “While both methods were effective for toric IOL correction, the Callisto significantly improved predictability from 94% to more than 99%,” he said.

Further, the Callisto eliminated the outliers, Dr. Black said.

“With the Callisto, there is no need for preoperative or intraoperative marking,” Dr. Black said, “and there is photographic documentation of the aligned IOL postoperatively.”

During a question-and-answer period, Dr. Black said if a patient falls between two IOL steps, he will leave the patient with a little bit of with-the-rule astigmatism “because I think that’s more realistic from a functional perspective,” he said.

Next: conclusion

Ideally, he said, lenses in the 0.25-D increments would resolve many of these dilemmas, but in the interim intraoperative aberrometry may be a useful tool.