Advanced toric IOL calculator improves refractive outcomes

A theoretical retrospective study found that the percentage of eyes with <0.5 D residual refractive astigmatism after toric IOL implantation improved by 19% using a new vergence-based algorithm for power calculations versus the previous fixed-ratio algorithm. 

This article was reviewed by Mark Packer, MD

The use of an advanced vergence-based algorithm to calculate power for the MX60T neutral aspheric monofocal toric IOL (enVista, Bausch + Lomb) significantly improved refractive outcomes when compared with a previous fixed-ratio algorithm, according to Mark Packer, MD.

Dr. Packer, president, Mark Packer, MD Consulting, Boulder, CO, described the new algorithm and presented findings from a theoretical study using historical data to compare predicted residual refractive error and astigmatism using the vergence-based algorithm versus the previous formula.

Unlike a fixed-ratio algorithm that assumes the toric power is always in the same place, the new formula takes into account the distance between the corneal plane and the IOL plane. 

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The new formula incorporates platform-specific inputs for surgically induced astigmatism (SIA) and posterior corneal astigmatism (PCA). 

Dr. Packer reported that the outcomes analysis showed that the percentage of eyes predicted to have <1 D residual refractive error improved from 55% using the fixed-ratio algorithm to 74% with the vergence-based formula.

The percentage of eyes with residual refractive astigmatism  ≤0.5 D also improved from 55% with the fixed-ratio algorithm to 74% using the vergence-based formula. 

“The percentage of eyes predicted to be left with 0.5 D or less residual refractive astigmatism with the vergence-based formula approaches the value of 80% that has been reported using other toric IOL formulas that are vergence-based and take into account posterior corneal astigmatism,” said Dr. Packer. “While the new formula seems to achieve its goal in reducing residual astigmatism, it is important to continue this project because I think the results can be further improved.”

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Developing algorithm

Data for the theoretical comparative study and some values for the new algorithm were derived from patients who were implanted with the MX60T or the monofocal nontoric MX60 (enVista, Bausch + Lomb) IOLs in their respective US FDA investigational device exemption clinical trials. 

A value of 0.46 D at 84.60° was used for SIA and represented the average of SIA values from patients implanted with the MX60 and MX60T IOLs in the clinical trials. The value of PCA was set at 0.3 D at 90.°

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To limit influence of biometric errors, the vergence-based portion of the algorithm was developed using values for anterior chamber depth and effective lens position that were derived from optical modeling averaged over the full range of dioptric powers of the toric lens.

Then a regression analysis was performed to uncover a scaling factor needed to predict the power relationship between the corneal and IOL planes over the implant’s spherical power range. 

Future refinements

The predictability of IOL calculation using the vergence-based formula could be improved in the future by using individual measurements for PCA rather than a population average.

“Although SIA is also an important factor to include in toric IOL calculations, the value used has to be based on historical data because patient-specific values are not known until after the operation,” Dr. Packer concluded. “The SIA for each case is multifactorial and depends on incision size, the injector used, incision location, the physical dimensions of the IOL, and corneal healing.” 

Read more by Cheryl Guttman Krader 

Mark Packer, MDE: mark@markpackerconsulting.com
This article is adapted from Dr. Packer's presentation at the American Academy of Ophthalmology 2019 annual meeting. Dr. Packer is a consultant to Bausch + Lomb.