Toric IOLs: Factoring in posterior cornea

June 15, 2014

The ratio of the anterior to posterior corneal curvature is not constant for astigmatism. By factoring in the contribution of the posterior cornea based on population data, the Baylor toric IOL nomogram can improve surgical outcomes following toric IOL implantation.

 

Take home

The ratio of the anterior to posterior corneal curvature is not constant for astigmatism. By factoring in the contribution of the posterior cornea based on population data, the Baylor toric IOL nomogram can improve surgical outcomes following toric IOL implantation.

 

Dr. Koch

By Cheryl Guttman Krader; Reviewed by Douglas D. Koch, MD

Houston, TX-Ignoring the posterior cornea’s contribution to total corneal astigmatism may be the hidden factor underlying unexpected refractive outcomes after toric IOL implantation, according to research conducted by Douglas D. Koch, MD.

Dr. Koch recently reviewed findings from studies he conducted with colleagues at Baylor College of Medicine characterizing corneal astigmatism in the population and its measurement with different devices, and described the Baylor toric IOL nomogram that was developed based on that research.

However, Dr. Koch pointed out that although the Baylor nomogram is a step forward to achieving more predictably accurate outcomes with toric IOLs, more work needs to be done in this area.

“Recent advances are helping to improve outcomes with toric IOL implantation, and there is also promising research underway. New power formulas are being introduced, and intraoperative aberrometry is proving helpful as well,” said Dr. Koch, professor of ophthalmology and The Allen, Mosbacher and Law Chair of Ophthalmology, Baylor College of Medicine, Houston, TX.

 “However, the availability of technology that can accurately measure total corneal astigmatism accurately in individual patients is the holy grail, and that is the focus of interesting research using optical coherence tomography and Scheimpflug systems.

“I consider the Baylor nomogram a placeholder,” he continued. “It is something I think is helpful for now, but when we are able to measure our patients accurately on a one-by-one basis, a regression-derived nomogram like ours will no longer be needed.”

 

Development

The path to development of the Baylor nomogram, which Dr. Koch created with Mitchell Weikert, MD, and Li Wang, PhD, began with findings from a study they had undertaken using a combined Placido-Scheimpflug corneal analyzer (Galilei, Zeimer Ophthalmic Systems) [J Cataract Refract Surg. 2012;38(12):2080-7]. Unlike the devices that are most commonly used to evaluate astigmatism, the combined platform measures posterior in addition to anterior astigmatism.

The results showed the fallacy of the assumption that the ratio of the anterior to posterior corneal curvature is constant for astigmatism and shed light on puzzling refractive outcomes being encountered after toric IOL implantation.

In analyzing more than 700 adult eyes, it was found that the steep corneal meridian was vertically oriented on the front surface of the cornea in about half of eyes, but on the back surface in a much higher proportion (87%). This finding explains why, if posterior corneal astigmatism is ignored, eyes with against-the-rule (ATR) anterior surface astigmatism would be undercorrected in most cases, while those with WTR anterior surface astigmatism would be overcorrected, Dr. Koch said.

“When the posterior cornea is steep vertically, it creates a net plus power along the horizontal meridian,” he explained. “Although that seems counterintuitive, remember that the posterior cornea is a minus lens, and when it has more curvature vertically than horizontally, it creates against-the-rule ocular astigmatism.”

Proof of principle was provided in another study evaluating outcomes of toric IOL patients [J Cataract Refract Surg. 2013;39(12):1803-9]. In calculating prediction errors associated with the use of preoperative keratometry data from different devices, the results indicated that when only the anterior cornea was measured, eyes having WTR astigmatism tended to be overcorrected-by 0.5 D on average-while eyes having ATR astigmatism were generally undercorrected, by 0.3 D on average.

 

A nomogram solution

The Baylor toric IOL nomogram identifies the appropriate effective IOL cylinder power at the corneal plane to implant for different ranges of WTR and ATR astigmatism. The latter values represent the vector sum of anterior corneal astigmatism and surgically induced astigmatism (SIA).

“Don’t forget to factor in your SIA,” said Dr. Koch.

The recommended effective IOL cylinder powers are based on achieving a target of 0.25 D to 0.5 D WTR astigmatism to account for a gradual ATR shift in astigmatism with age that was identified in the 2012 JCRS study.

“Targeting a small amount of WTR astigmatism when implanting a toric IOL will provide patients with the best clear distance vision for a longer period of time,” Dr. Koch said

According to the nomogram, the threshold for implanting a toric IOL is 1.7 D in eyes having WTR astigmatism, but just 0.4 D in eyes with ATR astigmatism.

In providing tips for achieving better toric IOL outcomes, Dr. Koch also pointed out that IOL power and anterior chamber depth impact the effective IOL toricity.

“Surgeons can account for these effects by using the Holladay II consultant formula for their calculations, and they are also incorporated in the toric IOL calculator from Abbott Medical Optics,” he said.

 

 

Douglas D. Koch, MD

E: dkoch@bcm.edu

Dr. Koch is a consultant to Abbott Medical Optics, Alcon, ReVision Optics, and Ziemer, and receives research support from i-Optics and Ziemer.