BLOG: Use total corneal, OCT lens measurements to elevate refractive cataract surgery
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Increasingly sophisticated diagnostic tools provide new opportunities for better managing astigmatism in patients with cataract but also raise new challenges for surgeons trying to effectively synthesize all the information available to us.
My Catalys femtosecond laser (Johnson & Johnson Vision) imports Cassini anterior keratometry measurements and can automatically plan arcuate incisions based on the Donnenfeld nomogram that is integrated into the system. This is a great time saver; however, there are still times when it may be valuable to adjust planned arcuate parameters, whether they were imported directly or manually calculated, based on posterior astigmatism measurements. What we want to avoid is overtreating corneal astigmatism and inadvertently flipping the axis.
I may back off or, on occasion, increase the length of my arcuate incisions based on the Cassini total corneal astigmatism (TCA), a measurement I find to be extremely reliable. In fact, the device checks its own accuracy and reports a confidence level (expressed as a percentage) for multiple values, including centration, posterior astigmatism, focus, corneal coverage, stability and transfer to femto.
A close look at the TCA is especially important in complex post-refractive eyes. In a recent post-hyperopic LASIK case, I had planned to implant a monofocal IOL with paired 40° arcs to correct 1.3 D of astigmatism at 90° measured in the office with two different biometers (Zeiss IOLMaster and Topcon Aladdin). Although both measurements showed classic central bow tie astigmatism, in a post-hyperopic-LASIK eye, this is often an artifact that, if fully treated, will overcorrect the true astigmatism. In this case, a Cassini measurement on the day of surgery showed the TCA was only 0.5 D. I adjusted the plan to a single small intrastromal arcuate incision to avoid overcorrection. In another case, the TCA measurement led me to choose a higher toric IOL power than originally planned (Figure 1).
So, I often use TCA as a “tiebreaker” for astigmatism correction planning and will also consider these keratometry measurements to make sure they are consistent with those being used in my IOL calculations. For example, prior to surgery, we obtain keratometry values from at least two different biometry devices. When there is a small discrepancy between these keratometry values, I will use the Cassini values on the day of surgery to adjust IOL power up or down by 0.5 D. In the past, I used intraoperative aberrometry as a tiebreaker. However, I always felt that there were too many unknown variables, such as IOP, speculum pressure and capsular bag anatomy.
I now like to look at the Catalys laser’s intraoperative spectral domain OCT imaging of the lens anatomy, including the lens meridian position (LMP) it calculates. LMP correlates with postoperative effective lens position (ELP). In other words, if the LMP value is higher than normal (greater than 5 mm), as in Figure 2, the lens will likely sit farther back in the capsule, which will lead to postop hyperopia. In deciding between two different IOL powers, LMP helps me make a quick decision that will result in the best visual result for that eye’s ELP.
Managing astigmatism is challenging, especially in complex eyes. But having the Cassini total corneal astigmatism and the Catalys OCT imaging of the lens provides much more information on which to base my management decisions.
References:
Gouvea L, et al. Spectral-domain OCT lens meridian position as a metric to estimate effective lens position. Presented at: American Society of Cataract and Refractive Surgery meeting; May 16-17, 2020 (virtual meeting).
Younger J. Ophthalmology Times. March 2018.
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