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Publication Exclusive: Extra effort needed to achieve ideal results in refractive cataract surgery patients
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The most demanding lens implant patient for me in regards to postoperative residual refractive error is the individual who has significant astigmatism and desires spectacle independent vision at all distances. While there are different surgeon preferences and lens implant approvals in each country, let’s presume we have access to and have chosen to use a bifocal or trifocal toric diffractive multifocal IOL.
My experience is that in order to achieve high patient satisfaction, I need to generate a refractive outcome with no more than 0.5 D of residual astigmatism and less than 0.5 D of residual spherical equivalent. There are many published series of refractive outcomes, including those from the United Kingdom, the Swedish registry and, closer to home, a large registry of cases accumulated by Guy Kezirian. These data sets suggest that with standard methods of optical biometry, astigmatism measurement and steeper meridian marking, the “typical surgeon” only achieves this outcome in 50% to 65% of cases.
A few thoughts on the challenges and some of the methods I have found helpful in improving my outcomes. In our practice, we use optical biometry whenever possible, which includes at least 95% of our patients. We will utilize immersion ultrasound biometry in cases in which we cannot get a reading with optical biometry. Optical biometry is very accurate and rarely the source of a meaningful postoperative refractive surprise. Measuring the mean keratometric power of the cornea, the magnitude of the astigmatism and the steeper meridian presents a much greater challenge. Each keratometer measures the corneal power at a different diameter, and all must be calibrated. Using a consistent method of keratometry and a consistent model of IOL and then collecting postoperative data to personalize one’s so-called A-constant can enhance outcomes.
Unfortunately, the patient with a history of corneal refractive surgery is a common challenge in my practice, and accurate measurement of the corneal power is especially difficult in these eyes. In addition, many of these patients have significant spherical aberration and coma; therefore, I rarely utilize a multifocal IOL for them. The ASCRS IOL calculator is an excellent tool in these patients, but one is still presented with a patient in whom residual refractive error is more common, and all my patients are counseled that they may need a postoperative refractive enhancement if they are highly motivated to have spectacle independence.
The steeper meridian is also a challenge. I measure several ways, using manual keratometry, the IOLMaster (Carl Zeiss Meditec), Humphrey or Tracy Placido disc topography, and the Pentacam (Oculus) or Orbscan (Bausch + Lomb). In addition, I look at the refraction. Unfortunately, in many patients, the steeper meridian varies significantly from one measurement to another. Clinician judgment is useful, but these variable measurements provide a challenge. My solution includes the use of intraoperative keratoscopy with a Mastel keratoscope and/or the Alcon WaveTec intraoperative aberrometer. My very best outcomes have been obtained through marking the steeper and flatter meridian before making my incisions with the Mastel keratoscope and then completing a final check with intraoperative aberrometry. This is, to be sure, a lot of work and requires two separate microscopes because there is no intraoperative aberrometer with a built-in surgical keratoscope.
Even with all this technology, my challenges are still significant. I do not personally have an instrument that reliably measures posterior corneal astigmatism, which can be significant. I rely on Doug Koch’s finding that the typical patient has against-the-rule posterior corneal astigmatism of about 0.3 D. This is convenient for me because my typical incision in the horizontal meridian flattens the cornea a similar amount. I therefore personally always do both my primary and secondary incision at the 180° meridian when treating astigmatism with a toric IOL. I assume these horizontal incisions will neutralize the posterior corneal astigmatism, but I know this is only a rough estimate and hope in the future to be able to measure it accurately.
Click here to read the full publication exclusive, Lindstrom's Perspective, published in Ocular Surgery News U.S. Edition, August 25, 2015.