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Achieving spectacle independence for distance vision after toric IOL implantation depends on the refractive outcome. Factors affecting residual astigmatism and studies evaluating the use of various surgical techniques and technologies relating to those factors are discussed.
Reviewed by Rudy M.M.A. Nuijts, MD, PhD
Maastricht, The Netherlands-A large majority of cataract surgery patients implanted with a toric IOL will achieve spectacle independence for distance vision, but a sizeable proportion are left with significant residual astigmatism and find themselves needing glasses at least sometimes.
Understanding the sources of residual astigmatism allows surgeons to choose techniques and technologies that will optimize outcomes for toric IOL surgery, according to Rudy M.M.A. Nuijts, MD, PhD.
“Results from a trial we conducted showed that 70% of patients who underwent bilateral toric IOL implantation could see 20/25 uncorrected at distance and 84% of patients were spectacle independent for distance vision,” said Dr. Nuijts, professor of ophthalmology, Maastricht University Medical Center, Maastricht, The Netherlands.
However, 26% of patients had more than 1 D of residual stigmatism and 54% were left with at least 0.5 D of residual astigmatism (Figure 1).
He reviewed factors affecting the astigmatic outcome, including preoperative measurement errors; surgically induced astigmatism (SIA); calculation inaccuracies, including the effect of posterior corneal astigmatism, and toric IOL misalignment.
Accuracy of preoperative measurements is critical, and different instruments can be used for determining keratometry values. Dr. Nuijts noted that a study performed by his group found there was not a large difference between the measurements obtained when using several different instruments for autorefraction, a manual keratometer, or the sim K value measured with a topographer or a Scheimpflug tomographer (Pentacam, Oculus).
However, the equivalent K value measured with the latter device was significantly lower than the values obtained with the other devices. The explanation for the difference is that the equivalent K incorporates astigmatism of the posterior corneal surface, Dr. Nuijts said.
He noted that the influence of the posterior cornea on total astigmatism has been described by Douglas Koch, MD, and colleagues. Essentially, the posterior cornea acts as a minus lens and is generally steep vertically, inducing against-the-rule (ATR) corneal astigmatism that compensates for some of the with-the-rule astigmatism on the anterior corneal surface.
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On a population basis, the magnitude of posterior corneal astigmatism is about 0.3 D, and in about 10% of eyes, the value exceeds 0.5 D.Results from various studies show that the accuracy of the astigmatic outcome with toric IOL implantation can be improved when the posterior corneal astigmatism is factored into power calculations, Dr. Nuijts said.
“Especially in cases with high anterior corneal astigmatism, the influence of the posterior corneal surface can be significant and has to be taken into consideration when performing the toric IOL power calculation,” said Dr. Nuijts.
He presented a case to illustrate this point (Figure 2) and discussed results of one study investigating showing that in eyes with ATR total corneal astigmatism, the proportion of eyes left with <0.45 D of refractive astigmatism decreased from 67% to 42% using a method that accounted for posterior corneal astigmatism.
Another study comparing different methods for measuring keratometry and different formulas for power calculations found that the most accurate results were achieved using an optical low coherence reflectometry device and a calculation method that compensates for the effect of posterior corneal astigmatism (Barrett calculator).
SIA also influences the astigmatic outcome and needs to be factored into toric IOL power calculations. Dr. Nuijts noted that the amount of SIA varies depending on incision size, location, and level of preoperative astigmatism.
However, he pointed out that in a study his group conducted evaluating the effect of incision size on SIA, the standard deviation of the average error was the same across the different incision sizes (0.52 to 0.54 D), indicating there is still a problem with predictability of SIA.
Misalignment of the toric IOL affects the astigmatic outcome as there is about a 3% loss of efficacy for every degree of misalignment. For surgeons who manually mark the eye, accuracy of toric IOL alignment depends on the accuracy of the preoperative markings, including both the reference and axis alignment marks.
Dr. Nuijts said that he likes to use a bubble marker, but noted that research conducted by Oliver Findl, MD, comparing marking at the slit-lamp, a pendular marker, bubble marker, and tonometer marker showed the most predictable outcomes were achieved using the pendular marker.
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Intraoperative digital guidance techniques have been developed to overcome the inaccuracies of manual marking.
However, there is still room for error when using this technology because digital marking cannot eliminate human error from malpositioning of the IOL relative to the intended meridian.
“A study we conducted found that even when using a digital marking system, there was still an average inaccuracy of 2.6° in this last step of toric IOL positioning,” Dr. Nuijts said.
In addition, there is still the potential for the toric IOL to rotate postoperatively, although on average, modern toric IOLs demonstrate excellent postoperative stability.
Rudy M.M.A. Nuijts, MD, PhD
This article was adapted from Dr. Nuijts’ presentation during the Refractive Surgery Subspecialty Day at the 2015 meeting of the American Academy of Ophthalmology. Dr. Nuijts is a consultant, lecturer, and or receives grant support from companies that market toric IOLs.