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Posterior corneal astigmatism key in calculating total astigmatism
Measuring only anterior corneal astigmatism may lead to overcorrection or undercorrection.
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Neglecting to measure posterior corneal astigmatism during surgery to correct astigmatism with toric IOLs may lead to overcorrection or undercorrection, according to a study.
The study found that relying on only anterior corneal measurements resulted in underestimating total corneal astigmatism by 0.22 D at 180°, surpassing 0.5 D in 5% of eyes.
Lead author Douglas D. Koch, MD, and colleagues at Cullen Eye Institute, Baylor College of Medicine, Houston, thought it would be intriguing and relevant to evaluate the posterior corneal surface with the Galilei Dual Scheimpflug Analyzer (Ziemer Group) and determine what effect, if any, the posterior corneal surface has on astigmatism.
Douglas D. Koch
“We have also been interested in looking at wavefront aberrations of the posterior corneal surface,” Koch said.
At the same time, the investigators incidentally began to identify patients who had unusual and unanticipated outcomes after receiving toric IOLs. “That motivated us to continue the study even further,” Koch told Ocular Surgery News.
Study findings
The prospective study, published in Journal of Cataract and Refractive Surgery, analyzed 715 corneas of 435 consecutive patients, ranging in age from 20 to 89 years (mean: 55 years), with a mean magnitude of posterior corneal astigmatism of –0.3 D. The authors measured both the anterior and posterior corneal surfaces for astigmatism.
“We looked at the characteristic of astigmatism of the anterior cornea and how it changes with time,” Koch said. “And then we looked at the characteristic of astigmatism on the posterior corneal surface and how that changes over time.”
As expected, the study found that younger patients tended to have with-the-rule corneal astigmatism on the anterior corneal surface; however, over time, with-the-rule astigmatism shifted to against-the-rule astigmatism, Koch said.
“There are exceptions, of course, but this is definitely the population trend,” Koch said.
The results for the posterior corneal surface were different.
“This surface is steep vertically in nearly all eyes, regardless of the patient’s age,” Koch said. “Optically, this means that the posterior corneal surface is a minus lens, so if it is steep vertically, it actually creates refractive power at 180°.”
Translated into a clinical setting, a patient who has +2 D of astigmatism on the anterior corneal surface could also have –0.5 D of astigmatism on the posterior corneal surface, which equals a net corneal astigmatism of +1.5 D.
“This has huge implications for using toric lenses,” Koch said.
Another finding of the study was that for patients with with-the-rule astigmatism, the amount of posterior corneal astigmatism was proportional to the amount of anterior corneal astigmatism. For example, 1 D of with-the-rule astigmatism on the anterior corneal surface might correlate with 0.4 D or 0.3 D on the posterior corneal surface. Likewise, 4 D anteriorly may be associated with as much as 1 D posteriorly.
“However, the relationship between magnitude of the astigmatism on the front and the back did not pertain to those eyes that had against-the-rule on the front,” Koch said.
Measuring astigmatism
To accurately measure total corneal astigmatism for toric IOLs, Koch recommended either the IOLMaster (Carl Zeiss Meditec) or the Lenstar (Haag-Streit), plus topography for the anterior surface. Posterior corneal measurements are also helpful, but more work is needed in interpreting them in light of clinical outcomes, Koch said.
Koch also created his own nomogram that can be substituted for a posterior corneal measurement. “This nomogram is pretty tight and fairly accurate,” he said.
In addition, measuring posterior corneal astigmatism can be accomplished by intraoperative aberrometry.
“[Intraoperative aberrometry] very nicely captures the front and the back, because it measures the entire refraction of the aphakic eye during cataract surgery,” Koch said.
To provide longer enjoyment of the kind of vision that a toric IOL provides, Koch said he prefers to leave patients slightly on the side of with-the-rule astigmatism, around 0.25 D, knowing that there will be a shift to the against-the-rule side over the years. – by Bob Kronemyer
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Jack T. Holladay, MD, MSEE, FACS
Using a toric IOL in which the IOL calculation for toricity is based on the anterior cornea alone will lead to an overcorrection in eyes that have with-the-rule astigmatism and undercorrection in eyes with against-the-rule astigmatism. Posterior corneal astigmatism must be measured and the net astigmatism used in the toric calculation. The surgeon must also use an exact toric calculator that does not use a constant ratio between the toricity of the IOL to the corneal astigmatism. Using an exact toric calculator and the net astigmatism will yield the best outcomes of little or no residual ocular astigmatism.
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