Posterior Corneal Astigmatism
Tal Raviv, MD
Toric IOLs are the stars of the US premium IOL market, with their market penetration and rate of growth surpassing presbyopia-correcting IOLs. However, we’ve all had cases where toric patients have residual astigmatism despite our seemingly perfect toric lens selection and placement.
Well, thanks to Doug Koch’s illuminating study,1 we surgeons now have a better idea of why we still have unanticipated astigmatic outcomes. All current toric IOL calculators factor in only the anterior corneal astigmatism (from keratometry or corneal topography). But, as Doug has taught us, the total corneal astigmatism is also affected by the posterior corneal astigmatism. This can change the magnitude and the axis of the measured anterior astigmatism.
At the recent ESCRS meeting in Amsterdam, an entire paper session was devoted to posterior corneal astigmatism. Although corneal tomography by Scheimpflug or OCT can measure posterior corneal curvature, there is no current gold-standard method to calculate the posterior corneal astigmatism, or more importantly, the true total corneal astigmatism. Indirectly, however, we do have intraoperative aberrometry when performed on the aphakic eye. This measures total ocular astigmatism (presumably mostly from the anterior and posterior cornea) and allows us to match an appropriate toric IOL to neutralize it. I’ve recently compared my refractive results on a series of toric IOL patients implanted before and after instituting aberrometry with the ORA System (WaveTec Vision). The results are very favorable with ORA (abstract submitted). I’ve had a number of patients in whom 1.00 D of topographic with-the-rule astigmatism ended up measuring 0.40 D intraoperatively, thus requiring only a spherical IOL, and vice versa. I use ORA with all of my toric IOLs (no financial interest in WaveTec), and I look forward to further advancements in measuring and factoring for posterior corneal astigmatism in our toric patients.
Are you incorporating posterior corneal astigmatism measurements in your toric IOLs? Take the survey!
PS
This was my first ESCRS meeting, and walking the exhibition hall was like walking 3 to 5 years into the future thanks to our current FDA regulatory environment. I passed by the Calhoun Vision exhibit where European doctors have already been ordering and using the Light Adjustable Lens for more than 1 year. There were at least three other optical biometers (in addition to our IOLMaster [Carl Zeiss Meditec, Inc.] and the Lenstar [Haag-Streit AG]) on sale, most combined into a unified corneal topography and tomography unit for full cataract analyses with one machine. And as if I wasn’t feeling depressed enough, I passed by a life-sizes model of Zeiss’ newest IOL: a trifocal toric microincisional lens that comes preloaded. Enough said.
