Posterior corneal astigmatism

Estimates of posterior astigmatism better than measurements for toric IOLs

Roibeard O’hEineachain

Posted: Monday, July 9, 2018

Tiago Ferreira MD, FEBOS-CR

Toric IOL calculation methods that use estimates of posterior corneal astigmatism appear to provide greater predictive accuracy than those that use direct measurements obtained with a Scheimpflug or colour-LED topography, 
said Tiago Ferreira MD, FEBOS-CR Hospital da Luz, Lisbon, Portugal.
“We still have limited accuracy in evaluating total corneal astigmatism and, at present, directly measuring toric total corneal astigmatism for toric IOL calculation is not superior to estimating it,” Dr Ferreira told the 22nd ESCRS Winter Meeting in Belgrade, Serbia.
The posterior surface of the cornea surface is the 
most important source of error in toric IOL power calculation. A toric intraocular lens selected based on anterior corneal measurements will overcorrect eyes with with-the-rule astigmatism and undercorrect eyes with against-the-rule astigmatism.
Therefore, consideration of the posterior corneal surface is necessary for precise toric IOL calculation. However, that adds another level of complexity to the calculation and it also requires a measurement method that is highly precise and repeatable. Until such technology becomes available, the current best approach may be using estimates of total corneal astigmatism, he explained.
He and his associates conducted a trial comparing the predictive accuracy of modern IOL calculation methods. That study found that the centroid prediction error was closer to zero in calculation formulas that took the posterior surface into account, 
such as the Barrett toric IOL calculator, compared to those formulas that only considered the effective lens position, such as the Holladay toric calculator.
However, it was also noteworthy that the formulas that had the lowest centroid prediction error were those using estimates of the posterior corneal refraction, namely, the Barrett toric calculator and the Abulafia-Koch formula, rather than those that used direct measurements with a Scheimpflug camera (Pentacam, Oculus), Dr Ferreira said (J Cataract Refract Surg 2017; 43:340–347).
A subsequent study supported those findings. It compared the predictive accuracy of toric IOL calculation methodologies that estimate the power of the posterior corneal surface with those that use real measurements. They again found that a higher proportion of eyes would achieve lower refractive errors with estimates of posterior corneal astigmatism than would be achieved with direct measurements (J Refract Surg 2017;33(12):794-80).
Dr Ferreira also presented preliminary findings from a study comparing the Barrett toric IOL calculator with calculations based on the real total corneal astigmatism measurements obtained using a Scheimpflug camera or a colour-LED topographer (Cassini; i-Optics). They found that the Barrett toric calculator performed best, closely followed by calculations based on colour-LED Cassini measurements, with the Pentacam-based calculations third.
Tiago Ferreira:

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