OD News Articles

25th March 2013

Posterior Corneal Astigmatism

by Michael Glanzer, OD Vancouver, Washington

As cataract and refractive surgery become increasingly intertwined, we are always on the lookout for ways to improve refractive outcomes for cataract patients. Correcting pre-existing astigmatism with toric IOLs or limbal relaxing incisions is a common way to achieve more precise results.

In this article, I will review an interesting paper presented by Douglas D. Koch, MD at last year’s ASCRS/ASOA Symposium & Congress and published in the Journal of Cataract & Refractive Surgery1Dr. Koch addresses the role of posterior corneal astigmatism and some methods that can improve visual outcomes of cataract surgery.

Clinical Importance

The clinical importance of posterior corneal astigmatism dates back to 1890 when Javal published his Memoires d’Ophthalmometrie2. He described what is now known as Javal’s rule showing the relationship between refractive astigmatism and keratometric astigmatism. For years, cataract surgeons have noticed a tendency to overcorrect with-the-rule (WTR) astigmatism and undercorrect against-the-rule (ATR) astigmatism without a clear explanation. An increased understanding of the posterior cornea may provide some answers.

Study

Dr. Koch and his colleagues at the Cullen Eye Institute, Baylor College of Medicine, in Houston, Texas set out to determine two things:

  • The contribution of posterior corneal astigmatism to the total amount of corneal astigmatism.
  • The error in estimating total corneal astigmatism using only anterior corneal measurements.

They retrospectively looked at the Galilei Dual Scheimpflug Analyzer3 measurements of 715 corneas on 435 consecutive patients that presented to their clinic for cataract and refractive surgery evaluation. Four corneal astigmatic values were obtained and analyzed:

  1. Total corneal astigmatism calculated using ray tracing
  2. Corneal astigmatism from simulated keratometry
  3. Anterior corneal astigmatism
  4. Posterior corneal astigmatism
Findings

Dr. Koch and his colleagues found that patients had, on average, 0.3 D of curvature vertically on the posterior cornea. The range was from 0.01 to 1.10 D, exceeding 0.50 D in 9% of eyes. Because the posterior cornea is a minus lens, a steeper vertical meridian produces refractive power 180 degrees away or horizontally. This gives an ATR refractive power effect.

New Understanding

According to the study, Failure to account for vertically steep posterior corneal curvature (and the ATR ocular astigmatism it produces) will cause:

  • overcorrection of eyes having WTR anterior corneal astigmatism
  • and under correction of eyes having ATR anterior corneal astigmatism

So for example, a patient who presents pre-operatively with 2.0 D of anterior WTR corneal astigmatism may really only have 1.5 D or even less total WTR corneal power because of the neutralizing effect of the ATR power of the posterior cornea.

This information lead Dr. Koch and his Baylor colleagues to create new toric IOL nomograms and has significantly affected their decision making process for recommending toric implants. After the study, they are using less toric IOLs in patients who have WTR astigmatism and more toric IOLs in those with ATR astigmatism.

Measurement Methods

The study also looked at the difference between estimating corneal astigmatism using simulated keratometry readings from the Scheimpflug analyzer versus total corneal power measurements on the same device. Total corneal power is measured using ray tracing, combining the contributions of both the anterior and posterior corneal surfaces as well as corneal thickness. The average estimation error found by using only simulated K values was 0.22 @180 with 5% of eyes having an error greater than 0.50 D.

Again, because the posterior corneal surface has negative power, steeper curvature in the vertical meridian creates ATR astigmatism, therefore, most total corneal power measurements show more power at 180 degrees than simulated K measurements. It is interesting to note that a similar study done by Ho et al4 found a mean vector error of 0.28 @177.2. They used the Pentacam, a rotating Scheimpflug device, but the similarity in findings is remarkable. Still to be determined is whether or not this 0.22 D error is actually clinically significant. And if so, for what percentage of patients.

Age Makes a Difference

One additional variable the study addressed was the relationship between age and astigmatic changes to the anterior and posterior cornea. The researchers found that as we get older, average astigmatism on the anterior corneal surface shifts from WTR to ATR. By contrast, the mean astigmatism on the posterior corneal surface essentially stays the same. This would indicate that in younger patients, the posterior corneal astigmatism partially compensates for anterior corneal astigmatism. But in older individuals, it would have an additive effect. This is something we might do well to keep in mind—particularly in younger cataract patients as we attempt to predict their refractive status years down the road.

Summary

Posterior corneal astigmatism plays a significant role in overall corneal astigmatism and may become an important pre-operative measurement in the management of our cataract surgery patients. As surgical techniques and IOL technology evolve, and the accuracy of our pre-operative measurements improves, our cataract patients should experience an ever-increasing level of visual satisfaction—thanks in part to the information Dr. Koch has shared.

References
  1. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012 Oct 12
  2. Javal E. Memoires d’Ophtalmometrie: Annotes et Precedes d’une Introduction. Paris, France, G. Masson, 1890; 131
  3. Ziemer Ophthalmic Systems, Port, Switzerland
  4. Ho J-D, Tsai C-Y, Tsai RJ-F, Kuo L-L, Liou S-W. Validity of the keratometric index: evaluation by the Pentacam roatating Scheimpflug camera. J Cataract Refract Surg 2008; 34: 137-145