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EW NEWS & OPINION 24 November 2015 by Ellen Stodola EyeWorld Staff Writer posterior corneal astigmatism could lead to errors in astigmatic correc- tion." Guy Kleinmann, MD, Rehov- ot, Israel, presented his paper, "The effect of high posterior corneal astigmatism on toric IOL pow- er calculation." There are several practical ways to factor in posterior corneal astigmatism today, he said. One is with the Baylor toric formu- la and another is with the Barrett toric calculator. The purpose of the study done by Dr. Kleinmann and colleagues was to evaluate the prevalence of high posterior cor- neal astigmatism (greater than or equal to 0.8 D) and to compare the prediction error of toric IOL cal- culations in those eyes with high posterior corneal astigmatism. To compare this, a number of methods were used, including vector analysis, the Holladay toric calculator, the Holladay toric calculator with the Baylor nomogram adjustment, and the Barrett toric calculator. The study was a retrospective comparative study at a single center with a single surgeon. It included extraction surgery performed from July 2012 to January 2015 involv- ing AcrySof toric IOL (Alcon, Fort Worth, Texas) implantation in patients with no previous ocular trauma, pathology, or surgery. In this series of patients, 12% of eyes had a "high" posterior corneal astigmatism (PCA). Additionally, no correlation was found between high PCA and the amount of the anterior astigmatism. The best results were found by using vector analysis, Dr. Kleinmann said. He concluded that when dealing with toric IOL power calculations, direct measurements of the PCA is recommended. In cases of high PCA, vector analysis should be considered, but if not available, the Baylor nomogram or Barrett toric calculator should also be considered, he said. EW Editors' note: The physicians have no financial interests related to their comments. the anterior and posterior astigma- tism to deduce the contribution of the posterior surface of the cornea. However, recent technologies have allowed for independent analysis of the posterior surface of the cornea. The purpose of his study was to evaluate the contribution of the anterior and posterior corneal astig- matism for total corneal astigmatism and to analyze the relationship between the different types of cor- neal astigmatism and age. The study looked at 123 eyes of 123 patients between ages 20 and 80, with a mean age of 54. Inclusion criteria included a corneal astigmatism of greater than 1 D. The study was an observational, cross-sectional, ana- lytical study. Axis and power mea- surement of the steepest and flattest corneal meridian tomography using Scheimpflug imaging was taken. Patients were observed between June and September 2014. Previous ocular trauma or surgery, corneal or other ocular disease or contact lens use within 2 weeks of the measurements excluded patients from the study. In the case series, Dr. Leite and colleagues found that the mean posterior corneal astigmatism was –0.40 D. There was statistically sig- nificant, weak, and negative correla- tion of anterior, posterior, and total astigmatism with age. Lower age values tended to be associated with a higher magnitude of astigmatism. With increasing age, the anterior corneal steep meridian shifted from vertical to horizontal, but with the posterior corneal steep meridian, it did not change. Dr. Leite said that the compensatory effect of the pos- terior corneal surface decreased with age, there was overcorrection in eyes with with-the-rule anterior corneal astigmatism, and there was under- correction in eyes with against-the- rule anterior corneal astigmatism. "We cannot predict the amount of posterior corneal astigmatism from anterior measurement only," Dr. Leite said. "In patients hav- ing cataract surgery, the incorrect estimation of total corneal astigma- tism that can occur by ignoring the curvature data would be calculated by applying the appropriate with- the-rule or against-the-rule coeffi- cient of adjustment. These adjusted keratometric values can then be used to recalculate an adjusted toric IOL incorporating the likely effect of posterior corneal curvature, he said. The outcome of the Goggin nomogram was tested on 31 consec- utive eyes, where IOL power was cal- culated using the Carl Zeiss Meditec (Jena, Germany) online calculator. All of the eyes received an astigmati- cally neutral incision, and 5 of these eyes received spherical IOLs. A con- trol group of 65 eyes was used with the same parameters, except all of these eyes received toric IOLs with unadjusted keratometric cylinder values instead of the spherical IOLs. In conclusion, Dr. Caputo said that the absolute error is approx- imately halved from a median of 0.45 D to a median of 0.23 D by adjusting the keratometric cylinder value with the coefficients presented in the Goggin nomogram. The me- dian remaining refractive cylinder in eyes receiving toric IOLs in this series was 0.25 D versus 0.51 D in the controls, he said. This reduc- tion of 50% is both statistically and clinically highly significant. The Goggin nomogram is successful in adjusting for the likely effect of pos- terior corneal curvature for toric IOL calculation, Dr. Caputo said. Further testing of the nomogram's superi- ority would require a randomized trial; however, a trial is not currently planned unless there is a worsening of results over time, he said. Ricardo Leite, MD, Braga, Portugal, presented on his paper, "Corneal posterior astigmatism: contribution for total corneal astigmatism and relation with age." Corneal astigmatism is a frequently encountered type of optical aberra- tion in the cornea and is the most significant factor in the overall astig- matism, he said. "It is also a signifi- cant factor in determining the axis and the amount of intraoperative correction of astigmatism." There is a fixed ratio assumption between This topic was a focus of an ESCRS free paper session M ore and more surgeons are realizing the impor- tance of and the role that posterior corneal astigmatism can play in measurements and outcomes of cataract surgery. During the 2015 European Society of Cataract & Refractive Surgeons (ESCRS) meeting in Barcelona, a number of free paper presentations focused on this topic. The speakers shared the details of studies that they have done on the topic of posterior corneal astigma- tism. Silvestro Caputo, MD, Ade- laide, Australia, presented a paper on "Outcome of adjustment of anterior corneal astigmatism values to incorporate the likely effect of posterior corneal curvature for toric IOL calculation." The incorrect estimation of total corneal astigmatism in toric IOL calculation leads to error in astigma- tism correction, he said. The study quantified the degree of over- or undercorrection of astigmatism in a population of eyes receiving IOLs. "What we observed was the system- atic overcorrection in with-the-rule eyes with IOL cylinder power of 2 D or less," he said. There was system- atic undercorrection in against-the- rule eyes with IOL cylinder of 2 D or less. There was no systematic error with IOL cylinder powers greater than 2 D or with oblique astigma- tism axis. A systematic coefficient of adjustment was derived to adjust the anterior corneal astigmatism values to incorporate the likely effect of posterior corneal curvature for toric IOL calculation. Using this "Goggin nomogram," adjustment is performed in eyes that would routinely require an IOL cylinder of 2 D or less. It's also necessary to decide on the "rule" of the cornea based on the anteri- or steep meridian and to exclude eyes with oblique corneal astigma- tism, Dr. Caputo said. New corneal Importance of posterior corneal astigmatism

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