EyeWorld India December 2013 Issue

16 December 2013 EWAP FEAturE Measuring - from page 15 Views from Asia-Pacific FAM Han Bor, MD Senior Consultant & Head, Cataract & Implant Service The Eye Institute @ Tan Tock Seng Hospital 11 Jalan Tan Tock Seng, Singapore 308411 Tel. no. +65-6357-7726 Fax no. +65-6357-7718 famhb@singnet.com.sg O ur understanding of ocular astigmatism has improved tremendously over the years due to various factors. The experiences gained from toric IOL implantations and the advancement in corneal tomography has also helped us toward that end. Patients’ expectations of better precision and our desire to achieve better outcomes with toric IOLs have driven us to strive for better understanding of corneal and ocular astigmatism. Analysis In analyzing our toric IOL outcomes in 268 toric IOLs, we compared the actual toric outcomes with the expected toric outcomes, calculated based on ELP of the IOLs with wound-induced astigmatism factored in. We noticed that with-the-rule (WTR) corneal astigmatisms tended to be over-corrected; while against-the-rule (ATR) corneal astigmatism under-corrected. (Figure) These outcomes, while cannot be entirely alluded to, were consistent with the contribution of posterior corneal astigmatism 1 . Case presentations 1. A patient implanted with a +2.50D toric IOL aligned at 150 presented at 1 month postoperatively with a manifest refraction: +1.75 -1.50 X 1000. Postoperative auto-keratometry (Canon RK-F1) measured 8.11mm @ 1000; 7.66mm @ 100. (Cylinder 2.44) Galilei G4 tomography total corneal power (TCP), which measures the posterior corneal power by ray-tracing, was 40.37 @ 1040; 43.66 @ 140. (Cylinder 3.29) The expected refractions: a. Based on the auto-keratometry was +1.40 -0.80 x 890. b. Based on Galilei G4 when posterior corneal astigmatism was taken into consideration, was +1.78 -1.57 x 1030. 2. Another patient implanted with a +2.25D toric IOL aligned at 100 presented postoperatively with a toric surprise of +1.5 -2.25 x 800 Postoperative cornea astigmatism by iTrace 5.11 (SimK) and Galilei G4 (TCP) were 43.98 @ 890; 46.89 @ 1790 (Cylinder 2.91) and 41.40 @ 890; 45.14 @ 1790 (Cylinder 3.74) respectively and the expected refraction based on keratometric cylinder and that on TCP were +1.03 -1.58 x 780; and +1.43 -2.37 x 820 respectively. In both instances, the TCPs-based expected refractions were closer to the manifest refractions. Both eyes had ATR anterior corneal astigmatisms that were enhanced by theposterior corneal astigmatisms. The above cases illustrate the importance of measuring posterior corneal astigmatism. In his papers, Koch et al. analyzed the contribution of posterior corneal astigmatism to total corneal astigmatism 1 and showed that toric IOL implantation was more precise when posterior corneal power was taken into consideration 2 . 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;38:2080–2087. 2. Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L, Correcting astigmatism with toric intraocular lenses: Effect of posterior corneal astigmatism. J Cataract Refract Surg . 2013;(in press). Editors’ note: Dr. Fam has no financial interests related to his comments. Figure. Double-angle plot of the centroids of both WTR and ATR. The table compares the centroids of WTR with ATR. In WTR, the actual postoperative steep meridian flips almost 90 degrees away from the expected indicating over-correction. The postoperative steep power in ATR remained on about the same meridian as the expected, but of a higher magnitude suggesting under-correction. Both ATR and WTR preoperative corneal astigmatisms were effectively reduced by the toric IOLs. Source: Fam Han Bor, MD