EyeWorld India March 2015 Issue

33 EWAP CATARACT/IOL March 2015 Views from Asia-Paci c Ronald YEOH, FRCS, FRCOphth, DO, FAMS Adj Ass Professor, Duke-NUS Grad Med School and Singapore National Eye Centre Consultant Eye Surgeon & Medical Director, Eye & Retina Surgeons #13-03 Camden Medical Centre, One Orchard Boulevard, Singapore 248649 Tel. no. +65-67382000 Fax no. +65-67382111 ersyeoh@gmail.com T he traditional way of measuring corneal astigmatism was based on the anterior surface. This was adequate because of the consistent anterior-posterior ratio. 1 With the development of toric IOLs and the increasing expectations of refractive cataract surgery, there is an emphatic need for greater precision and improved outcomes. Despite aligning the toric IOL correctly, many surgeons noticed that the actual postoperative results are not consistent with the expected calculated outcomes. It is not uncommon to find over- or undercorrections postoperatively. These inconsistencies have led many to look for the answers. In the last few years, the importance of posterior corneal astigmatism has been brought to our attention. In his 2012 ASCRS Innovator’s Lecture on “Contribution of posterior corneal astigmatism to total corneal astigmatism”, Dr. Douglas Koch highlighted it as the cause for under- and overcorrection of corneal astigmatism post toric IOL implantation. 2 He emphasized that ignoring posterior corneal astigmatism will results in overcorrection in with-the-rule astigmatism and under-correction in against-the-rule astigmatism. This is because in most eyes, the posterior cornea is steep vertically, effectively resulting in a net against-the-rule astigmatism. In our own analysis of 268 toric IOLs, we found that there is a consistent over-correction for with-the-rule astigmatism and under-correction for against-the-rule astigmatism. Chart 1 and Table 1 show the centroids (average) of preoperative cylinders, expected cylinders (based on ELP and corrected for wound induced astigmatism) cylinders, postoperative manifest astigmatism and the absolute prediction cylindrical errors. Preoperative corneal cylinders have a range up to 6.0 D. Most of the preoperative cylinders were within 3.0 D. Postoperative, most of the cylinders were corrected to within 2.0 D of cylinders, indicating the effectiveness of toric IOLs. The average preoperative astigmatism was less than 0.25 D against-the-rule. The expected astigmatism was almost neutral. The actual postoperative results showed an against-the-rule bias. This resulted in a similar prediction error. Chart 2 provides a closer look at eyes with preoperative with-the-rule astigmatism. Chart 3 and Table 3 show the analysis for eyes with against-the-rule astigmatism. As for eyes with preoperative against-the-rule astigmatism, there was an undercorrection of about +0.35 D against-the-rule. From the analysis above, there were an overcorrection of WTR eyes and undercorrection of ATR eyes. This was consistent with the ndings of other authors. While posterior corneal astigmatism was not directly measured here, this was consistent with most studies on posterior corneal astigmatism. Several strategies have been suggested to correct the effect of posterior corneal astigmatism, including the Baylor normogram suggested by Dr. Douglas Koch 3 and Professor Graham Barrett’s toric calculator. At the same time, the advent of Schleimp ug imaging systems and other topographic systems has allowed us to measure the total corneal astigmatism. This can potentially improve the accuracy of toric IOL selection and orientation in the correction of astigmatism. The issue of posterior corneal astigmatism may also be circumvented with the use of intraoperative aberrometry. It will be very interesting to see a robust comparison of all these modalities in the future. References 1. Yeoh R. Intraoperative miosis in femtosecond laser-assisted cataract surgery. J Cataract Refract Surg. May 2014;40(5):852. 2. Yeoh R. Hydrorupture of posterior capsule in femtosecond laser cataract surgery. J Cataract Refract Surg . April 2012;38(4):730. Editors’ note: Dr. Yeoh is on the Alcon (Fort Worth, Texas/Hünenberg, Switzerland) and Abbott Medical Optics (Santa Ana, Calif.) speaker panels but has no nancial interests related to his comments.