EyeWorld India March 2014 Issue

33 EWAP CATARACT/IOL March 2014 Views from Asia-Pacific FAM Han Bor, MD Senior Consultant & Head, Cataract & Implant Service, The Eye Institute @ TanTock Seng Hospital 11 Jalan Tan Tock Seng, Singapore 308411 Tel. no. +65 6357-7726 Fax no. +65 6357-7718 famhb@singnet.com.sg Chart2. Table 2. On average, these eyeswith preoperativewith-the-rule astigmatismwereovercorrected.The centroidwas flippedby almost 90 0 .The actualpostoperativemanifest astigmatismwhen comparedwith the expected outcomes showed an error that is biased towards against-the-rule. Chart 2. Table 2. On average, these eyes with preoperative with-the-rule astigmatism were overcorrected. The centroid was flipped by almost 90 0 . The actual postoper tive manifest astigmatism when compared with the expected outcomes showed an error that is biased towards against-the-rule. 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 1. Chart 2. Chart 3. Table 1. Table 2. Table 3. 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 findings 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 GrahamBarrett’s toric calculator. At the same time, the advent of Schleimpflug 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. Lim KL, Fam HB. Relationship between the corneal surface and the anterior segment of the cornea: An Asian perspective. J Cataract Refract Surg . 2006;32:1814–1819. 2. 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. 3. 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.