EyeWorld India March 2019 Issue

16 EWAP FEATURE March 2019 measurements, Dr. Koch shared a few pearls. • Do at least two measurements with different devices. His group does three or four measurements. “One of the devices can be a biometer, particularly biometers that have multiple LEDs, like the IOLMaster 700 and the LENSTAR, both of which have more than six [LEDs],” he said. • Perform topography. “I like reflection topography for the anterior surface. It is more accurate than elevation measurements, and Placido rings give valuable information about surface quality,” he said. • Repeat topography when necessary. “Make sure that all of your measurements closely match. If they don’t, re-evaluate the corneal surface and repeat the measurements,” Dr. Koch advised. Final pearls Drs. Koch and Hill reflected on other important issues for posterior corneal astigmatism, such as whether preop measurements suffice and when posterior measurements aren’t necessary. “Good preop measurements, combined with an advanced toric calculator such as the Barrett or the Abulafia-Koch/Hill-RBF calculator, are capable of a 90% accuracy of 0.50 D or less of residual refractive astigmatism,” Dr. Hill said. “Correctly used, intraoperative aberrometry can also achieve values such as this for residual refractive astigmatism.” Most of the time, Dr. Koch does not think that intraoperative aberrometry improves the accuracy of toric IOL selection and alignment. “For posterior corneal astigmatism in particular, its magnitude in a normal eye may be below the threshold of noise for intraoperative aberrometry so I am not sure that it adds more at this point than our current nomograms,” he said. It is important not to insert values for total corneal astigmatism into toric IOL calculators that use anterior corneal measurements to estimate posterior corneal astigmatism. “If a measuring device provides a true, or net, corneal power, the posterior cornea has already been incorporated into this value,” Dr. Hill said. “Adding a posterior corneal algorithm would account for the posterior cornea a second time and make the calculation inaccurate.” “If you have against-the-rule astigmatism, the posterior cornea often doesn’t contribute a lot, typically around 0.2 D,” Dr. Koch said. “But you have to add in lens tilt. Postoperative IOL tilt (which can be predicted from preoperative crystalline lens tilt 1,2 ) introduces up to 0.2 D of additional against-the- rule refractive astigmatism. 3 ” Measurements will continue to improve and evolve, including the impact of lens tilt, Dr. Koch predicted. “In the future, our preoperative measurements will include total corneal astigmatism and crystalline lens tilt. Knowing the latter and the type of IOL and its meridional alignment, we will be able to predict the refractive impact of IOL tilt. These steps will take us to a new level of accuracy in optimizing our treatment of patients’ astigmatism,” he said. EWAP References 1. Hirnschall N, et al. Prediction of postoperative intraocular lens tilt using swept-source optical coherence tomography. J Cataract Refract Surg. 2017;43:732–736. 2. Wang L, et al. Evaluation of crystalline lens and intraocular lens tilt using a swept- source optical coherence tomography biometer. J Cataract Refract Surg. Accepted for publication. 3. Weikert MP, et al. Astigmatism induced by intraocular lens tilt evaluated via ray tracing. J Cataract Refract Surg. 2018; 44:745–749. Editors’ note: Dr. Hill has financial interests with Alcon (Fort Worth, Texas) and Haag-Streit. Dr. Koch has financial interests with Alcon, Johnson & Johnson Vision (Santa Ana, California), and Carl Zeiss Meditec. Contact information Hill: hill@doctor-hill.com Koch: dkoch@bcm.edu LEE Mun Wai, MD Medical Director Lee Eye Centre 44-46 Persiaran Greenhill, Ipoh, Perak, Malaysia 30450 F or the refractive cataract surgeon, correcting astigmatism with toric IOLs is a fundamental component of our armamentarium. Aside from having a good, consistent, and reproducible surgical technique, the following are essential prerequisites for optimal outcomes: 1) Accurate IOL prediction (using optical biometry) and a toric IOL calculator which accounts for posterior corneal astigmatism (PCA) 2) An IOL with predictable performance and good rotational stability 3) Accurate IOL alignment (preferably with a digital system) The Barrett Toric Calculator It is common practice for patients to be considered for toric IOL only if they are found to have >1 D of corneal astigmatism. In my practice, however, all patients planned for cataract surgery will have their keratometry readings put through this calculator and often times a toric IOL is recommended even when the magnitude of corneal astigmatism is less than 1 D. This is particularly important when a presbyopic IOL is planned as any residual astigmatism degrades the quality of vision and results in an unhappy patient! $ UHODWLYHO\ QHZ DQG VLJQLÀFDQW DGGLWLRQ WR WKLV FDOFXODWRU LV WKH . &DOFXODWRU IXQFWLRQ ZKLFK DOORZV WKH LQSXW RI .HUDWRPHWU\ UHDGLQJV IURP PXOWLSOH PDFKLQHV DQG WKLV UHVXOWV LQ DQ ,QWHJUDWHG . UHDGLQJ XVHG IRU FDOFXODWLRQ which further improves the accuracy of toric IOL prediction. For those who have machines which are capable of measuring PCA, the option of using these measurements is available with this calculator by clicking on the Measured PCA option. But as mentioned in this article, one has to be wary not to double count as this would result in erroneous calculations. A recent study (by Dr. Graham Barrett) 1 compared toric IOL prediction XVLQJ PHDVXUHG SDUDPHWHUV IURP D 6FKHLPSÁXJ GHYLFH ZLWK WKH WKHRUHWLFDO prediction using the Barrett Toric Calculator and this showed that 70% of eyes were within 0.5 D of predicted residual astigmatism using Measured PCA vs 79% of eyes using Theoretical PCA but the differences were not VWDWLVWLFDOO\ VLJQLÀFDQW 5HFRJQL]LQJ WKH VLJQLÀFDQFH RI SRVWHULRU FRUQHDO DVWLJPDWLVP 2 has had a major impact on toric IOL prediction and credit goes to Dr. Graham Barrett who makes this calculator free, simple to use and readily available via the APACRS website (www.apacrs.org) . It has greatly improved my outcomes as I routinely achieve less than 0.5D residual refractive astigmatism in at least 85% of my toric IOL patients. References 1. Free paper presentation at ESCRS 2018 (Vienna). .RFK '' HW DO &RQWULEXWLRQ RI SRVWHULRU FRUQHDO DVWLJPDWLVP WR WRWDO corneal astigmatism. J Cataract Refract Surg. 2012;38:2080-2087. (GLWRUV· QRWH 'U /HH GHFODUHG QR UHOHYDQW ÀQDQFLDO LQWHUHVWV 9LHZV IURP $VLD 3DFLÀF Maximizing posterior – from page 15