EyeWorld Asia-Pacific June 2018 issue

June 2018 58 EWAP REFRACTIVE can be accurately described using exact ray tracing and modern bio- metric techniques.” Nevertheless, the error as- sociated with lens position is significant, necessitating a reliable prediction of the ALP. Dr. Olsen proposed a method to directly measure the dimensions of the crystalline lens in another study of 2,043 cases in which he evaluated the accuracy of the C constant for ray tracing assisted IOL power cal- culations using the Olsen formula, compared with the Haigis, Hof- fer Q, Holladay 1, and the SRK/T formulas. He found that the IOL tended to locate within the empty capsule bag at a certain location that could be described with a constant. 3 “Modern OCT techniques confirm the concept of the C constant,” Dr. Olsen explained. “We demonstrated IOL position using the C constant in a fairly consistent pattern in 1,730 eyes and compared the predicted to the observed value of ACD with a correlation coefficient of r=0.9. Al- most 90% of the cases were within ±0.25 mm. This is something we can use in IOL power calculations. We got rid of the K reading and the axial length, and we replaced the prediction of the IOL position by the anterior chamber depth and the lens thickness. The C constant defines the IOL position in terms of the crystalline lens anatomy, not dependent on the K reading or the axial length. This is especially important in post-LASIK cases. The C constant defines the position of the IOL as a fraction of capsular bag thickness. Our work shows good outcomes for different axial lengths in short but espe- cially long eyes.” Recent studies have shown good results using Dr. Olsen’s method. In one investigation that compared the prediction accuracy in nine IOL power calculation for- mulas using two optical biometers, the Olsen formula was the most accurate with optical low coher- ence reflectometry (OLCR) meas- urements and significantly better than the best formula with partial coherence interferometry (PCI) measurements, regardless of AL. 4 In another study on prediction accuracy of pre-installed formulas on two optical biometers, investi- gators found that the OLCR device outperformed the PCI device using the Olsen formula, which also ranked first for both short and long eyes. 5 Finally, a study that in- cluded more than 13,000 cataract surgeries using monofocal IOLs revealed that the Olsen method was more than 80% within 0.5 D, while most other formulas did worse. Error versus axial length showed ray tracing outcomes to be among the best compared to other formulas, especially in long eyes. 6 Fellow eye “We saw in almost 400 eyes that there is a correlation coefficient of roughly 0.9 between the error of the left and right eyes. If you take the difference between the right and left IOL position, you will get more than 95% within ±0.25 mm. This is even better than what I can predict from preop data. That means I can use the observed IOL position in the first eye in the planning for the second eye,” Dr. Olsen explained, referring to his study that tested the hypothesis that the refractive outcomes of the first eye could be used to improve the IOL power calculation of the second eye. 7 EWAP References 1. Olsen T, et al. Ray-tracing analysis of the corneal power from Scheimpflug data. J Refract Surg. 2018;34:45–50. 2. Olsen T, et al. Ray-tracing analysis of intraocular lens power in situ. J Cataract Refract Surg. 2012;38:641–7. 3. Olsen T, et al. C constant: new concept for ray tracing-assisted intraocular lens power calculation. J Cataract Refract Surg. 2014;40:764–73. 4. Cooke DL, et al. Comparison of 9 in- traocular lens power calculation formulas. J Cataract Refract Surg . 2016;42:1157–64. 5. Cooke DL, et al. Prediction accuracy of preinstalled formulas on 2 opti- cal biometers. J Cataract Refract Surg. 2016;42:358–62. 6. Melles RB, et al. Accuracy of intraocular lens calculation formulas. Ophthalmol . 2018;125:169–178. 7. Olsen T. Use of fellow eye data in the calculation of intraocular lens power for the second eye. Ophthalmol . 2011;118:1710–5. Editors’ note: Dr. Olsen has financial interests with Haag-Streit and Pha- coOptics (Aarhus, Denmark). Contact information Olsen: t@olsen.dk Beaver-Visitec Int. Page: 39 www.beaver-visitec.com Diamatrix Ltd. Page: 23 www.diamatrix.com Oculus Optikgeräte Page: 20 www.oculus.de Ziemer Page: 72 www.ziemergroup.com ASCRS Page: 59 , 67 , 71 www.ascrs.org APACRS Page: 2, 5, 7, 14, 30 www.apacrs.org EyeWorld Page: 10, 17, 33 , 41 , 45, 62 www.eyeworld.org Index to Advertisers Ray tracing – from page 57