EyeWorld Asia-Pacific December 2015 Issue

41 EWAP CATARACT/IOL December 2015 the biggest contribution to the correlation coefficient came from the axial length, followed by the anterior chamber depth. The other predictors hardly improved the correlation coefficient (less than 1%). This is why we kept axial length and anterior chamber depth as predictors for the true and later the effective lens position,” Dr. Haigis said. A study by Norrby published in 2008 in the Journal of Cataract & Refractive Surgery demonstrated that the prediction of the ACD was only part of the source of the error—other factors were also involved. Norrby found that the largest contributors of error were preoperative estimation of postoperative IOL position, at 35%, postoperative refraction determination, at 27%, and preoperative axial length measurement, at 17%. “[Norrby] showed that the corneal power in normal patients was down around the 10% percentile, and other things were down about 10%, and the biggest one of those was pupil size, and it was about 8%,” Dr. Holladay said. Biometry Dr. Holladay has studied the measurements of preoperative and postoperative ELP with ultrasound to determine biometry measurements. He found that the actual ELP, or the measured (postoperative) position of the IOL, did not improve the prediction error when error existed in the measurement of the axial length and K reading. He said the results reinforced the idea that axial length and K readings need to be improved for the best ELP results and new formulas might be needed to include errors made in axial length and K readings to optimize ELP results. “There’s no question that improving the precision of measurements that we make is going to reduce our prediction error,” he said. “And one of those variables is the ELP.” Dr. Holladay said the high resolution B-scan Artemis 3 high- frequency ultrasound (ArcScan, Morrison, Colo.) can provide a good prediction of the postoperative ELP from the equatorial plane of the crystalline lens and other dimensions that cannot be seen with light (Scheimpflug and OCT). “But we’re [also] going to have to develop some new formulas to utilize that accurate ELP that optimize the outcome based on the fact that we’re still making errors on axial length and K reading. That’s what we’re working on,” he said. Dr. Haigis said that optical biometry for calculating IOL power is near optimum. It has allowed for axial length measurements, including in long eyes, and accurate measurement of the ocular compartment dimensions. “Accuracy is no longer limited by the instrument’s hardware, but by physiological processes: We can measure how the axial length of the human eye changes during the day,” Dr. Haigis said. “We are almost there, but improvements are on the horizon,” Dr. Hoffer concluded. EWAP Reference Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg . 2008;34(3):368–376. Editors’ note: Dr. Hoffer owns the registered trademark name Hoffer when used commercially in biometers. He has no financial interests related to this article. Dr. Holladay is the developer of the Holladay IOL Consultant programs and has financial interests with ArcScan. Dr. Haigis has no financial interests related to this article. Contact information Haigis: wh@ocucalc.de Hoffer: KHofferMD@aol.com Holladay: holladay@docholladay.com 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 E ffective lens position (ELP) is a postoperative value that is predicted based on preoperative biometric measurements. It is a statistical prediction of an optical value, with certain optical assumptions in a physiological environment. Hence, it is subjected to physiological variations. Any errors in the measurements would certainly affect its outcomes as well. As new technology evolves and biometric measurements get more precise, prediction of ELP surfaces remains a major stumbling block in the quest for better outcomes, as shown by Dr. Norrby. The expanded range of eyes, including post-refractive surgery eyes, being implanted with IOLs and the heightened expectation of good outcomes in these eyes have also increased the issue of ELP prediction. In the article, Drs. Haigis, Hoffer, and Holladay explain quite comprehensively the issues with ELP prediction. Presently, there are two distinct approaches towards addressing this issue. One approach is using formulas that utilize more parameters, such as Holladay 2, newer Olsen, and Barrett formulas. These additional parameters better define and address the physiological variations across an expanded range of eyes. However, combining multiple measurements propagates their measurement errors as well, as pointed out by Dr. Haigis. Other newer formulas such as the Hoffer H5, Olsen, and Barrett have better predictive algorithms for ELP. Another approach is using the modern powerful computational power to predict the ELP and calculate the IOL power, such as the radial basis function led by Dr. Warren Hill. However, the importance of good preoperative measurements cannot be underestimated, as mentioned by Dr. Holladay. Optical biometers are preferable to ultrasound. Applanation ultrasound should best be avoided. Biometers should be calibrated regularly. Multiple measurements should be taken. Taking precise measurements and using the appropriate IOL power calculation formulas will go some distance towards good outcomes. Editors’ note: Dr. Fam declared no relevant financial interests. internal geometry of the eye any longer, thus producing an incorrect ELP,” Dr. Haigis said. Other factors According to Dr. Hoffer, based on results today, axial length and preoperative ACD are probably the best parameters to use as predictors for ELP. Dr. Hoffer is the author of the Hoffer Q and Hoffer H-5 formulas. Dr. Haigis said that he and others have studied parameters from the influence of anterior chamber depth, lens thickness, axial length, corneal radius of curvature, and height of the corneal dome on the predictability of the true, postoperative pseudophakic anterior chamber depth. “Five constants are associated with this approach. We found that