Eyeworld

EW CATARACT 49 "There's no question that improving the precision of mea- surements 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 dimen- sions 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 opti- mize the outcome based on the fact that we're still making errors on axi- al 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, includ- ing in long eyes, and accurate mea- surement 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 im- provements are on the horizon," Dr. Hoffer concluded. EW 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 de- veloper of the Holladay IOL Consultant programs and has financial interests with ArcScan. Dr. Haigis has no finan- cial interests related to this article. Reference Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg 2008;34(3):368–376. Contact information Haigis: wh@ocucalc.de Hoffer: KHofferMD@aol.com Holladay: holladay@docholladay.com 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, postopera- tive pseudophakic anterior chamber depth. "Five constants are associated with this approach. We found that the biggest contribution to the correlation coefficient came from the axial length, followed by the anterior chamber depth. The other predictors hardly improved the cor- relation 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 estima- tion of postoperative IOL position, at 35%, postoperative refraction determination, at 27%, and preop- erative axial length measurement, at 17%. "[Norrby] showed that the cor- neal 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 mea- surements of preoperative and post- operative 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 predic- tion 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.

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