Eyeworld

This is "far more accurate than the first-order approximations used by the thin lens formulas, and it has the advantage over thick lens for- mulas that higher order aberrations (HOA) can be taken into account." "[Ray tracing offers] full inde- pendence from perioperative data; surgeons just have to enter the axial length and software will predict the position of the lens, its power, and the final refraction, taking into consideration the corneal power and asphericity," Dr. Savini said. Dr. Olsen added that ray tracing "is recognized as the most effec- tive tool in optical engineering—it makes sense to use this technique on the eye." "Most formulas only use a single constant, the A-constant or the anterior chamber depth [ACD] constant, which normally is de- rived from the actual postoperative refraction, back-calculating what the constant should have been to ensure an average prediction error of zero," Dr. Olsen said. "When this 'opti- mization' has been accomplished, the performance of the formula is compared using metrics of the scat- ter like the standard deviation, the mean absolute error, or the median error." But the C-constant defines the physical IOL position from the pre- operative ACD and lens thickness, Dr. Olsen said, meaning it is not de- pendent upon the K-reading or the axial length. In a recent study, 2 Dr. Olsen and Peter Hoffmann, MD, evaluated 2,043 cases to determine the accuracy of the C-constant for ray tracing and found it a "prom- ising concept." In their analysis, using the C-constant for "unbiased prediction of the true position of the IOL," the Olsen formula also mod- ifies exact ray tracing to correct for spherical aberrations of the cornea and IOL. This approach correspond- ed to a 15% reduction in mean abso- lute errors, and the number of large errors (1.0 D) dropped 39% in public hospital series and 85% in private clinic series when compared to the SRK/T formula. "A great deal of the improved accuracy was the result of improved accuracy in predicting the IOL po- sition," in normal eyes over a large axial length range, Dr. Olsen said. Both physicians said the future looks promising for even more accu- rate IOL power calculations. EW References 1.Savini G, Hoffer KJ, Casamenti V. Clinical Comparison of Methods to Calculate IOL Power After Myopic LASIK and PRK. Paper presented at: American Society of Cataract & Refractive Surgery. April 2014, Boston. 2. Olsen T, Hoffmann P. C constant: New con- cept for ray-tracing assisted intraocular lens power calculation. J Cataract Refract Surg. 2014 2014;40:764–773. 3. Olsen T. Use of fellow eye data in the calcu- lation of intraocular lens power for the second eye. Ophthalmology. 2011;118(9):1710–1715. 4. Olsen T, Funding M. Ray-tracing analysis of intraocular lens power in situ. J Cataract Refract Surg. Apr 2012;38(4):641–647. Editors' note: Dr. Olsen is the founder of IOL Innovations and has financial interests with Haag-Streit (Koniz, Switzerland). Dr. Savini has no finan- cial interests related to his comments. Contact information Olsen: tkolsen@dadlnet.dk Savini: giacomo.savini@alice.it " [Ray tracing] is recognized as the most effective tool in optical engineering—it makes sense to use this technique on the eye. " –Thomas Olsen, MD

• Cover