Eyeworld

3 EW NEWS & OPINION by Rich Daly EyeWorld Contributing Writer Optical power calculation tools on ASCRS site expands to five March 2017 as well as the recommended toric cylinder for an IOL exchange or piggyback. The ophthalmologist's The new Barrett Rx Formula by Graham Barrett, FRANZCO, provides lens calculations for three different lens implantation scenarios T he Barrett Rx Formula is the latest surgical tool added to the ASCRS website, ex- panding its offerings to five calculators. The Rx formula, which was add- ed to the ASCRS website in January, aims to assist surgeons under three IOL placement scenarios: IOL ex- change, piggyback IOL, and a three- part solution to a toric IOL surprise (the correct orientation, the optimal toricity, and any changes required to the spherical component in order to achieve the desired refractive result). The calculator is derived from the Barrett Universal II formula and provides two options that can be se- lected by the user. The default "ELP" option first calculates the effective lens position (ELP) for the actual postop refractive outcome and uses this as the basis for the calculation. "As such, the results may differ from Holladay R, or the vergence formulas, which use the manufac- turer's anterior chamber depth as the basis for the ELP and use that as the base to recommend piggy- back or IOL exchange IOL power to correct the postop refractive error," said Graham D. Barrett, FRANZCO, Lions Eye Institute, Perth, Austra- lia. "The predicted outcome with the latter is therefore unchanged regardless of the power of the IOL implanted for a particular refractive outcome. The Rx formula, howev- er, allows for surgeons to select an 'IOL' option where the calculation assumes the predicted ELP was correct and utilizes the IOL constant of the selected IOL as the basis for the ELP for calculating the required piggyback or IOL exchange power." Dr. Barrett said surgeons should choose the option to select an IOL when treating post-refractive eyes or if the error is thought to be due to a mix up in IOLs or a mislabeled IOL. "Here, the piggyback IOL pre- diction will be similar to Holladay R and the vergence formula," Dr. Barrett said. The formula requires two lens factors/A constants: one for the actu- al IOL implanted and the other for the IOL selected for IOL exchange. The constant for the IOL can be ad- justed as explained in a pop-up win- dow if the IOL exchange is planned for placement in the sulcus. The Rx formula will predict the spherical equivalent IOL power, continued on page 10

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