Orbscan II-assisted intraocular lens power calculation for cataract surgery following myopic laser in situ keratomileusis (an American Ophthalmological Society thesis).

PURPOSE This study tests the hypothesis that the keratometric value derived from Orbscan II mean power maps, when used in an intraocular lens (IOL) calculation formula, at a specific measurement zone, will accurately determine the power of an IOL for planned cataract surgery in patients who have undergone prior myopic laser-assisted in situ keratomileusis (LASIK). METHODS This is a two-part study conducted in a referral practice. Experiment 1 is a prospective study of 59 eyes of 30 patients undergoing LASIK. The change in Orbscan mean power maps at five central zones (1.0, 1.5, 2.0, 2.5, and 3.0 mm) was compared with the refractive change from LASIK to determine the optimum Orbscan correlation zone. In Experiment 2, the power of the LASIK-altered cornea was measured by Orbscan and applied to IOL calculations for 17 eyes of 13 patients undergoing cataract surgery. RESULTS In Experiment 1, analysis at the Orbscan 1.0-mm measurement zone demonstrated overestimation of the refractive change, whereas the 2.5-mm and 3.0-mm zones demonstrated underestimation. The 1.5-mm and 2.0-mm zones best approximated the net refractive change following LASIK. In Experiment 2, the Orbscan power at 1.5 mm was selected for IOL calculations to minimize undercorrections. The refractive error following cataract surgery ranged from -0.75 to +0.90 diopters (average, +0.05 diopters). Eight eyes were overcorrected (average, -0.52 diopters) and nine eyes were undercorrected (average, +0.54 diopters). CONCLUSIONS The 1.5-mm Orbscan II zone measures the effective power of the LASIK-altered cornea. When applied to an IOL calculation formula, it accurately predicts the IOL power for planned cataract surgery.