ARTICLES Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia Mujtaba A. Qazi, MD, Irwin Y. Cua, MD, Cynthia J. Roberts, PhD, Jay S. Pepose, MD, PhD PURPOSE: To assess the accuracy of Orbscan II slit-scanning videokeratography for intraocular lens (IOL) calculation in eyes with previous photorefractive surgery for myopia. SETTING: Private practice, St. Louis, Missouri, USA. METHODS: Corneal power (K) was measured by manual keratometry, Placido-based videokeratogra- phy (Atlas), slit-scanning videokeratography (Orbscan II), and contact lens overrefraction in 21 post- photoablation eyes having cataract surgery. Postoperative data collected after phacoemulsification were used to back-calculate corneal power (BCK). The BCK values were statistically compared at 3.0 to 6.0 mm central Orbscan II curvature and power measurements, including total axial power, total tangential power, total mean power, and total optical power. Similar comparisons were made to Atlas curvature at the 0.0 to 10.0 mm zones. RESULTS: The mean corneal power after refractive surgery based on BCK values using the Holladay 2 formula (BCK H2) was 39.35 diopters (D) G 2.58 (SD). The mean manual value (40.52 G 1.95 D) and Atlas-based values were statistically higher than BCK H2 values (P<.001). The mean corneal power cal- culated from historical data was 39.33 G 2.70 D (P Z .83 to BCK H2; n Z 19) and from contact lens overrefraction, 41.38 G 3.11 D (P Z .19; n Z 5). Orbscan II parameters (n Z 21) of the total mean power (3.0 mm, 39.10 G 2.63 D), total tangential power (3.0 mm, 39.11 G 2.60), total axial power (5.0 mm, 39.19 G 2.55 D), and total optical power (3.0 mm, 39.08 G 2.78 D; 4.0 mm, 39.39 G 2.76 D) were statistically similar to both the historical and BCK H2 values (P>.11). If used prospectively, 80.9% of eyes would have been within G0.50 D of the targeted refraction using a 4.0 mm total optical power, 76.2% using a 5.0 mm total axial power, and 42.1% using the historical method. CONCLUSION: The Orbscan II 5.0 mm total axial power and 4.0 mm total optical power can be used to more accurately predict true corneal power than the history-based method and may be particularly useful when pre-LASIK data are unavailable. J Cataract Refract Surg 2007; 33:21–30 Q 2007 ASCRS and ESCRS Manual and automated keratometry are the most fre- quently used techniques for measuring central corneal refractive power (K) to calculate intraocular lens (IOL) power in cataract surgery. Although these instruments offer good accuracy when applied to healthy, unoperated cor- neas with regular astigmatism, their use in measuring cor- neal power in patients with irregular astigmatism 1 or who have had keratorefractive surgery 2–12 can lead to significant residual postoperative refractive errors. These ‘‘surprises’’ stem partly from the fact that the manual keratometer aver- ages measurements from only 4 discrete paracentral points on the anterior cornea and that it assumes a spherical cen- tral cornea, with the posterior cornea having a radius of curvature 1.2 mm smaller than its anterior counterpart. Eyes that have had keratorefractive laser surgery have altered corneal asphericity 13 and variations in the relation- ship between the anterior and posterior corneal radii of curvature. 14,15 Q 2007 ASCRS and ESCRS Published by Elsevier Inc. 0886-3350/07/$-see front matter doi:10.1016/j.jcrs.2006.08.026 21 J CATARACT REFRACT SURG - VOL 33, JANUARY 2007