Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia
Accepted 13 August 2006.
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 videokeratography (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) ± 2.58 (SD). The mean manual value (40.52 ± 1.95 D) and Atlas-based values were statistically higher than BCK H2 values (P<.001). The mean corneal power calculated from historical data was 39.33 ± 2.70 D (P = .83 to BCK H2; n = 19) and from contact lens overrefraction, 41.38 ± 3.11 D (P = .19; n = 5). Orbscan II parameters (n = 21) of the total mean power (3.0 mm, 39.10 ± 2.63 D), total tangential power (3.0 mm, 39.11 ± 2.60), total axial power (5.0 mm, 39.19 ± 2.55 D), and total optical power (3.0 mm, 39.08 ± 2.78 D; 4.0 mm, 39.39 ± 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 ±0.50 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.
From a private practice (Qazi, Pepose) and the Department of Ophthalmology and Visual Sciences (Qazi, Pepose), Washington University School of Medicine, St. Louis, Missouri, and the Department of Ophthalmology and Biomedical Engineering Department (Roberts), Ohio State University, Columbus, Ohio, USA; St. Luke's Medical Center (Cua), Quezon City, Philippines
Corresponding author: Jay S. Pepose, MD, PhD, Pepose Vision Institute, 16216 Baxter Road, Suite 205, Chesterfield, Missouri 63017, USA.
Supported in part by the Midwest Cornea Research Foundation, St. Louis, Missouri, USA.
Dr. Pepose received research and travel support from Bausch & Lomb. Dr. Roberts is a consultant to Bausch & Lomb. No other author has a financial or proprietary interest in any material or method mentioned.
ABSTRACT