Twenty-nine healthy subjects were enrolled in a study of 2 types of misalignment during Orbscan topography acquisition. The first was rotation, which was defined as angular deviation of the subject's visual axis away from the central fixation target, and the second was translation, defined as off-center displacement of the joined half slits on the computer screen during alignment by the technician. The effect of vertical and horizontal misalignments, by rotation or translation, was assessed. Study parameters included simulated keratometries (SimKs), mean corneal power (mean power), and corneal apex (R) and the asphericity factor (Q) values. Inferior–superior (I–S) and temporal–nasal (T–N) indices were calculated. Differences in parameter values between aligned and misaligned positions were studied. A mathematical model was also developed to simulate rotational and translational misalignment and consolidate clinical experimentation.
Results
The mean SimK values were slightly but significantly affected by rotation. The I–S value increased significantly with downward rotation and decreased with upward rotation of the eye, while T–N values increased significantly with temporal rotation and decreased with nasal rotation. The R and Q values increased with rotation. Overall, the topography parameters were minimally affected by translation. Comparable results were obtained with the theoretical model.
Conclusions
Rotational misalignment of the eye during acquisition influenced topography more than the translational misalignment during focusing by the technician. Rotational misalignment induced corneal asymmetry on elevation and curvature maps as well as pseudokeratoconus patterns.
From the Department of Ophthalmology (Hick), University of Liège, Liège, Belgium; the Departments of Ophthalmology (Hick, Laliberté, Meunier, Brunette), Computer Sciences and Operations Research (Meunier), and Mathematics and Statistics (Chagnon), University of Montreal, and Maisonneuve-Rosemont Hospital (Laliberté, Brunette), Montreal, Quebec, Canada
Corresponding author: Isabelle Brunette, MD, FRCSC, Ophthalmology Research Unit, Maisonneuve Rosemont Hospital, 5415, de L'Assomption Boulevard, Montreal, Quebec H1T 2M4, Canada.
No author has a financial or proprietary interest in any material or method mentioned.
Supported by the FRSQ Research in Vision Network, Montreal, Quebec, and the Canadian Institutes for Health Research, Ottawa, Ontario, Canada; the CIHR Institute of Neurosciences, Mental Health and Addiction, the Léon-Frédéricq Foundation, University of Liège, Belgium; and a mandate for clinical research, CHU Liège, Belgium; Le Fonds Besner-Valois and Le Fonds de recherche en ophtalmologie de l'Université de Montréal, University of Montreal, Montreal, Quebec, Canada.
Presented at the World Cornea Congress V and ASCRS Symposium on Cataract, IOL and Refractive Surgery, Washington, DC, USA, April 2005.
ABSTRACT