Evaluating the reproducibility of topography systems on spherical surfaces
L. J. Maguire, S. E. Wilson, J. J. Camp and S. Verity
Department of Ophthalmology, Mayo Clinic, Rochester, MN 55905.
Newly devised software was used to compare the ability of the Topographic
Modeling System-1 (Computed Anatomy, New York, NY) and the Corneal Analysis
System (EyeSys Laboratories, Houston, Tex) to reproduce power measurements
on spherical surfaces. Reproducibility results were compared for spheres of
40.00, 42.50, and 44.00 diopters. The program determines the absolute
difference in corneal power at defined keratoscope positions for paired
examinations of the same eye. Four examinations of each sphere were
obtained with each instrument. Individual points were sampled at specific
keratoscope locations at 30 degrees-semimeridional intervals. The program
compared variability of measurements at four defined ranges of distance
from the vertex normal: within 0.60 mm, 0.61 to 1.5 mm, 1.51 to 2.5 mm, and
2.5 mm or greater. The Corneal Analysis System showed significantly greater
variability of readings obtained within 0.60 mm of the vertex normal for
all three spheres (P = .001 by Duncan's multiple comparison procedure),
whereas the Topographic Modeling System-1 showed equally consistent
readings within 0.60 mm as it did between 0.61 and 1.5 mm from the vertex
normal.
