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Repeatability of corneal topography: the "corneal field"
K Zadnik1, N E Friedman, D O Mutti
1School of Optometry, University of California, Berkeley 94720, USA.
Background:
The purpose of this study was to determine the interoccasion repeatability of keratometry, photokeratography, and videokeratography and to describe the "corneal field," a scheme for explaining videokeratography results.
Methods:
A single examiner obtained corneal curvature measurements with a keratometer, a photokeratoscope, and the TMS-1 in 29 adult patients on two occasions.
Results:
Estimates for the repeatability of keratometry were +/- 0.49 and +/- 0.65 diopters (D) for the horizontal and vertical meridians respectively. The repeatability of photokeratography was +/- 0.90 and +/- 1.21 D. We presented a rational method for presenting numeric videokeratographic data by temporally and spatially averaging corneal curvature values and grouping them into 24 regions. The repeatability of videokeratography with the TMS-1 was +/- 0.50 D centrally, +/- 0.65 D paracentrally, and +/- 0.80 to +/- 1.00 D in the midperiphery.
Conclusions:
Repeatability of the photokeratoscope for central measurements is considerably worse than the keratometer. The repeatability of videokeratography is worse toward the periphery. Refractive surgeons and contact lens investigators need to be aware of these limitations so that true change can be distinguished from measurement error.