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The effect of poor fixation on computer-assisted topographic corneal analysis. Pseudokeratoconus
1Duke University Eye Center, Durham, North Carolina 27710.
Introduction:
The effect of changing the alignment of the corneal surface in relation to the plane of the videokeratoscope mires in computer-assisted topographic corneal analysis was evaluated.
Methods:
The corneas of five subjects were analyzed with the visual axis at 0 degrees, 2.5 degrees, 5 degrees, and 10 degrees below the central axis of the videokeratoscope. The relative steepness of the inferior and superior cornea was compared using inferior-superior (I-S) values at each deviation from the central axis. A similar analysis was performed on three aspheric test surfaces with different central curvatures.
Results:
The difference in the relative steepness of the inferior and superior cornea was found to increase in proportion to the deviation from alignment with the central axis in both the test subjects and the aspheric test surfaces.
Conclusion:
This change in relative steepness produces a pattern that mimics keratoconus and can be seen at deviations of less than 5 degrees in some patients. The increase in the I-S value was significant at 5 degrees of deviation (P < 0.05). It also was found that this effect increased as the central curvature of the aspheric surfaces increased.
