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Corneal topography bow-tie pattern: artifact of videokeratoscopy?
P R Keller1, P G Reid, P P van Saarloos
1Lions Eye Institute, Centre for Ophthalmology and Visual Science, University of Western Australia, Nedlands.
Purpose:
To test the hypothesis that the bow-tie corneal topography pattern results from corneal asphericity in the presence of astigmatism.
Methods:
Astigmatic color-coded power maps using different shape factors were computer generated. Each simulation was based on the calculation of dioptric power at 20 points along each of 180 hemimeridia, for a total of 3600 points. The calculations were made independent of the capture or measurement of video-keratographs. These simulations were compared to power maps taken from clinical records.
Results:
A shape factor of 1.00 resulted in a spherocylinder color-coded map with straight-edged sectors of power. The familiar bow-tie pattern was generated using an elliptical model with a shape factor of less than 1.00. This pattern was reversed by modeling the cornea as an oblate ellipsoid using a shape factor greater than 1.00.
Conclusions:
By simple alteration of the amount of corneal asphericity through manipulation of the shape factor, computer simulation showed that this surface characteristic is responsible for the bow-tie pattern observed in corneal topography power maps.