CORNEAL TOPOGRAPHY BOW-TIE PATTERN - ARTIFACT OF VIDEOKERATOSCOPY

Purpose: To test the hypothesis that the bow-tie corneal topography pa ttern results from corneal asphericity in the presence of astigmatism. Methods: Astigmatic color-coded power maps using different shape fact ors were computer generated. Each simulation was based on the calculat ion 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 c ompared to power maps taken from clinical records. Results: A shape fa ctor of 1.00 resulted in a spherocylinder color-coded map with straigh t-edged sectors of power. The familiar bow-tie pattern was generated u sing an elliptical model with a shape factor of less than 1.00. This p attern was reversed by modeling the cornea as an oblate ellipsoid usin g 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 i s responsible for the bow-tie pattern observed in corneal topography p ower maps.