Corneal topography bow-tie pattern: Artifact of videokeratoscopy?

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 videokeratographs. 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.