Corneal topography bow-tie pattern: artifact of videokeratoscopy?
OBJECTIVE
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.
Keller
P R
PR
Lions Eye Institute, Centre for Ophthalmology and Visual Science, University of Western Australia, Nedlands.
Reid
P G
PG
van Saarloos
P P
PP
D016477:Artifacts; D001251:Astigmatism / Q000175:diagnosis; D003198:Computer Simulation; D003315:Cornea / Q000473:pathology*; D019781:Corneal Topography; D006801:Humans; D008954:Models, Biological