The relation between corneal and total astigmatism.
Using computer-assisted videokeratoscopy we measured corneal astigmatism and compared these results over a range of corneal zone diameters with total ocular astigmatism derived by subjective refraction. Videokeratoscopes permit a more detailed analysis of the power distribution within a given corneal surface area, enabling comparison to the total astigmatism for equivalent aperture sizes. Although there were significant individual variations, the group average data supports the traditional view of a linear relation between corneal and total astigmatism. This was true across the range of apertures tested from 2 to 7 mm, with the coordinates of the relation being consistent with that of the modified Javal's rule; namely a slope of 1 and an intercept of approximately 0.50 D against-the-rule residual astigmatism.
Keller
P R
PR
Centre for Opthalmology and Visual Science, University of Western Australia, Perth, Australia.
Collins
M J
MJ
Carney
L G
LG
Davis
B A
BA
van Saarloos
P P
PP
D000328:Adult; D001251:Astigmatism / Q000473:pathology / Q000503:physiopathology*; D003315:Cornea / Q000473:pathology / Q000503:physiopathology*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D018715:Microscopy, Video; D012029:Refraction, Ocular; D012044:Regression Analysis