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Optical zone centration in keratorefractive surgery. Entrance pupil center, visual axis, coaxially sighted corneal
1Department of Ophthalmology, St. James's University Hospital, Leeds, England.
Purpose:
To study the differences in optical zone marking using the geometric corneal center, entrance pupil center, visual axis, and the coaxially sighted corneal reflex as centration points.
Methods:
A modified autokeratometer was used to photograph the cornea in 50 volunteers under standardized levels of illumination, with the subject fixing on the keratometer target. These photographs enabled us to mark the above-mentioned centration points and measure the direction and degree of decentration.
Results:
From the corneal intercept of the visual axis, the entrance pupil center was found up to 0.75 mm (mean, 0.34 mm) temporally, the corneal reflex was found up to 0.62 mm (mean, 0.02 mm) nasally, and the geometric corneal center was found up to 1.06 mm (mean, 0.55 mm) temporally.
Conclusion:
The ideal physiologic centration point is the corneal intercept of the visual axis. The decentration from the visual axis was least if the coaxially sighted corneal reflex was used for centration.