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Shape and radius of posterior corneal surface.

BACKGROUND
The posterior corneal surface is often ignored in predictive models concerned with refractive surgery. In previous studies, the radius of this surface has been measured in a variety of ways, primarily over a large chord diameter of the surface, with the common assumption that the surface is spherical. The asphericity of this surface has not been adequately addressed in the past.


METHODS
An algorithm is derived for the calculation of posterior corneal surface apical radius and using characteristics of the anterior corneal surface and topographic corneal thickness variation. Anterior corneal asphericity was measured using a commercially available photoelectric keratoscope. Using a marked soft contact lens, a simple method of locating noncentral corneal sites is described as an aid to ultrasonic pachometry.


RESULTS
In a group of 20 normal subjects ranging in age from 19 to 23 years, the average posterior corneal surface apical radius and asphericity (p) was 5.80 mm (SD = +/- 0.42) and 0.64 (SD = 0.37) for the vertical meridian, and 5.82 mm (SD = +/- 0.40) and 0.52 (SD = 0.30) for the horizontal meridian. Average central corneal thickness was 533 mu (SD = +/- 19).


CONCLUSIONS
The average asphericity values are below 1, hence the posterior corneal surface is described as a flattening ellipse. The rate of flattening of this surface is greater than the rate of flattening along the averaged anterior corneal surface.