Relationship between anterior corneal asphericity and refractive variables.

Background: The anterior corneal surface is closely modelled by a conic section that is fully described by asphericity (Q) and the apical radius of curvature. Computerized corneal topographers have allowed for more accurate and complete descriptions of corneal shape. Our objective was to compare anterior corneal asphericity (Q) values determined for different corneal diameters in eyes of different refractive state.

Methods: Q-values were determined in 118 eyes of 118 subjects using both a videokeratoscope (Atlas Mastervue, Humphrey Instruments-Zeiss) and Vol-CT 6.89 software (Sarver & Associates Inc.), which estimates Q-values for several corneal diameters (3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm and 8.0 mm) using topographic data obtained with the instrument. For comparisons, Q-values were stratified three ways: by refractive error (myopic, emmetropic or hyperopic eyes), corneal power (low, intermediate and high) and corneal astigmatism (low, intermediate and high).

Results: Mean corneal asphericity was -0.35 +/- 0.03, differing significantly from reported data (Student's t-test). Asphericities determined using both methods did not vary significantly with regard to refractive error or corneal power, but did differ among the corneal astigmatism groups (p < 0.01). A trend was observed towards more negative Q-values with increasing corneal diameter, but differences in corneal asphericity according to corneal diameter were only significant in the astigmatism group (p < 0.01).

Conclusion: Q-values varied according to the refractive properties examined. However, the relationship between refractive state and corneal asphericity was found to be determined more by the geometric properties of the eye (corneal power and axial length) than by manifest refraction.