Axial, instantaneous, and refractive formulas in computerized videokeratography of normal corneas.

PURPOSE

To compare the values for corneal power determined by the axial, instantaneous and refractive formulas when imaging normal human corneas using computerized videokeratography.

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.

METHODS

This prospective clinical trial involved 60 corneas of 30 normal volunteers. Computerized videokeratography was performed to determine corneal power at the center and the 1, 3, 5, and 7 mm zones using the 3 formulas.

RESULTS

Mean central corneal power was 42.86 diopters (D) with each of the formulas. The mean corneal powers for the axial, instantaneous, and refractive formulas were 43.09, 43.21, and 42.98 D at the 1 mm zone; 43.10, 42.92, and 43.46 D at the 3 mm zone; 42.75, 41.63, and 44.02 at the 5 mm zone; 42.21, 40.30, and 44.79 D at the 7 mm zone, respectively. The differences among powers for the 3 formulas at the 3, 5, and 7 mm zones were statistically significant (P < .01).

CONCLUSION

In normal corneas, clinically significant differences exist in the corneal power values calculated by the axial, instantaneous, and refractive formulas.