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

J Cataract Refract Surg. 1998 Sep;24(9):1184-90. doi: 10.1016/s0886-3350(98)80009-6.


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.

Publication types

  • Clinical Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Cornea / anatomy & histology*
  • Corneal Topography / methods*
  • Female
  • Humans
  • Male
  • Mathematics
  • Middle Aged
  • Prospective Studies
  • Refraction, Ocular*