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Comments
The power of the cornea is conventionally measured by keratometry. Original
keratometers (Javal) measured the total corneal power. That method incorrectly assumed
that the back and front corneal radii were equal and therefore used a tear film index of
1.336. Later, to "standardize" the results, an arbitrary index of refraction of 1.3375 was
used so that a radius of 7.5 mm would yield 45.0 diopters (13). The keratometer
measures the anterior radius of corneal curvature, expressed in millimeters, which is
translated into diopters by considering the entire corneal power to be at the anterior
corneal surface. The relationship between the keratometric readings (K in diopters) and
the value of the anterior corneal radius (r in millimeters) is:
K = 1000 (1.3375 1) ÷ r
The refractive index of the cornea has been measured to be 1.376 by Gullstrand (14) and
1.3771 by Le Grand (15) who used a sodium light (0.59 µ) for a more accurate
measurement. The relationship between the real value of the power of the cornea's
anterior surface (D in diopters) and the keratometric readings (K in diopters) (16) is:
D = 1.1173 x K
The change in the ratio of the front to the back curvature after Lasik makes the assumed
index of refraction incorrect. Since the corneal power is measured at the anterior corneal
surface, each diopter of myopic correction should decrease the measurement at the
corneal apex by a factor of 1.1173.
