keratometria

Keratometry, also known as keratometria, is the measurement of the curvature of the anterior surface of the cornea. It provides an estimate of the corneal refractive power and is commonly expressed in diopters. Keratometry converts the radius of curvature into diopters using the standard formula with an assumed corneal refractive index, typically around 1.3375. In practice, keratometric power is calculated as P = (n − 1) / r, with r in meters, resulting in values that usually fall in the low 40s to mid-40s diopters range for a normal central cornea.

Measurements are taken along the flattest (K1) and steepest (K2) meridians, yielding the magnitude of corneal astigmatism and its axis. Central keratometry focuses on the central 3 millimeters of the cornea, where power is most influential for vision and contact lens fitting.

Keratometry is performed with manual keratometers or automated devices, and many modern systems integrate keratometry with corneal topography or anterior segment imaging. While traditional keratometry emphasizes the anterior corneal surface, newer technologies may also assess broader areas of the cornea or include posterior surface data to provide a more comprehensive refractive evaluation.

Applications of keratometry include fitting contact lenses, especially rigid gas-permeable lenses, and planning refractive surgical procedures such as LASIK or PRK, where accurate corneal power and astigmatism measurements guide treatment design. It is also used in intraocular lens power calculations and in screening for corneal irregularities such as keratoconus.

Limitations include its focus on the anterior surface, reliance on a fixed index, and susceptibility to tear film quality and fixation errors. It may be less reliable for irregular or post-surgical corneas.