Keratometri

Keratometry, or keratometer, is a diagnostic technique used to measure the curvature of the anterior surface of the cornea. It provides an estimate of corneal refractive power, typically reported in diopters (D), and the amount of corneal astigmatism. The test is widely used in refractive and cataract surgery planning, and in fitting rigid contact lenses.

Principle: Most keratometers use the reflection of mires projected onto the cornea. The instrument analyzes the ring patterns reflected from the corneal surface to determine the two principal meridians of curvature. From these, the keratometric powers K1 and K2 are calculated, and mean keratometry (K) and corneal astigmatism (K2 − K1) are derived. A standard corneal refractive index of about 1.3375 is assumed, which relates corneal curvature to dioptric power. Modern devices are automated, sometimes combining keratometry with other measurements, while older manual keratometers rely on mechanical alignment.

Typical values: normal mean K is around 42 to 44 D, with corneal astigmatism commonly 0 to 3 D. The measurement reflects only the anterior corneal surface and can be influenced by tear film, dry eye, or irregular astigmatism; it does not assess posterior corneal curvature or higher-order aberrations.

Indications: preoperative assessment for cataract surgery and IOL power calculation; planning for refractive surgery; contact lens fitting; monitoring corneal diseases such as keratoconus.

Limitations: assumes a fixed refractive index and a spherical posterior surface; less reliable in irregular corneas; results may vary between devices and require confirmation with corneal topography or tomography.