is the measurement of the anterior corneal curvature and is traditionally performed with a manual keratometer. This device, also known as an ophthalmometer
, was developed by von Helmholtz in 1880. It is an instrument that gives 2 corneal curvature values (maximum and minimum) 90 degrees apart.
The two basic keratometers are the Helmholtz type and the Javal-Schiotz type. Both use the relationship between object size, image size, and distance, to calculate corneal curvature. The former is the more familiar to most ophthalmologists. It is a one-position device that uses adjustable image size and consists of aligning plus sign and minus sign mires. The latter is a two-position instrument that uses adjustable object size and requires alignment of a red square and green staircase design.
Keratometers measure the size of an image reflected from two paracentral points on the cornea. The instrument contains doubling prisms to stabilize the image allowing more accurate focusing. The anterior corneal curvature is then obtained from the convex mirror formula and corneal power is calculated empirically using Snell’s law of refraction with simplified optics. The keratometer measures the anterior corneal surface but uses an assumed index of refraction (1.3375 rather than the actual 1.376) to account for the small contribution from the posterior corneal surface, the corneal thickness, and also to allow 45 D to equal 7.5 mm radius of curvature (K (diopters) = 337.5/r).
These simplifications and assumptions create a number of limitations. The keratometer only measures a small region of the cornea (i.e., 2 points at the 3-4 mm zone), and this measured region is different for corneas of different powers. No information is provided about the cornea central or peripheral to these points. Furthermore, the keratometer assumes that the cornea has a symmetric spherocylindrical shape with a major and minor axis separated by 90 degrees. It also does not account for spherical aberration, and it is susceptible to focusing and misalignment errors. Finally, distortion of the mires precludes accurate measurement of irregular corneas and cannot be quantified.
Despite these drawbacks, the manual keratometer provides accurate information for the majority of our patients. It is the standard instrument in ophthalmology for measuring astigmatism. Clinically, this corneal curvature data is primarily used for contact lens fitting, IOL calculations, and corneal refractive surgery. It is also helpful for detecting irregular astigmatism, which is visible as distortion or irregularity in the appearance of the mires.
Alternatives to the traditional keratometer are automated instruments that provide keratometry readings alone or in addition to a number of other functions. These include autorefractors that measure refraction, corneal topographers that map the anterior corneal surface, and the IOLMaster that calculates axial length, anterior chamber depth, white-to-white distance, and IOL powers for cataract surgery. The advantage of corneal topography is the ability to measure and quantify irregular astigmatism, which cannot be done with a keratometer. Although topography devices can average numerous corneal curvature measurements over a variety of central optical zone diameters, it is important to remember that the simulated keratometry readings (SimK) that these machines provide are essentially what a manual keratometer would estimate the corneal curvature to be at approximately the 3 mm zone. The IOLMaster provides more accurate keratometry readings than those from a manual keratometer because the corneal curvature data is obtained from 6 points closer to the center of the cornea (2.5 mm zone) and three consecutive readings are averaged.
Regardless of the device you choose to use, it is important to obtain accurate and consistent readings. To this end, the instrument should be calibrated on a regular basis, the same designated individual(s) should operate the instrument, multiple readings should be obtained, and consideration should be given to taking measurements with more than one device particularly when planning for the surgical correction of astigmatism.