Melanie Corbett - Corneal Topography: Principles and Applications

In 1619, Scheiner made the first measurements of corneal shape []. He held up a series of convex mirrors of different curvatures next to the eye, until he found one which gave an image of the same size as the image from the cornea.

In the 1820s, Cuignet developed a keratoscope through which he observed the reflected image of an illuminated target held in front of the patients cornea. His major problem was in the alignment of the light, target and observer with the patients visual axis. This was overcome in 1882 by Placido , who placed an observation hole in the centre of the target []. His target was a disc bearing alternating black and white concentric rings; and this pattern still forms the basis of many topography systems today.

Quantification of corneal curvature became possible in 1854 with the development of the keratometer (ophthalmometer) by Helmholtz []. The distance between two pairs of reflected points gave the spherocylindrical curvature of the central 3 mm of the cornea, in two meridians. In order to increase the area of the cornea analysed, Javal (1889) attached a Placido-type disc to his keratometer. Its telescopic eyepiece gave him the additional benefit of a magnified keratoscopy image. He realised the need to fix the image and measure the size of the rings, but this was not practical until Gullstrand (1896) applied photography to keratoscopy (photokeratoscopy). Numerous attempts were made to quantify keratographs by comparison with photographs of spheres of known radius, but all techniques were laborious and time-consuming.

Little progress was then made in corneal topography until the middle of this century, when interest was rekindled by the introduction of contact lenses. Contact lens fitting requires knowledge of the curvature of the midperipheral cornea. The keratometer could provide this information for relatively normal corneas with only regular astigmatism and is still suitable for contact lens fitting in uncomplicated cases today.

With the development of microsurgical techniques for cataract extraction, corneal grafting and incisional refractive surgery, interest turned to the optical power provided by the cornea. Measurements of cornea curvature can be converted to dioptric power using the standard keratometric index.

As the visual results of these procedures have improved, fine-tuning of the refractive outcome has become increasingly important. It became necessary to develop means of assessing the topography of the whole corneal surface with great detail and accuracy. Photokeratoscopy provided qualitative information about a large area of the cornea, but it was only as a result of developments in computing that quantitative analysis of these images could be performed using videokeratoscopy. Several devices were developed based on the principle of projection rather than reflection to generate true height data, but in practice, mainly those using Scheimpflug technology are in general use.