The healthy central cornea is aspheric and prolate (the central curvature is steeper than the periphery). Attempts to measure the cornea were made as early as the 1600s by Scheiner, who compared reflections produced by glass spheres whose diameters were known to the reflections from the anterior surface of the cornea. The central keratometric values vary between 40 D to 47 D (mean 43D). The standard keratometry device measures two points at a 2.25-4 mm zone in the central cornea.
The radius of curvature of the central part of the anterior surface of the cornea is measured by the size of the reflection of an image projected by the keratometer using the formula r=2uI/o or the radius of curvature= 2 x distance between the reflective surface and the object x image size /object size (r=radius of curvature, u=distance between the reflective surface and the object, I=image size, and o= object size).
The different keratometers, such as the Javal Schiotz and the Bausch and Lomb keratometer, were adequate to calculate the dioptric power of the cornea for intraocular lens power calculation and contact lens fitting. However, with increased keratorefractive procedures, the evaluation of paracentral and peripheral cornea is of increased significance. The Placido ring was the first instrument used to analyze the anterior surface of the cornea. Then, computerized videokeratography (CVK) was developed to map the corneal power and shape.
Principles of topography - A luminous object is projected onto the cornea, and its reflection is analyzed. The methods of topography include
Placido method,
Slit scanning technique, and
Schiempflug method.
The Placido method analyzes only the anterior surface of the cornea, whereas the other two methods also map the posterior corneal surface and measure the corneal thickness. The Placido method involves projecting a series of concentric black and white rings onto the cornea and capturing their reflection by a camera placed in the center of the rings. This is the first Purkinje reflex. The power of the central 3 mm of the cornea is extrapolated, resulting in the Sim (simulated) K (keratometry) values. The rings are centered on the visual axis and not the pupil. The distance between the visual axis and the center of the pupil is the angle kappa (K). Algorithms calculate the power of the cornea at each point depending on the deformation of the mires. The Placido-based topography measures the refractive status of the cornea accurately but does not, however, reflect the true shape of the cornea.
Corneoscope and corneopter: This Placido-based device projects black and white mires on the cornea. Then a black and white or a color polaroid photograph is taken at 4.8 X magnification. The dioptric power, and the radius of curvature at a particular point, are read by comparing the amount of magnification needed to match the photographed rings to a standard set of rings on the comparator screen. The steeper the cornea, the closer the rings. Variations between the normal cornea, pseudocones, suture-induced astigmatism, and true keratoconus can be recognized.
The commonest causes of pseudocones are pellucid marginal degeneration, contact lens-induced irregular astigmatism, and corneal or limbal trauma. The corneal power is inversely related to the area which can be examined. A larger area of a flatter cornea can be examined compared to a steep cornea.
The slit scanning technology is based on measuring the dimensions of a slit scanning beam projected on the cornea. Orbscan initially was only a slit scanning technology to measure corneal thickness as well as curvature of the anterior and posterior surfaces of the cornea. Elevation information was acquired directly, and curvature information was derived from this. Orbscan II incorporated a Placido disc attachment to get curvature measurements directly. The Orbscan IIz is a further improvement because it incorporates the Shack-Hartmann aberrometer in the Zyoptix workstation.
The reflection of the illuminated Placido disc onto the cornea is stored, followed by the projection of 40 slits, 20 from the right and the left (each slit 12.5 mm high x 0.30 mm wide) at an angle of 45 degrees to the instrument axis. The backscattered images are captured by the device's video camera. Acquisition of images is followed by assessment of images and compensation for eye movements by a proprietary technique. Processing of the data is done by the machine to construct the anterior and posterior elevation maps and the curvature maps.
The Schiempflug technology uses a rotating camera to capture the reflection of a bright slit beam that sweeps across the cornea. A 3-D reconstruction of the anterior and posterior elevation is done, and the pachymetry is calculated. The Schiempflug principle is credited to Theodore Schiempflug, who devised a method to correct distortion in aerial photography. A single rotating camera and static camera are present in the Pentacam device.
The Schiempflug and the Placido-based technologies are combined in machines like the Sirius, TMS 5, and Galilei. The Galilei machine uses two rotating cameras (dual Schiempflug analyzer), whereas the pentacam has a single rotating camera.
Tomography vs. Topography
A topographer analyzes only the front surface of the cornea. Tomography is derived from the word "Tomos " (Greek for cut or section) and "graphein" (to write). A 3-dimensional image of the anterior and posterior surface of the cornea and corneal thickness is measured as opposed to topography, where only the anterior surface of the cornea is mapped.
Tear film analysis is possible with the Placido-based systems. Non-invasive tear film break-up time (NITBUT) can be measured for various purposes by Placido-based topographers or by Placido disc alone. Projecting rings on the corneal surface and measuring the time to break or distortion of the mires can effectively measure the tear film break-up time.
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