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Working Principle
Placido based devices
Placido disc based devices – These work on the Reflection principle. The anterior surface of the cornea acts like a convex mirror and hence the size of the image formed by it is determined by its curvature. A steeply curved cornea will produce a smaller image, while a flatter cornea will produce a larger image – of the same object positioned at the same distance from the cornea. These devices thus, measure the slope of the cornea and compute the curvature. These work true to the principle of the Placido disc. They have different projection devices that use lighted circular rings of varying sizes and numbers. These rings are reflected by the convex patient cornea and through an opening in the center of the target, the images are obtained using an acquisition camera. The images are digitized, after allowing operator verification, and proprietary algorithms are used to determine the radius of curvature of the innermost ring. Once this is determined, the distance of the next reflected ring from the first ring is used to determine the curvature of this ring an so on – until the most peripheral ring is reached.
Usually these values are determined for a finite number of points on the circle represented by each ring. Thus, the cornea between rings is not imaged and there is no actual data for these points. Thus, the apex of the cornea which is inside the innermost ring, is also not visualized or measured. Since reflection of images is a tear film based property of the cornea, in eyes which have irregularities of the corneal surface – or other tear film problems, distortion of the reflected images results in inaccurate estimation of corneal curvature.

Once the curvature of the points is determined, the corneal dioptric power at that point is determined using the formula -
Dioptric power of the Cornea = refractive index of cornea – refractive index of air / radius of curvature in meters
= (1.376 – 1.0) / 7.6 x 10-3
= 49.45D

However, since the measured radius of curvature is of the anterior corneal surface, the conversion provides the power of the anterior corneal surface. Since there is negative refraction when light passes into the eye, through the posterior corneal surface – this is accounted for in most  keratometers, by using an effective corneal refractive index instead of the true refractive index. Although this effective index can vary in different instruments, the commonly used value is 1.3375. Hence in the above example, the dioptric power of the cornea – after conversion, would be
= (1.3375 – 1.0) / 7.6 x 10-3
= 44.4D

This inability to measure the true power of the posterior corneal surface remains a weakness of Placido based systems. With the increasing use of corneal refractive procedures, the importance of this measurement has gained increasing acceptance and has led to the development of newer instrumentation that can be used for this purpose. These instruments include the Orbscan – based on the slit scanning principle, in which an edge point in the corneal surface is triangulated by mathematically intersecting the diffuse reflected camera edge ray with the calibrated slit-beam surface.

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Advantages of this approach include direct measurement of corneal elevation (without conversion from obtained curvature values), and the ability to measure convex surfaces (which can often defy the algorithms used in Placido based systems).Since the local elevation features on a corneal surface are typically very small relative to the much larger curvature of the cornea itself, the corneal global curvature must first be removed. This is done by measuring elevation relative to some close-fitting reference surface, a process that unfortunately, also induces some distortion. In order to obtain this comparison surface, a best-fit sphere is obtained. This minimizes the square difference between the two surfaces, but this is done only within a specified zone –known as the fit-zone. There are a variety of surfaces that are possible, like the ellipse, but in general, a sphere is used to match the entire corneal surface. Such surfaces can also be floating, in which the center location is unconstrained, or axial or pinned. In general, the floating option is used as it has the least error.

The anterior surface of the cornea initially was calculated in this manner; however, since the calculation from the reflected images used by corneal topography is more precise, the current version of Orbscan is using the latter method and is a combination of reflective corneal topography and optical slit design.
The Pentacam and Scheimpflug photography - The Scheimpflug principle is a geometric rule that describes the orientation of the plane of focus of an optical system (such as a camera) when the lens plane is not parallel to the image plane. In this scenario, an oblique tangent can be drawn from the image, object and lens planes, and the point of intersection is the Scheimpflug intersection, where the image is in best focus. The principle is named after Austrian army Captain Theodor Scheimpflug, who used it in devising a systematic method and apparatus for correcting perspective distortion in aerial photographs.

With a rotating Scheimpflug camera, the Pentacam can obtain 50 Scheimpflug images in less than 2 seconds. Each image has 500 true elevation points for a total of 25,000 true elevation points for the surface of the cornea. The Pentacam actually has 2 cameras. One is for detection and measurement of pupil, which helps with orientation and fixation. The second camera is used for visualization of the anterior segment. The Pentacam is able to image the cornea such that it can visualize anterior and posterior surface topography, including curvature, tangential, and axial maps.

Advantages of the Pentacam include the following: (1) high resolution of the entire cornea, including the center of the cornea; (2) ability to measure corneas with severe irregularities, such as keratoconus, that may not be amenable to Placido imaging; and (3) ability to calculate pachymetry from limbus to limbus. The Pentacam can also provide corneal wavefront analysis to detect higher-order aberrations.

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Problems with the Pentacam include eye movement during the 2 second measurement process, although this is unlikely to be a large amount. However, calculation of corneal power from elevation measurements has several limitations. Hence, the ultimate solution may be to use both a Placido based image analysis for corneal power requirements and to interpret these in light of data above corneal elevation from devices like the Pentacam – for both the anterior and posterior corneal surfaces. Although the instruments described are the prototype devices for the measurement principles propounded, and the ones most in use today, other devices exist.

These include a variety of instruments using the Placido principle

The use of elevation data to represent the corneal elevation data in comparison to a reference surface results in points being labelled as higher or lower than the reference plane. The point higher are depicted on color-coded maps as red, while those that drop below the reference surface are shown in blue. This can lead to confusions when comparing with a Placido based corneal power map, because the areas with a steeper curvature or higher dioptric power are shown in red, while flatter curvatures and lesser powers are shown in blue.

The systems that measure the anterior and posterior corneal surfaces also allow computing of corneal pachymetry as differences between the two surface elevations, and also provide a host of other information including limbus to limbus distance measurement, and anterior chamber and in Scheimpflug devices, lens densitometry data that are not available with Placido systems.

CLINICAL APPLICATIONS

1. Corneal Ectasia
a. Early diagnosis of various corneal ectasias like keratoconus, pellucid marginal degeneration
b. Monitor progress of the condition over time
c. Use data to study efficacy of treatment options like collagen cross linking

2. Corneal diseases
a. To study the corneal changes in various conditions like Terrien’s marginal degeneration, epithelial dystrophies, Salzmann’s nodular degeneration, and pterygium
b. Use topography to monitor the efficacy of surgical approaches used to manage the above corneal conditions, such as lamellar keratoplasty, excimer PTK, superficial keratectomy, or pterygium surgery.

3. Contact lens fitting
a. Some of these devices offer contact lens fitting modules - these programs allow the operator to choose a contact lens based on the corneal curvature and the software will then allow the simulation of a fluorescein fitting strategy. Based on the pattern the lens can be altered and this simulation usually allows the chair time during actual contact lens fitting to be reduced significantly.

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