1.4Specular Microscopy

As seen with confocal microscopy, specular microscopy of KC shows signs of altered endothelium cell morphology. There is a significant increase in polymegathism compared with normal controls and a significant decrease in hexagonality in the keratoconic cornea. Higher pleomorphism is seen in KC.

1.5Corneal Topography

1.5.1Instruments Measuring Corneal Surface

1.5.1.1 Curvature-Based Instruments

The normal corneal outer surface is smooth; corneal irregularities are neutralized by the tear film layer. The anterior surface acts as an almost transparent convex

mirror; it reflects part of the incident light. Many instruments have been developed to assess the anterior surface by measuring the reflected light. These noncontact instruments use light target (in different shapes) and a microscope or other optical systems. The instruments are either quantitative or qualitative, and either reflection-based or projection-based. These instruments are as follows:

The keratometer: It is a quantitative reflection-based instrument.

The photokeratoscope: It is a qualitative reflectionbased instrument.

The computerized videokeratoscope: It is a projec- tion-based topographer consisting of a Placido disk with a central camera (Fig. 1.21).

1.5.1.2 Elevation Based Topographers

Placido-based (or curvature-based) systems rely on the data collected from the anterior surface of the cornea either with reflection-based or projection-based systems. Additionally, without the information about the posterior surface, complete evaluation of corneal pachymetry is not possible. Of course, ultrasonic pachymetry can give central and few paracentral measurements, but now we need full pachymetry map. Moreover, the posterior surface of the cornea is being more appreciated as a sensitive indicator of corneal ectasia and can often be abnormal in spite of a normal anterior corneal surface. It is now recognized that while the refractive power of the cornea is mostly

Fig. 1.21 Curvature-based topographer. The computerized videokeratoscopy is a projection-based topographer consisting of a Placido disk with a central camera. The curvature-based topographer evaluates the anterior corneal surface

determined by the anterior surface, the biomechanical behavior of the cornea is at least equally determined by both surfaces.

On the other hand, in the curvature-based systems, the elevation map of the anterior surface is derived from the curvature map, while it is directly calculated in the elevation-based systems.