CURVATURE BASED INSTRUMENTS
The normal corneal outer surface is smooth; corneal irregularities being 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 non-contact 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 reflection-based instrument (Fig. 1.1).
- The computerized videokeratoscope: it is a projection-based topographer consisting of a Placido disk (Fig. 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 pachymetric evaluation of the cornea is not possible. Of course, ultrasonic pachymetry can give us central and few paracentral measurements, but now we need full pachymetric 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 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. For full discussion of the curvature and elevation maps please refer to my book “Corneal Topography in Clinical Practice” published by Jaypee Brothers 2009.
Description of the Unit
The OCULUS Pentacam/Pentacam HR is a rotating Scheimpflug camera (Figs 1.3 and 1.4). The rotational measuring procedure generates Scheimpflug images in three dimensions, with the dot matrix fine-meshed in the center due to the rotation. It takes a maximum of 2 seconds to generate a complete image of the anterior eye segment. Any eye movement is detected by a second camera and corrected for in the process to some extent. The Pentacam calculates a 3-dimensional model of the anterior eye segment from as many as 25.000 (HR: 138.000) true elevation points.
The topography and pachymetry of the entire anterior and posterior surfaces of the cornea from limbus-to-limbus are calculated and depicted.
Fig. 1.3: The Pentacam system. Central slit light with lateral concentric rotating Scheimpflug camera.
The analysis of the anterior eye segment includes a calculation of the chamber angle, chamber volume and chamber height and a manual measuring function at any location in the anterior chamber of the eye. In a moveable virtual eye, images of the anterior and posterior surface of the cornea, the iris and the anterior and posterior surfaces of the lens are generated. The densitometry of the lens is automatically quantified.
The Scheimpflug images taken during the examination are digitalized in the main unit and all image data are transferred to the PC.
When the examination is finished, the PC calculates a 3D virtual model of the anterior eye segment, from which all additional information is derived.
Fig. 1.5: The image in the ordinary camera. The main disadvantage is limited depth of focus because the picture plane, the objective plane and the film plane are parallel.
Fig. 1.6: The Scheimpflug camera. Higher depth of focus, sharp image but distorted. The picture plane, the objective plane and the film plane cut each other in one line or one point of intersection.
To understand Scheimpflug principle, see Figs 1.5 and 1.6. Fig. 1.5 illustrates the image in the normal camera; notice that the three planes (The picture plane, the objective plane and the film plane) are parallel. Fig. 1.6 illustrates the Scheimpflug camera. The Scheimpflug law says: To get a higher depth of focus, move the three planes, provided that the picture plane, the objective plane and the film plane have to cut each other in one line or one point of intersection. The advantages of the Scheimpflug Camera are: higher depth of focus and sharp picture, but distorted.