Day to Day Clinically Relevant Corneal Elevation, Thickness, and Curvature Parameters Using the Orbscan II Scanning Slit Topographer and the Pentacam Scheimpflug Imaging Device

Abstract

The introduction of different techniques and computerized devices into clinical ophthalmology has significantly improved our knowledge of the eyes, optics, and eye conditions. Today, corneal topography is performed with a wide range of devices that implement a variety of techniques. Advance computerized analysis systems provide us with simple and quick evaluation procedures, yet the sophisticated data and clinical information that is generated can only be interpreted with adequate knowledge of the system itself as well as the accepted normal ranges of various properties assessed with these systems. Two computerized topography systems that are in common use are the Orbscan (Bausch and Lomb Inc., Rochester, NY, USA) and the Pentacam (Oculus GmBH, Wetzlar, Germany). The Orbscan is a slit-scanning device and the Pentacam is Scheimpflug imaging device. In this review, we present a brief description of both technologies, the techniques implemented in each device and the acquisition process with each. This will be followed by a list of corneal parameters that need to be assessed in screening patients for refractive surgery. We will discuss how these parameters are displayed, how each parameter may serve as clinic criteria, and how data should be interpreted. We will also try to provide evidence regarding the accuracy of different measurements, and the comparability of the two devices.

Keywords: Corneal Curvature; Corneal Elevation; Orbscan; Pentacam; Posterior Ectasia; Scheimpflug; Slit Scanning; Topography.

Figure 1

An Orbscan Quadmap of a normal right eye. The four maps, clockwise, from the top left include the anterior elevation, posterior elevation, pachymetry, and axial power. See text for more information

Figure 2

An Orbscan dual display of a keratoconic left eye. The anterior elevation map (left) is displayed on a 10 µm (0.010 mm) step scale and the posterior elevation map is on a 20 µm step scale. In both maps, there are more than 3 colors in the central zone; a sign indicative of keratoconus. Note that larger scale steps would increase the scale range and show abnormal areas with less alarming colors (See Figure 3).

Figure 3

The Orbscan quadmap of the keratoconic eye in figure 2. Both elevation (anterior, top right; posterior, top left) maps are set on a 5µm step scale, and the abnormally elevated parts in the center are mapped with red. Note the best fit sphere diameter/power (7.60 mm /44.4 D anterior, 6.07 mm /55.6 D posterior) and the high elevation readings at the point of the cursor (46 µm anterior, 73 µm posterior)

Figure 4

A dual posterior elevation display of a right eye before (left) and 6 weeks after (right) keratorefractive surgery for myopia. The scale step and range in these two maps are similar. Note the change in central zone posterior elevation which can be interpreted as posterior corneal bulging or a forward shift

Figure 5

A dual pachymetric display of a right eye before (left) and after (right) keratorefractive surgery for myopia. The preoperative central corneal thickness with the Orbscan was 506 µm, compared to 490 µm with an ultrasound pachymeter (overestimation). After surgery, the values were respectively 347 µm and 395 µm (underestimation)

Figure 6

An Orbscan quadmap of an astigmatic left eye. The four maps, clockwise, from the top left include the keratometric axial, tangential, optical, and mean power. In this same order, note the decrease in the axial bias and bow pattern

Figure 7

An Orbscan quadmap of the keratoconic eye in Figures 2 and 3. Note that the location of the highest point on the posterior corneal surface (top left) corresponds with the thinnest point on the pachymetry map (top right), and the steepest point of the cornea in the mean power map (bottom right). The keratometric axial power map (bottom left) shows an asymmetric bow tie, and fails to locate the cone accurately. Also note that the axial power map here is different from that in Figure 3, because it is based on Placido data

Figure 8

A Pentacam refractive 4-map of the same normal right eye in Figure 1. The four maps, clockwise, from the top left include the sagittal power, anterior (front) elevation, posterior (back) elevation, and pachymetry. See text for a more detailed description

Figure 9

A Pentacam dual display of the same right eye in Figure 4 on the same days before (left) and after (right) keratorefractive surgery for myopia. Note the similarity in posterior corneal elevation maps (top), and the change in corneal thickness maps (bottom) as a result of central ablation

Figure 10

A Pentacam 4 map of the same left keratoconic eye. Note that applying a best fit toric ellipsoid reference surface to the anterior (top left) and posterior (bottom left) elevation map eliminates the toric effect of corneal astigmatism and gives a better demonstration of the exact location of abnormally heightened areas

Figure 11

Two Pentacam single pachymetry maps of a single right eye. The actual corneal thickness map (left) demonstrates that the thinnest point is 549 µm thick, and only 5 µm thinner than the center of the cornea. The relative pachymetry map (right) shows that the abnormal area is located slightly more inferiorly, and is about 6.0% thinner than normal

Figure 12

A Pentacam Refractive display of the same keratoconic left eye in Figures 3 and 10. The top middle graph demonstrates the corneal thickness spatial profile. The pachymetry at the thinnest point is 457 µm and the increase in more peripheral areas (red curve) fails to move parallel to normal (black dotted curves). Compared to 4-map displays, additional information include the value of different indices provided in the central box, and the eccentricity values along major meridians (top right). According to values of different indices, keratoconus in this eye is classified as stage 3 (KC3)

Figure 13

A Pentacam Pachymetric display of the same normal right eye in Figures 1 and 8. The corneal thickness at the thinnest point is 534 µm and it increases in the periphery following the normal pattern. The bottom table gives corneal thickness values at different rings and its progression in percentages

Figure 14

A Pentacam topometric display of the same normal right eye in figures 1, 8, and 13. The top two maps show the sagittal curvature of the anterior (left) and posterior (right) corneal surfaces. The table in the middle gives eccentricity figures in different peripheral areas and degrees. Indices are shown in the middle box in the bottom. Note that none of the indices are highlighted, as they are all within the normal range