For example, distorted or wavy mires suggest surface irregularities, such as those caused by surface dryness (Figure 2B). Minor, insignificant variations are present in every cornea but may not be detectable on qualitative inspection of a Placido disc image. Distinct, well-formed mires are referred to as "crisp" and suggest the underlying corneal surface to be regular and smooth.Īs an ideal sphere, the cornea should have mires equally spaced over its entire surface (Figure 2A). The mires can also give information regarding the quality of the surface. Areas where the mires are more widely spaced are flatter. Similar to a topographic map of a mountain, areas where the mires appear closer together correspond to steeper corneal curvature. The projected concentric rings are referred to as "mires." The shape of the corneal surface can be inferred by inspection of these mires. Placido disc images can be interpreted both qualitatively and quantitatively. This method corrects for the non-planar shape of the cornea and, thus, allows greater accuracy and resolution in creating a 3-D map of the cornea (2-3). A third technique, known as Scheimpflug imaging, uses a rotating camera to photograph corneal cross-sections illuminated by slit beams at different angles (e.g., Pentacam). This method uses rapidly scanning projected slit beams of light and a camera to capture the reflected beams to create a map of the anterior and posterior corneal surface. Many topography machines rings (e.g., Atlas, NIDEK OPD-Scan) still utilize Placido discs but take the technique a step further, providing a computerized quantitative assessment of the corneal surface to provide more detailed information than one can appreciate by simply looking at the reflected.Ī second technique for corneal topographic assessment is the scanning slit technique (e.g., Orbscan). The posterior corneal surface cannot be characterized using Placido disc technology or a hand-held keratoscope. Similarly, hand-held keratoscopes (Figure 1B-C) are practical instruments that display concentric rings at the slit lamp for quick assessment of topographical changes (e.g., astigmatism induced by sutures). The pattern of light reflection reveals the shape of the anterior surface of the cornea (1). As the image from the Placido disc is projected on the cornea, some of the light is reflected off the tear film-air interface like a mirror. This technique characterizes the corneal surface by assessing the reflection of a set of concentric rings off the anterior corneal surface. The first advancement in assessing the shape of the anterior corneal surface was made in the late 1800s with the development of the Placido disc (Figure 1A) (1-2). Advances in digital photography and computer processing have vastly increased the utility of corneal topography (1). Devices now are able to characterize both the anterior and posterior corneal surfaces, creating a three dimensional map. Originally, corneal topography was only used to describe the anterior surface of the cornea. Corneal Topography and Tomography Basic PrinciplesĬorneal topography is used to characterize the shape of the cornea, similar to how one would characterize a mountain using a topographic map.
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