Compared to ultrasound pachymetry, the topographical systems provide a thicker value of the corneal thickness, as the anatomical limits for measurements are different. Ĭorneal topographic assessment frequently involves the analysis of four maps: the pachymetric map, the anterior elevation map, the posterior elevation map and a map of corneal curvature. Modern capture and analysis systems that combine the Scheimpflug rotating camera, the Placido disk and the slit scanning, such as the Galilei® (Ziemer, Switzerland) and Sirius® (Schwind Eye-Tech-Solutions, Germany), were created to combine the advantages of topographers and tomographers. In contrast, Scheimpflug technology provides less information on possible distortion of the anterior corneal face compared to the Placido disc. The technology of corneal tomographers uses a Scheimpflug type rotating camera that allows the analysis of both anterior and posterior faces of the cornea, by direct measurements, not only by mathematical assessments as in the case of topographers. The term “tomography” is also derived from the Greek words “tomos” (section) and “graphein” (to write). Ĭorneal tomographers are new (third) generation devices, their technology allowing digital reconstruction of the structures of the anterior segment, without using a curvature system. Second-generation corneal tomographers, Orbscan® type (Bausch and Lomb, USA), assesses the corneal elevation through optical sections obtained by combining Placido disc and slit scanning technologies, thus being able to characterize the posterior face of the cornea. The Placido disc method is the most widely used. There are several generations of corneal topographers, according to the technology they use. First-generation corneal topographers are devices that project a system of concentric light circles (the Placido disk) onto the corneal surface, measure their angle of reflection and thus calculate the corneal curvature at various points, providing information on the corneal shape. In ophthalmology, the term “corneal topography” is improperly used in order to characterize the shape of the corneal surface and its radii of curvature. The term “topography” is derived from the Greek words “topos” (place) and “graphein” (to write) and refers to the study and representation of forms, being used especially in geography and astronomy. Current technologies of corneal topography
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