![]() ![]() The second approach is to look at the values corresponding to the thinnest location (TL). #Images of pellucid marginal degeneration plusThe first is to look at the highest plus value within the central 5-mm zone abnormal values are greater than 12 μm and greater than 15 μm on the anterior and posterior elevation maps, respectively. There are two methods to study the values. The parameters of anterior and posterior corneal elevation maps should be studied using the best-fit toric ellipsoid (BFTE) reference surface. The normal shape is the hourglass (Figure 3) abnormal shapes include irregular, tongue-like extensions and isolated islands (Figure 4). The shape should be studied using the best-fit sphere (BFS) reference surface. The superior point may rarely have a higher value than the inferior one in this case, the difference should be less than 2.50 D. Normally, the inferior point has a higher value than the superior one, and the difference between the inferior and superior should be less than 1.50 D. Note the two opposing points on the 5-mm central circle on the steep axis in Figure 1. There are also abnormal shapes that the clinician should learn to recognize (Figure 2). In this Figure, the two segments S and I are equal in size, and their axes are aligned. The normal shape seen on this map is the symmetric bow tie (Figure 1). In each map, both shape and parameters should be studied, as described below. ![]() The four most important tomographic maps are the anterior sagittal curvature, anterior and posterior corneal elevation, and pachymetry maps. Despite these capabilities, corneal tomography must be complemented by other investigations. Corneal tomography includes not only these topographic maps, but also corneal pachymetry and other maps and profiles of both corneal surfaces.Ĭorneal tomography, the most important screening test for refractive surgery, can be used to detect abnormalities, diagnose and classify early cases of ectatic corneal diseases, diagnose postkeratorefractive ectasia, and help determine which refractive procedure is best for a given patient. Corneal topography is an older term, now applied to the maps produced by Placido– disc-based machines and consisting of anterior sagittal (axial) and anterior tangential (instantaneous) curvature maps. TOPOGRAPHY VERSUS TOMOGRAPHYĬorneal tomography is a relatively new term used to describe the maps and images generated by Scheimpflugbased imaging devices. Consequently, our understanding of refractive surgery has advanced and many new concepts have been introduced, resulting in the need for a systematic approach to the clinical interpretation of corneal imaging. Since the introduction of topography to ophthalmology, corneal and anterior segment imaging has been further advanced through technological developments including tomography, anterior segment optical coherence tomography (AS-OCT), and the newest devices measuring corneal biomechanics. Skillful interpretation of corneal and anterior segment imaging is key to successful refractive surgery. What Determines the Success of a LASIK Practice? Skillful Interpretation of Corneal Imaging #Images of pellucid marginal degeneration fullIncreasing Endocapsular Support in Eyes With Localized Zonular WeaknessĪlmost Full Circle: The CCC and Its ComplicationsĬataract Removal in a Glaucomatous Eye With a Small Pupil The Physiologic Mechanism of AccommodationĬataract Surgery in Eyes With Nonprogressive Keratoconus ![]()
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