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SM Lee, DG Hwang; Is The Thinnest Point Of Keratoconus Corneas Always At The Apex? . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3237.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To compare, in keratoconus corneas, the relative positions of three differently defined apical points: the point of maximum steepness as measured by the Placido disk-based Tomey TMS 2N, the elevation apex as determined by the B&L Orbscan II, and the point of maximum thinning as determined by the Orbscan II. Methods: Twenty eyes of 14 patients with keratoconus were evaluated using both the TMS 2N and Orbscan II units. The keratometric apex was defined as the point of steepest curvature as determined by the TMS 2N. The elevation apex was defined as the point of maximum elevation as determined by the Orbscan II anterior float map. The "pachymetric apex" was defined as the point of maximum thinning on the Orbscan II pachymetry map. For each eye, the position of each type of apex relative to the map center was measured. Results: Inferior displacement of the cone apex was most common, followed by lateral and superior cone positions. The location of the apex relative to the map center was 1.82 +/- 0.84 (mean +/- SD, in mm) for the TMS 2N-derived keratometric apex, 1.32 +/- 0.39 for the Orbscan II-derived elevation apex, and 1.20 +/- 0.40 for the Orbscan II-derived pachymetric apex. The mean distance between the elevation apex and pachmetric apex was 0.626 +/- 0.32 mm. In the subgroup of mild to moderate cones (sim K value < 50D, n=8), the discrepancy in location of these various apices was statistically significant (p < 0.001), with mean TMS apex at 2.41 +/- 0.64mm, elevation apex at 1.48 +/- 0.39mm, and pachymetric apex at 1.40+/- 0.32mm from center. By contrast, in the subgroup of more severe cones (any sim K value ≷ 50D, n=12), there was less discrepancy in the three apex locations (keratometric apex 1.34 +/- 0.66mm, elevation apex 1.20 +/- 0.34mm, and pachymetric apex 1.04 +/- 0.37mm from center.) Conclusion: In keratoconus corneas, the location of the point of maximum thinning frequently differs from the point of maximum elevation or steepening. The apex as defined by the TMS 2N was more inferiorly displaced than either the elevation apex or the thinnest point as determined by the Orbscan II. This may be an artifact created by the position dependence of Placido-based topographers, which can show inaccuracies in determining the topography of a surface with an eccentrically displaced apex. Because of these limitations, scanning slit systems such as the Orbscan II may be of particular value in assessing and monitoring the severity and progression of keratoconus.
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