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Repeatability of field average vs single point measurements of maximal corneal curvature in keratoconus using a dual Scheimpflug-placido topography system

Poster Details

First Author: S.Awwad LEBANON

Co Author(s):    C. Mehanna   R. Chalhoub   A. Salloum   M. Yehyia           

Abstract Details


To evaluate whether an average of several measured points around the maximal keratometric point (Kmax) would yield a better repeatability, making it a more reliable parameter in the assessment of keratoconus disease progression than Kmax.


American University of Beirut Medical Center


50 eyes with keratoconus were evaluated with 3 consecutive scans using the Galilei dual scheimpflug and placido tomographer. The data was extracted and single point Kmax was compared with an area centered around the Kmax, with variable diameters of 0.1 mm to 0.5 mm. The Intraclass coefficient of coefficient (ICC) and the intra-subject standard deviation (Sw) was calculated for each area of measurement.


The mean single point Kmax for the first, second and third measurements were 50.61 +/- 3.66, 50.55+/03.70, 50.44+/-3.63 D, respectively. The ICC and the Sw for Kmax, Kmax 0.1, Kmax 0.2, Kmax 0.3, Kmax 0.4, Kmax 0.5, were as follow: 0.962, 0.950, 0.930. 0.930, 0.930, and 0.928, respectively, and 0.711, 0.779, 0.888, 0.872, 0.844, 0.829, respectively.


Sampling and averaging more points around the Kmax did not improve the repeatability of the maximal keratometric area value. This might be due, in part, to the fact that Placido and Scheimpflug devices sample the cornea via a set of concentric circles. Averaging a fictional circle with a defined diameter leaves non-sampled areas that vary with the variable Kmax position. A matrix style of corneal representation, such as the one provided by optical coherence tomography, might fit better our proposed method.

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