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Confidence intervals for assessing ectatic changes post cross-linking using the ABCD grading system

Poster Details

First Author: H. Jang USA

Co Author(s):    M. Belin   M. Borgstrom   F. Hafezi   E. Flockerzi   B. Seitz        

Abstract Details


The ABCD progression display (Pentacam, Oculus GmbH, Wetzlar, Germany) monitors for ectasia progression and demonstrates statistically significant changes by displaying 80% and 95% one-sided confidence intervals (CI) of each measured parameter. The parameters are anterior (A) and posterior (B) radii of curvature taken from a 3.0 mm optical zone centered on the thinnest point and thinnest pachymetry reading (C). Currently, CI after corneal cross-linking (CXL) is not displayed, as measurement noise post-CXL has not been established. Our study aimed to measure noise of post-CXL eyes with a minimal follow-up of one year and provide CI for this group.


Pentacam measurements of post-CXL eyes were taken at ELZA Institute (Zurich, Switzerland) and Homburg Keratoconus Center (Homburg, Germany). Statistical analyses were performed at the University of Arizona, Tucson, USA.


Patients from ELZA Institute (Zurich, Switzerland) and Homburg Keratoconus Center (Homburg, Germany) with a minimum of 12 months post-CXL were enrolled. Three separate Pentacam measurements were taken for each eye, removing the patient from the Pentacam device between each measurement. A minimum 7.5 mm of coverage and an acceptable quality score were required. Pooled variance, standard deviations (SD), and one-sided CI were computed. Site specific and time specific comparisons were made using nonparametric statistics.


60 eyes (38 Zurich, 22 Homburg) of patients aged 29.5 ᄆ 12.3 (range 11-62) were enrolled. Patients were 26.1 ᄆ 19.3 months (range 12-28 Zurich, 16-115 Homburg) post-CXL. The values of ABC parameters and their respective noise measured as SD were all comparable between sites. A subgroup analysis of eyes from Homburg comparing 16-35 months to 38-115 months post-CXL showed no difference in SD. There was no correlation between time post-CXL and SD in all parameters. The 80% CI of ABC parameters were 0.0283, 0.0492, and 3.682, respectively. The 95% CI were 0.0553, 0.0962, and 7.196, respectively.


Measurement noise of each parameter on the ABCD progression display was analyzed in eyes post-CXL with at least 12-month follow-up period. Noise after 12 months was not correlated with time post-CXL in this group. CI of each measured parameter were computed as above and will be incorporated into the next iteration of the Belin ABCD progression display.

Financial Disclosure:

receives consulting fees, retainer, or contract payments from a company producing, developing or supplying the product or procedure presented

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