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P. Eze, L. Szczotka-Flynn, A. M. Mahmoud, C. J. Roberts; Orbscan Measured Posterior Corneal Elevation as an Index to Screen for Keratoconus in a Normal Population. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3539.
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© ARVO (1962-2015); The Authors (2016-present)
Posterior corneal elevation is generally noted to be higher in keratoconus (KCN) when compared to normal corneas. It has been suggested to be the best performing single index on the Orbscan system for detecting KCN when the highest elevation on the posterior surface is greater than 50 microns above the best-fit sphere. There are many clinical circumstances in which presumed normal patients should be screened for KCN. For our purposes, we desired a tool to screen presumed normal subjects for a control cohort in a KCN genetic case-control study.
The posterior corneal elevation was measured using the Orbscan II system in 162 right eyes of presumed normal patients. These 162 subjects were enrolled in the LASH contact-lens study and all had clinically normal corneas, refractive error correctable to 20/20, astigmatism less than or equal to 1D and no clinical signs of keratoconus. The highest posterior elevation was recorded in two ways: 1) as reported by the instrument using original software (v. 3.00E) and 2) after reprocessing the slit images at Ohio State University using a beta version (v 3.12) edge tracker software and an algorithm which records the highest spot within the central 6mm zone of posterior elevation. The 50 micron single metric cutoff was applied to the originally processed data.
3 eyes had visibly erroneous maps and were not included in the analysis. In the remaining 159 eyes, the originally processed data revealed a mean posterior elevation of 35.4 +/-10.0 microns (range 8-61 microns). 8.2% (13/159 eyes) of these presumed normal eyes had posterior elevation data greater than 50 microns. In the reprocessed dataset, the mean posterior elevation was 17.8 +/- 4.8 microns (range 7-34.4 microns).
Differing Orbscan processing software yields significantly different posterior elevation results. When applying a suggested single metric cutoff using posterior corneal elevation, about 8% of presumed normal subjects are classified as KCN suspects and thus could not be used as part of a control cohort in a KCN study.
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