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A. M. Mahmoud, M. D. Twa, J. S. Pepose, M. A. Qazi, P. Kollbaum, C. J. Roberts; Influence of Region of Interest and Area Analyzed on the Calculation of the Highest Elevation Above the Posterior Best Fit Sphere. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1019. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate how changes in the region of interest (ROI) of the Orbscan II posterior corneal surface best fit sphere (pBFS) affect calculation and repeatability of the magnitude of the highest elevation above the pBFS (PE) in normal and keratoconic corneas.
Orbscan data from 19 normal and 18 keratoconic eyes were retrospectively identified. Three scans of each eye had been acquired. The data were processed with The Ohio State Corneal Topography Tool. The pBFS and PE were determined for each fit using the following ROI’s for the sphere-fit calculation: 6.0, 7.0, 8.0, 9.0 and 10.0 mm diameters. The PE was determined using an automated search routine over the central 2.5 mm of the posterior surface, to locate the 0.5 mm diameter zone with the highest elevation. The actual area available for analysis (AA) for the pBFS calculations for ROIs 6.0 to 9.0 mm were determined. Repeated multi-variate analysis of variance was used to compare the PE for repeated scans and different ROIs. Also, the PE for keratonic and normal eyes were compared. Lastly, the average AA was calculated for each ROI.
The repeated measures analysis showed no difference (p = 0.45) in PE by scan for each ROI for both groups. There was a significant difference by ROI. (p < 0.0001) in PE for both groups. The mean PEs in normal eyes were 0.008, 0.011, 0.016, 0.021 and 0.023 mm above the pBFS at 6.0, 7.0, 8.0, 9.0 and 10.0 mm diameter ROI, respectively. For keratoconus, the mean PEs were 0.045, 0.056, 0.066, 0.074, and 0.076 mm as ROI diameter increased from 6.0 to 10.0 mm diameter ROI, respectively. For all ROI, PE values in keratoconics were statistically different from normal eyes (p < 0.0001). The mean AAs in normal eyes were 100%, 100%, 99.5%, and 94.6% at 6.0 to 9.0 mm diameter ROI, respectively. The mean AAs in keratoconic eyes were 98.6%, 97.9%, 95.1%, and 86.6% at 6.0 to 9.0 mm diameter ROI, respectively.
Attention to the zone of analysis and corneal coverage is critical in describing the elevation of the posterior cornea. The default ROI is often large, where coverage may be an issue, which could potentially produce an artificially low PE. Standardization of the highest posterior elevation above the pBFS calculation and minimum AA are recommended prior to attempting to differentiate between normal and diseased eyes based upon this metric.
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