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K. C. Baker, Y.-L. Chen, L. Shi, J. W. L. Lewis, L. Kugler, M. Wang; Does the Posterior Corneal Elevation Provide the First Indication of Keratoconus?. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4963. doi: https://doi.org/.
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The purpose of this study is to examine optical characteristics of keratoconus (KC) and forme fruste keratoconus (FFKC), and investigate whether an irregular shaped posterior cornea is a better indicator than the anterior surface that is often used in KC screening.
We included Pentacam data of 18 KC and 22 astigmatic eyes in this study. Within the KC eyes, 8 are mild with manifest best corrected visual acuity (BCVA) of 20/25 or better. The FFKC diagnosis relied on the other eye's condition from the same subject, other KC indications, and family history. The remaining KC eyes are comprised of 5 moderate with a BCVA of 20/30-20/40, and 5 that are worse than 20/40. The 22 astigmatic eyes all had a BCVA of 20/25 or better. The posterior and anterior elevation maps are decomposed into Zernike polynomials to obtain the root mean squared error (RMSE) above the 2nd order that represents high-order complex patterns. The difference between the 2 RMSE values is used as a third indicator. The extreme RMSE values of the astigmatic eyes are used as the screening criteria (i.e. 100% specificity).
The 7mm diameter cornea decomposition found that the anterior RMSE missed 7 of the 18 KC eyes (61% sensitivity); 5 in the FFKC, 1 in moderate, and 1 in advanced. Both posterior and the RMSE difference missed 4 of the 18 (78% sensitivity); 3 FFKC and 1 moderate KC. The 6mm result shows that the anterior missed 7 of the 18 (61% sensitivity); 5 FFKC, 1 moderate, and 1 advanced. The posterior and the RMSE difference missed 6 of the 18 (67% sensitivity); 5 FFKC and 1 moderate KC. The 5mm result found that the anterior missed 8 of the 18 (56% sensitivity); 5 FFKC, 2 moderate, and 1 advanced. The posterior missed 6 of the 18 eyes (67% sensitivity) and the RMSE difference missed 5 of the 18 (72% sensitivity).
1. The posterior cornea surface alone provides more significant indication of keratoconus than the anterior surface. 2. Analysis with a larger cornea area improves detection sensitivity. 3. Either anterior or posterior cornea irregularity is a sufficient indicator of early KC, but sometimes additional information is required to clinically diagnose FFKC.
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