Abstract
Purpose: :
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.
Methods: :
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).
Results: :
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).
Conclusions: :
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.
Keywords: keratoconus • computational modeling • cornea: basic science