Technical variability of cornea parameters derived from anterior segment OCT fitted with Fringe Zernike polynomials

Abstract

Background This study uses bootstrapping to evaluate the technical variability (in terms of model parameter variation) of Zernike corneal surface ft parameters based on Casia2 biometric data. Methods Using a dataset containing N = 6953 Casia2 biometric measurements from a cataractous population, a Fringe Zernike polynomial surface of radial degree 10 (36 components) was ftted to the height data. The ft error (height – reconstruction) was bootstrapped 100 times after normalisation. After reversal of normalisation, the bootstrapped ft errors were added to the reconstructed height, and characteristic surface parameters (fat/steep axis, radii, and asphericities in both axes) extracted. The median parameters refer to a robust surface representation for later estimates of elevation, whereas the SD of the 100 bootstraps refers to the variability of the surface ft. Results Bootstrapping gave median radius and asphericity values of 7.74/7.68 mm and −0.20/−0.24 for the corneal front surface in the fat/steep meridian and 6.52/6.37 mm and −0.22/−0.31 for the corneal back surface. The respective SD values for the 100 bootstraps were 0.0032/0.0028 mm and 0.0093/0.0082 for the front and 0.0126/0.0115 mm and 0.0366/0.0312 for the back surface. The uncertainties for the back surface are systematically larger as compared to the uncertainties of the front surface. Conclusion As measured with the Casia2 tomographer, the ft parameters for the corneal back surface exhibit a larger degree of variability compared with those for the front surface. Further studies are needed to show whether these uncertainties are representative for the situation where actual repeat measurements are possible.