Posterior corneal shape: Comparison of height data from 3 corneal topographers

Purpose: To compare the ability of 3 clinical corneal topographers to describe the posterior corneal shape.

Setting: University Medical Center Groningen, the Netherlands.

Design: Prospective observational study.

Methods: Corneas of healthy participants were measured twice with a dual Scheimpflug instrument (Galilei G2), a scanning-slit system (Orbscan llz), and a single Scheimpflug instrument (Pentacam HR). Height data describing the posterior corneal shape were fit with Zernike polynomials. Mean values with standard deviations (SD), test-retest variability (coefficient of repeatability [CoR]), and interdevice variability were determined for the defocus Z(2,0), astigmatism Z(2,-2) and Z(2,2), and higher-order terms coma Z(3,-1) and Z(3,1), trefoil Z(3,-3) and Z(3,3), and spherical aberration Z(4,0) coefficients for 5.5 mm and 8.0 mm diameters.

Results: For the 5.5 mm diameter, CoRs ranged from 0.3 to 4.3 mu m with the dual Scheimpflug instrument, 1.6 to 5.2 mu m with the scanning-slit system, and 0.3 to 2.0 mu m with the single Scheimpflug instrument. The CoR was similar for the Scheimpflug instruments (P=.43) but poorer for the scanning-slit system (P

Conclusions: Repeatability in assessing the posterior corneal shape was generally good for the Scheimpflug instruments but poor for the scanning-slit system. Interdevice variability between the Scheimpflug instruments compromised the interchangeability of higher-order coefficients. For astigmatism, CoR and 95% limits of agreement of the Scheimpflug instruments typically corresponded to 0.1 diopter per astigmatism term. (C) 2017 ASCRS and ESCRS