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Background The aim was to assess the intra-session, inter-session and inter-observer repeatability of curvature and aberrometric measurements of Pentacam-derived posterior corneal surface in normal control ( CG), keratoconic ( KCG) and after corneal collagen cross-linked ( CXLG) groups. Methods This was a prospective, clinic-based, observational study that was conducted at the Eye Institute of Thrace ( EIT) in Alexandroupolis, Greece. Sixty-two normal eyes formed the control group, 33 were in the keratoconic group, while 34 eyes, which already had received corneal collagen cross-linking formed the CXLG. Curvature and aberrometric measurements were obtained using the Pentacam by two operators within each of the two sessions. Precision (Pre), repeatability ( COR) and the intra-class correlation coefficient ( ICC) were calculated for evaluating intra-session and inter-session repeatability. Bland-Altman analysis was used for assessing inter-observer repeatability. Results Simulated keratometric parameters K1 and K2, asphericity at the central 8 mm zone (Q8) and central steepest radius of best fit toric ellipsoid reference body (Rs), achieved an acceptable level of accuracy. Aberrometric parameters were repeatable, reliable and reproducible only in terms of the root mean square ( RMS) errors (coma, coma-like and the RMS of higher order aberrations or HOA RMS). For both observers, best precision and lowest range of agreement ( ROA) was found for HOA RMS parameters in all groups (less than 0.27 and less than 0.29 for the control group, less than 0.95 and less than 0.72 for the keratoconic group and less than 0.67 and 0.53 for the collagen cross-linked group). Conclusion The Pentacam system provides precise measurements of corneal posterior keratometry, reference body radius and asphericity for large diameter analysis. Although Zernike coefficients for individuals presented poor repeatability, the coma, coma-like and HOA RMS seem to be accurate indices in characterisation of back corneal aberrations. [ABSTRACT FROM AUTHOR]

Purpose: To determine the variability in Scheimpflug image–derived posterior elevation measurements in eyes with keratoconus or corneal collagen crosslinking (CXL). Setting: Eye Institute of Thrace, Alexandroupolis, Greece. Design: Prospective observational study. Methods: Two operators in 2 sessions obtained posterior elevation measurements using the Pentacam Scheimpflug system. Variability in measurements was assessed using Bland-Altman analysis and intraclass correlation coefficients (ICCs). Measurements were associated with a series of Scheimpflug-derived keratoconus indices using the Pearson correlation and multivariate linear-regression analysis. Results: Mean interobserver differences for the central elevation point (e1) and the highest elevation point (HEP) in the keratoconus group were −0.9 μm (95% limits of agreement [LoA], 5.7 to −7.5 μm) and 1.7 μm (95% LoA, 26 to −22.6 μm), respectively. In the CXL group, the mean differences were 0.2 μm (95% LoA, 6.0 to −5.5 μm) and −6.1 μm (95% LoA, 16.8 to −29 μm), respectively. The ICC values in the keratoconus group were satisfactory for most elevation points; the ICC values were unsatisfactory for a series of points in the lower corneal hemisphere. The ICC values for points in both hemispheres were unsatisfactory in the CXL group; HEP had adequate reproducibility. The e1 and HEP parameters correlated significantly with most Scheimpflug-derived indices (P<.05) in the keratoconus group but not in the CXL group. Regression analysis indicated that the central keratoconus index was the primary determinant of the variability in keratoconus eyes. Conclusions: Scheimpflug-derived posterior elevation measurements had acceptable reproducibility in keratoconus eyes and post-CXL eyes. Less consistent measurements were obtained in the lower hemisphere. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. [Copyright &y& Elsevier]