Multivariate Analysis of the Spatial Distance Between the Corneal Thinnest Point and Maximum Keratometry in Keratoconic Corneas: Association with Disease Severity.

Background

This study quantified the planar distance (D, mm) between the corneal thinnest point (TP) and maximum keratometry (Kmax) and examined its association with tomographic features and keratoconus severity.

Methods

Tomographic data from 140 patients (280 eyes) with bilateral manifest keratoconus were obtained with a Scheimpflug camera (Pentacam HR; Oculus, Wetzlar, Germany). D was calculated from the Cartesian coordinates of TP and Kmax. A multiple linear regression model included simulated anterior average keratometry (SimKavg), anterior and posterior radii of curvature (ARC and PRC), and anterior Q-value (Q-ant). Inference used patient-clustered robust standard errors. Internal validation comprised repeated grouped 10-fold cross-validation and patient-level bootstrap resampling. Performance was assessed using R2, root-mean-square error (RMSE), mean absolute error (MAE), and Bland-Altman limits of agreement.

Results

The model explained 51.8% of the apparent variance in D (adjusted R2 = 0.511; p < 0.001). Apparent RMSE and MAE were 0.568 and 0.428 mm. Mean grouped cross-validated performance was R2 = 0.472, RMSE = 0.594 mm, and MAE = 0.443 mm. The out-of-fold bias was -0.005 mm, with 95% limits of agreement from -1.162 to +1.151 mm. ARC, SimKavg, and Q-ant remained significant with patient-clustered inference, whereas PRC did not (p = 0.111).

Conclusions

D was associated with keratoconus severity and could be estimated with moderate internal performance from routinely available tomographic variables. Collinearity, fellow-eye dependence, and limits of agreement of approximately ±1.1 mm restrict interpretation of individual coefficients and preclude interchangeability with direct measurement. External and longitudinal validation is required before clinical use.