Prediction of anterior ocular surface sagittal heights using Placido-based corneal topography in healthy eyes

To investigate the validity of Placido-based corneal topography parameters to predict corneoscleral sagittal heights measured by Fourier-based profilometry at various diameters.

Minimal (Min_sag ), maximal (Max_sag ) sagittal height, toricity (Max_sag  - Min_sag ) and axis of the flattest meridian (Min_sag ) of 36 subjects (mean age 25.4 SD ± 3.2 years; 21 female) were measured using the Eye Surface Profiler and analysed for diameters (chord length) of 8 to 16 mm (in 2-mm intervals). Furthermore, corneal central radii, corneal astigmatism, eccentricity and diameter were measured using the Keratograph 5 M.

Using multiple linear regression analysis, the best equation for predicting the sagittal heights for 8 mm (r²  = 0.95), and 10 mm (r²  = 0.93) diameters included corneal central radii and eccentricity. The best equation for predicting sagittal heights for 12 mm (r²  = 0.86), 14 mm (r²  = 0.78) and 16 mm (r²  = 0.65) diameters included corneal central radii, eccentricity and corneal diameter. Corneal astigmatism was significantly correlated with sagittal height toricity for 8 and 10 mm diameters (r²  = 0.50 and 0.29; p < 0.01), while no correlation was observed for 12, 14 and 16 mm diameters (p = 0.18 to p = 0.76). The axis of the flattest corneal meridian measured by Placido-based topography was significantly correlated with the axis of the flattest meridian measured by Fourier-based profilometry for 8, 10 and 12 mm diameters (r²  = 0.17 to 0.44; p < 0.05), while there was no correlation for 14 and 16 mm diameters (p = 0.48 and p = 0.75). For a typical soft contact lens diameter of 14 mm, 78% of the variance could be determined with a corneal topographer and 68% with keratometry and corneal diameter measurement.

The combination of corneal central radii, eccentricity and corneal diameter measured by Placido-based topography is a valid predictor of the corneoscleral sagittal height in healthy eyes. Scleral toricity and axis of the flattest meridian seem to be independent from Placido-based corneal parameters and requires additional measuring tools.