Stromal and epithelial surface smoothness in normal and post-PRK corneas in vivo by very high-frequency ultrasound

Purpose; Degradation of stromal surface smoothness produced by PRK results in a diffractivc surface given different refractive indeces for epithelium and stroma. Very high-frequency ultrasound (VHFU) has been used to map surface height of epithelial and stromal layers. We used this method to quantify surface smoothness in vivo, for stromal and epithelial surfaces in normal and post-PRK corneas. Methods: VHFU (50 MHz) scanning was performed in ten normal and ten post-PRK corneas (post-op > 6 months). Mean lateral point resolution was 180 microns. Surface height reproducibility was ±1.5 microns. Two dimensional plots of epithelial and stromal surface height using a standardized gray-scale were analyzed using a 2D Fast Fourier Transform (FFT). A power spectrum profile was derived from each. The Hurst Exponent (H), a quantitative statistic describing surface smoothness, was calculated from the power spectra. Results; For normals, the stromal (Bowman's) surface was found to be smoother than the epithelial surface (tissue surface, not tear film) with H(stroma) - 0.629 vs. H{epithelium) = 0.623. After PRK, both epithelial and stromal surfaces became rougher although the epithelium in these cases was performing a 'smoothing" function in relation to the stromal surface with H(stroma) 0.587 and H(epithelium) - 0.592. There was a statistically significant decrease in slromal surface smoothness (p=0.0355) and a trend describing a reduction in epithelial surface smoothness (p=0.062) after PRK. The standard deviation of smoothness (H) was greater for normals than PRK corneas. Conclusions: This method demonstrated expected changes in stromal surface smoothness after PRK. However, we discovered a possible reduction in epithelial surface smoothness after PRK. and found that Bowman's layer is smoother than the epithelial surface in the normal state.