principal curvature for 3d images

I am trying to obtain the principal curvatures (K1,K2) for each voxels of a 3d image. These values are calculated based on Gaussian and mean curvatures . I have convolved my 3d image with a 3D Gaussian smoothing filter (sigma=0.5). Then I computed the first and second-order partial derivatives of the smoothed image. There are some zero values in the obtained results, so I have problems in doing the remain works based on the formulas mentioned in the paper. Any guidance/ help that I can get on this is much appreciated!

My reference paper is “Three-Dimensional Computer-Aided Diagnosis Scheme for Detection of Colonic Polyps" published in IEEE TRANSACTIONS ON MEDICAL IMAGING, VOL. 20, NO. 12, DECEMBER 2001.