Change in alkaline phosphatase (ALP) activity induced by cyclic mechanical stress. (A) Quantitative analysis of ALP activity in non-thoracic ossification of ligament flavum (TOLF) and single- and multiple-level TOLF (data are the means ± standard deviation; n=6 patients/group). *P<0.05, ***P<0.001 vs. non-TOLF at the same time point. ###P<0.001 vs. single-level TOLF at the same time point. (B and D) Images of ALP activity staining: (B) gross observation, (C) light microscopy, and (D) with nuclear counterstaining (×40 magnification for panels C; and ×100 magnification for panels D). (E and F) Semi-quantitative analysis of the area of (E) ALP staining and (F) stained cells (data are the means ± standard deviation; n=10 micrographs/group). **P<0.01, ***P<0.001 vs. non-TOLF at the same time point. ##P<0.001, ###P<0.001 vs. single-level TOLF at the same time point.