(a) Signal intensity (an index of stiffness) correlated significantly with the modified Mankin's score (plug site, open circles; corresponding site on the sham side, closed circles; n = 44, P = 0.0176, r = -0.36). (b) Signal duration (an index of surface irregularity) correlated significantly with the score (n = 44, P = 0.0269, r = 0.34). Signal intensity also correlated with the score categories (c) 'cartilage structure' (n = 44, P = 0.0343, r = -0.32) and (d) 'cartilage cells' (n = 44, P = 0.0499, r = - 0.30) of the modified Mankin's score. (e) When mean values of signal intensity of the plug site were calculated among each of the six groups, the mean was significantly and strongly correlated with the total modified Mankin's score (n = 6, P = 0.0130, r= -0.91). D0, day 0; 2-w, 2 weeks; 4-w, 4 weeks; 8-w, 8 weeks; 12-w, 12 weeks; 24-w, 24 weeks.