Western blot analysis with antibody specific to MMP-13. Each experimental group of MMP-13 protein was normalized against GAPDH and expressed as the percentage of the control group. *P<0.05 vs. control group; #P<0.05 vs. OVX group. Values are expressed as the mean ± standard deviation. OVX, ovariectomized; US, ultrasound; ERT, estrogen replacement therapy; MMP-13, matrix metalloproteinase-13; GAPDH, glyceraldehyde 3-phosphate dehydrogenase.