MMP-13 expression in the PCL-rupture side and the control side. A. In the control side, a very small number of MMP-13 positive-staining cells were observed. B. On 4 weeks after PCL-rupture, a small number of MMP-13 positive-staining cells were observed. C. On 8 weeks after PCL-rupture, a small number of MMP-13 positive-staining cells were observed. D. On 12 weeks after PCL-rupture, more MMP-13 positive-staining cells were observed with strongly-staining image. E. On 16 weeks after PCL-rupture, strongly-staining image was observed. F. On 24 weeks after PCL-rupture, MMP-13 positive-staining cells decreased.