MMP9 and osteonecrosis: MMP-9-deficient mice showed severe defects in skeletal development and diminished angiogenesis, thus leading to reduced restoration of microvascular perfusion capacity in response to ischemia [38]; moreover, MMP-9 mRNA levels were lower in a mouse model of glucocorticoid-induced osteonecrosis compared with controls [39], while its upregulation (e.g., after treatment with exosomes released by bone mesenchymal stem cells in steroid-induced femoral head necrosis) promoted osteogenesis and improved osteonecrosis [40].