CXCR4 and osteoarthritis: SDF-1 can interact with CXCR4-specific receptors on the surface of cartilage to activate the SDF-1/CXCR4 signaling pathway, which activates the extracellular signal-regulating enzyme (Erk) and related kinase (p38 mitogen-activated protein (MAP) kinase) signaling pathways, promoting the release of matrix metalloproteinases (MMP) from the cartilage matrix, which degrade the type II collagen and aggrecan substrates in the cartilage matrix, ultimately degenerating the articular cartilage and inducing osteoarthritis [18,19,20].