SPP1 and neoplasm: The binding of OPN with integrins initiates the activation of several downstream signaling effectors, such as phosphatidylinositol 3 kinase (PI3K) /protein kinase B (AKT), focal adhesion kinase (FAK)/AKT and nuclear factor kappa-B (NF-κB), leading to cell proliferation, migration, epithelial–mesenchymal transition (EMT), inflammation, neurotoxic microglial phenotype, tumor growth, migration and invasion, as well as angiogenesis within the chronic subdural hematoma (CSDH) outer membrane [15,16,17,18,19,20].