MMP9 and COVID-19: Notably, biomarkers such as serum procollagen type I N-propeptide (PINP) [47,48], serum total alkaline phosphatase (ALP) [17,49], osteocalcin [50,51], matrix metalloproteinases (MMP-3 and MMP-9) [52,53,54], and osteopontin (OPN) [55,56] have emerged as critical indicators reflecting the extent of COVID-19’s impact on skeletal and joint tissues.