Magnesium is a critical component of collagen formation and repair. Chronic magnesium deficiency can lead to defective collagen formation over time. Basically what this means is that when magnesium deficiency is present, it is believed that the old collagen cells that need to be replaced get replaced with unhealthy calcified cells rather than flexible new healthy cells. In the case of connective tissue such as joints, ligaments and tendons, this can lead to a susceptibility to joint problems, tendonitis and ligament issues, osteoarthritis, and other connective tissue dysfunction.
Senni, Karim, Alexandrine Foucault-Bertaud, and Gaston Godeau. "Magnesium and connective tissue." Magnesium research 16, no. 1 (2003): 70-74.