In the Injury Theory, it's damage to the arterial endothelium, followed by platelet activation, then smooth muscle cell migration to injury, then macrophages, with resulting "foam cells." Engorged foam cells burst, starting the injury cycle all over again
M. Lin, A. Abatan, and W. Zhang. Proc. SPIE Vol. 3671, p. 297-304, Smart Structures and Materials 1999: Smart Systems for Bridges, Structures, and Highways, Steve C. Liu; Ed., volume 3671 of Presented at the Society of Photo-Optical Instrumentation Engineers (SPIE) Conference, page 297-304. (May 1999)