What arterial plaque actually is

The artery is lined by a thin active layer, the endothelium. Where blood flow is turbulent (branch points, curves) it becomes leaky and inflamed, and ApoB-carrying particles (LDL and its relatives) pass into the wall and get trapped — the 'response-to-retention' model, the initiating step. Trapped particles are oxidised, which the body reads as danger.

Oxidised particles recruit immune cells (monocytes) that become macrophages, which gorge on the trapped cholesterol and turn into foam cells — the earliest visible lesion (a 'fatty streak', present in many people by their 20s). Foam cells die, spilling a soft lipid/necrotic core; smooth-muscle cells lay a fibrous cap over it. The whole thing is self-amplifying: more particles in the blood means more retained, more inflammation, more plaque. Evidence ●●●●●

Soft (non-calcified) plaque is lipid-rich, inflamed, thin-capped — the young, dangerous, rupture-prone form (it shows as 'low-attenuation' plaque on a CT scan). Dense calcified plaque is older and largely a healing/stabilising signature — mineralised, quieter, less likely to rupture. A key nuance: spotty micro-calcification flags active instability, while large confluent dense calcium marks stability. This distinction runs through the whole imaging story.

The lesion that kills is usually not the tightest narrowing. It's the thin-cap fibroatheroma — a large soft lipid core under a fibrous cap thinner than 65 microns, riddled with inflammation. Under stress the cap tears, the core hits flowing blood, and a clot forms in minutes. Because these plaques are soft rather than bulky, they are often non-obstructive — they barely narrow the artery, so they look reassuring on an angiogram or stress test. Evidence ●●●●●

Roughly 60–70% of heart attacks arise from lesions narrowing the artery less than 50%. The landmark PROSPECT study found future events were predicted by total plaque burden, a small lumen, and a thin-cap fibroatheroma — not by the tightest stenosis. This is the single most important reframing in the field: how much plaque you have, and how soft it is, matters more than the 'percent narrowing' — and it's exactly why a calcium score of zero can miss risk (covered in the scans section).