Calcification

Calcification is the deposition of calcium salts in body tissues. In most cases the mineral is calcium phosphate in the form of hydroxyapatite. Calcification can be part of normal physiology or arise as a pathologic process.

Physiological calcification includes mineralization during bone and tooth formation. Endochondral ossification and dentin/enamel mineralization rely on controlled deposition of calcium salts, phosphate, and organic matrices.

Pathological calcification is often categorized as dystrophic or metastatic. Dystrophic calcification occurs in damaged or necrotic tissue with normal serum calcium and phosphate levels, and is commonly seen in atherosclerotic plaques, damaged heart valves, necrotic tumors, and damaged soft tissues. Metastatic calcification results from systemic disturbances of calcium–phosphate metabolism, leading to deposition in otherwise normal tissues. It is associated with hypercalcemia or hyperphosphatemia, as seen in hyperparathyroidism, vitamin D intoxication, sarcoidosis, or advanced kidney failure.

Mechanisms involve local supersaturation of calcium phosphate, nucleation on damaged matrices, and reduced activity of mineralization inhibitors such as extracellular pyrophosphate and fetuin-A. Tissue injury, aging, and chronic inflammation can promote ectopic calcification.

Clinical significance and imaging: calcifications often appear radiopaque on X-ray or CT and can be detected by ultrasound. They occur in arteries (vascular calcification), heart valves, kidneys (nephrolithiasis or nephrocalcinosis), and soft tissues such as tendons (calcific tendinopathy) or the brain (basal ganglia calcifications in certain disorders). Management focuses on treating the underlying metabolic disturbance or tissue damage and addressing complications.