Extraskeletal (Ectopic) Calcification and Ossification

A significant number and variety of disorders cause extraskeletal deposition of calcium and phosphate (Table 1). In some, mineral is precipitated as amorphous calcium phosphate or as crystals of hydroxyapatite; in others, osseous tissue is formed. The pathogenesis of ectopic mineralization is generally attributed to one of three mechanisms (Table 1). First, a supranormal “calcium-phosphate solubility product” in extracellular fluid can cause metastatic calcification. Second, mineral may be deposited as dystrophic calcification into metabolically impaired or dead tissue despite normal serum levels of calcium and phosphate. Third, ectopic ossification (or true bone formation) occurs in a few disorders for which the pathogenesis is becoming increasingly understood. Discussed briefly in this introduction are these three mechanisms for extraskeletal calcification or ossification. Subsequently, there follows a description of disorders that illustrate each pathogenesis.