ABSTRACT
Calcium pyrophosphate dihydrate (CPPD) crystals form extracellularly, most
commonly in cartilage matrix. The essential anionic component of CPPD crystals
is inorganic pyrophosphate (PPi). Each unit cell of crystalline CPPD contains one
pyrophosphate (P2O7) for every 2 calciums and 2 waters of hydration, yielding
a structural formula of Ca2P2O7$2H20. Theoretically, perturbations of matrix or elevations of either calcium or PPi concentrations within cartilage could favor
CPPD crystal formation and growth. Changes in organic matrix components
and in PPi have been most prominently implicated in the pathogenesis of
CPPD formation, while there is little support for elevations of ionized calcium as
a predisposing factor. Subsequent discussion will focus on PPi metabolism and
matrix factors that may influence crystal formation.
