Calcium-containing crystals, such as calcium pyrophosphate dihydrate and ba
sic calcium phosphate, have profound in vitro biologic effects. They promot
e synovial hyperplasia and synthesis of metalloproteases able to degrade ca
rtilage matrix. Clinical observations support the thesis that these in vitr
o effects also occur in vivo. Prospects for successful interventions in the
crystal deposition diseases depend upon better understanding of the mechan
isms of crystal formation and activation pathways that lead to their biolog
ic effects.