What is the relation between crystals and osteoarthritis?

The nature of the relation between osteoarthritis and the various forms of calcium crystals that are found within osteoarthritic joints continues to challenge and confound researchers. The most basic question is whether such crystals are directly relevant to the development of osteoarthritis, or are merely a byproduct or marker of the disease itself. The past year has produced several studies that elucidate important aspects of the molecular and cellular mechanisms of calcium pyrophosphate dihydrate and apatite crystal formation. Such studies may yield novel targets for therapeutic intervention in crystal-associated osteoarthritis. Other recent studies have provided further understanding of the mechanisms by which crystals induce inflammation. Arthroscopic assessment of patients with knee osteoarthritis refractory to traditional therapy suggests that the combined absence of chondrocalcinosis on plain films and identifiable crystals on compensated polarized light microscopy of synovial fluid from arthrocentesis may not be adequate to exclude clinically relevant crystalline deposition and inflammation. Clinical criteria are needed to identify patients with occult crystalline disease who, by virtue of crystal-induced inflammation, require more aggressive anti-inflammatory therapy than those with noninflammatory osteoarthritis.