Synovial inflammation, crystals, and osteoarthritis.

Inflammation is now generally accepted as a component in the course of most patients with symptomatic osteoarthritis (OA), although the stage at which it occurs and how it affects the course has not been resolved. When joint effusions are present in OA they almost always contain more leukocytes than normal synovial fluid, although often still falling into what was considered a noninflammatory range. Multiple factors can contribute to this low grade inflammation, with crystals being only one of many such factors needing consideration. Crystals of calcium pyrophosphate dihydrate (CPPD) and apatite are more frequent in more severe radiographic OA, but whether these are a result, a contributing factor, or both is not known. Experimental studies and the few clinical observations do not show that either CPPD or apatite consistently cause either inflammation or more rapid destruction in OA. These attempts at correlations are plagued with technical questions about what crystal numbers may be meaningful, and about techniques for crystal identification. Continued efforts are indicated to identify a treatable component of OA.