Low incidence of calcium pyrophosphate dihydrate crystal deposition in rheumatoid arthritis, with modification of radiographic features in coexistent disease.

A negative correlation between rheumatoid arthritis (RA) and calcium pyrophosphate dihydrate (CPPD) crystal deposition was demonstrated in separate controlled radiographic and synovial fluid surveys of RA patients aged 55-75 years. Knee chondrocalcinosis was detected in 14% of 135 normal controls and 28% of 87 post-meniscectomy ("joint damage") controls (P less than 0.05), but only 3% of 100 RA and 75 osteoarthritis patients revealed CPPD crystals in 1% and 23%, respectively (P less than 0.01). Ten subjects with coexistent RA and CPPD deposition were also studied; 7 showed radiographic features atypical of RA, including patchy, asymmetric disease, retained bone density, prominent osteophytosis, well-corticated cysts, and paucity of progressive erosive disease. It is suggested that rheumatoid joint damage, unlike that in osteoarthritis, is not conducive to CPPD crystal formation. When RA and CPPD coexist, atypical radiographic features reflecting a hypertrophic reparative response may occur.