Arthritis can result from intra-articular deposition of crystals:
Monosodium urate
Calcium pyrophosphate dihydrate
Basic calcium phosphate (apatite)
Rarely, others such as calcium oxalate crystals
Definite diagnosis requires synovial fluid analysis. Polarized light microscopy is used to specifically identify most crystals; basic calcium phosphate crystals are of ultramicroscopic size and require other methods. Crystals may be engulfed in white blood cells or may be extracellular; either is diagnostic of crystal-induced arthritis. The presence of crystals does not exclude the possibility of simultaneous infectious or other inflammatory forms of arthritis. Noninvasive identification of monosodium urate and calcium pyrophosphate dihydrate crystals is possible using ultrasonography, but not all ultrasonographers have sufficient expertise to make this diagnosis reliably. Dual-energy CT imaging can also be used to detect uric acid or calcium deposition, but neither imaging is 100% sensitive.
Crystal-induced arthritides include the following:
