Crystals from intraarticularly injected corticosteroids may be found in synovial fluid. The crystals are birefringent and can be confused with sodium urate and calcium pyrophosphate dihydrate crystals. Left, Low-power compensated polarized microscopy demonstrates rod-shaped triamcinolone hexacetonide crystals. A leukocyte near the middle of the field has phagocytosed one of these crystals. The crystals may be blunt, squared, or tapered at the end and range in length from approximately 3 µm to 16 µm. Right, These betamethasone acetate crystals are rod -haped, have blunted ends, and are negatively birefringent. They range in length from approximately 10 µm to 20 µm. These and other corticosteroid crystals may take on different sizes, shapes, and characteristics that are partially the result of synovial fluid chemistry, such as pH. The in vivo phagocytosis of these and other corticosteroid crystals may cause rupture of synovial fluid leukocytes, releasing mediators of inflammation, and producing a ""postinjection flare.“ Self Learning Series. Philadelphia, Pa: American College of Physicians; 1978.