Image Description
Gout results from prolonged untreated hyperuricemia. Uric acid is the end product of purine metabolism and interestingly humans are the only mammals lacking the enzyme uricase. Gout may result from overproduction or underexcretion of uric acid. High urate levels in plasma result in supersaturation of synovial fluid which is a poorer solvent causing precipitation of crystals in and around the synovium and superficial soft tissues with lower temperature. The image shows several gouty tophi surrounded by foreign body giant cells.
About the Disease
Crystal-induced arthritis is a group of acute and chronic inflammatory joint disorders caused by deposition of crystalline material within and around the joints. The prototypic example is gout. Endogenous Crystals: The crystals may be monosodium urate (Gout), calcium pyrophosphate dehydrate (Pseudogout), or calcium phosphate. Exogenous Crystals: The exogenous material that may trigger arthritis include corticosteroids, talcum, and biomaterials used in joint prosthesis (silicone, polyethylene, and methyl methacrylate). The underlying metabolic abnormality in gout is hyperuricemia. High plasma urate levels lead to supersaturation of synovial fluid which is a poor solvent for monosodium urate (MSU). The lower temperature in peripheral joints further decreases solubility and promotes crystal formation. Materials in and around joints, including collagen fibers, chondroitin sulfate, proteoglycans and cartilage fragments act as nuclei for crystals. Once deposited, the MSU crystals trigger cytokine-mediated tissue injury and inflammation that eventually destroys the articular cartilage.