The tophus is the cardinal feature of advanced gout. This review summarises recent research into the biology, impact and treatment of tophaceous gout. Microscopically, tophi are chronic foreign body granuloma-like structures containing collections of monosodium urate (MSU) crystals surrounded by inflammatory cells and connective tissue. Extracellular trap formation mediated by neutrophil interactions with MSU crystals may be a central checkpoint in tophus formation. Gouty tophi impact on many aspects of health-related quality of life. Tophi are also implicated in the development of structural joint damage and increased mortality risk in people with gout. Effective treatment of tophaceous gout requires long-term urate-lowering therapy, ideally to achieve a serum urate concentration of <5 mg/dL (300 μmol/L). Recent advances in gout therapeutics have expanded urate-lowering therapy options for patients with severe tophaceous disease to allow faster regression of tophi, improved health-related quality of life and, potentially, improved structural outcomes.

BackgroundGout is a chronic disease of monosodium urate (MSU) crystal deposition. Although hyperuricaemia is the central risk factor for development of gout, not all people with hyperuricaemia have subclinical MSU crystal deposition or indeed, symptomatic disease. The aim of this systematic literature review was to identify factors that contribute to MSU crystallization.MethodsA search was conducted of the electronic databases PubMed, Science Direct and Scopus. Articles were included if they contained original data related to MSU crystallization. The methods and results were summarized and categorized into articles describing at least one of the three key steps in MSU crystallization (reduced urate solubility, nucleation and growth).ResultsA total of 2175 articles were initially identified in our systematic search with 35 of these articles included in the final analysis. Elevated urate concentration was identified as a central factor driving all three stages of MSU crystallization. Factors that were found to consistently reduce urate solubility were reduced temperatures, pH 7–9 and various ions including sodium ions. Connective tissue factors including bovine cartilage homogenates and healthy human synovial fluid and serum all enhanced urate solubility. MSU nucleation was found to be increased by a number of factors, including sodium ions, uric acid binding antibodies, and synovial fluid or serum from patients with gout. Other than elevated urate concentrations, no other specific factors were identified as promoters of MSU crystal growth.ConclusionsIncreased urate concentration is the key factor required at each stage of MSU crystallization. Different proteins and factors within connective tissues may promote MSU crystallization and may be important for determining the sites at which MSU crystallization occurs in the presence of elevated urate concentrations.Electronic supplementary materialThe online version of this article (doi:10.1186/s12891-015-0762-4) contains supplementary material, which is available to authorized users.

The clinical course of gout is initially characterized by acute self-limited joint inflammation, but long-standing disease is often associated with chronic inflammation followed by the development of erosive joint damage, which can result in long-term functional impairment. Preventing joint damage is now a major focus of therapeutic intervention in gout. New light has been shed on the mechanisms leading to cartilage and bone damage in patients with this disease. Here, we discuss basic science studies focusing on the cellular immunology of bone and cartilage in gout and the effects of monosodium urate crystals on signaling pathways, cytokine release and the function of osteoclasts, osteoblasts and chondrocytes. We then explore the use of advanced imaging modalities (including MRI, ultrasonography, CT and dual-energy CT) to investigate pathology in gout, as they provide new ways to visualize joint tissues. These modalities vary in their ability to detect the various pathological features of gout and have different clinical applications. Imaging provides information about the inflammatory nature of the joint lesion, position and size of tophaceous deposits, and extent of bone and cartilage damage. Imaging is also increasingly being used to monitor the progression of joint damage and regression of tophi with effective urate-lowering therapy.

MSU crystals have profound inhibitory effects on osteoblast viability and differentiation. These data suggest that bone erosion in gout occurs at the tophus-bone interface through alteration of physiological bone turnover, with both excessive osteoclast formation, and reduced osteoblast differentiation from mesenchymal stem cells.

These data indicate that MSU crystals directly interact with tenocytes to reduce cell viability and function. These interactions may contribute to tendon damage in people with advanced gout.