title: Urate crystal deposition and bone erosion in gout: 'inside-out' or 'outside-in'? A dual-energy computed tomography study.
abstract: It is currently unknown whether bone erosion in gout occurs through an 'inside-out' mechanism due to direct intra-osseous crystal deposition or through an 'outside-in' mechanism from the surface of bone. The aim of this study was to examine the mechanism ('outside-in' vs. 'inside-out') of monosodium urate (MSU) crystal deposition in bone erosion in gout. Specifically, we used three-dimensional dual-energy computed tomography (DECT) to analyse the positional relationship between bone and MSU crystal deposition in tophaceous gout, and to determine whether intra-osseous crystal deposition occurs in the absence of erosion.
  
  
  
  One hundred forty-four participants with gout and at least one palpable tophus had a DECT scan of both feet. Two readers independently scored all metatarsal heads (1433 bones available for scoring). For bones in contact with urate, the bone was scored for whether urate was present within an erosion, on the surface of bone or within bone only (true intra-osseous deposit). Data were analysed using generalised estimating equations.
  
  
  
  Urate in contact with bone was present in 370 (54.3 %) of 681 joints with urate deposition. For those bones in contact with urate, deposition was present on the surface of bone in 143 (38.6 %) of 370 joints and within erosion in 227 (61.4 %) of 370. True intra-osseous urate deposition was not observed at any site (p < 0.0001). For all bones with apparent intra-osseous deposition in one plane, examination in other planes revealed urate deposition within an en face erosion.
  
  
  
  In tophaceous gout, MSU crystal deposition is present within the joint, on the bone surface and within bone erosion, but it is not observed within bone in the absence of a cortical break. These data support the concept that MSU crystals deposit outside bone and contribute to bone erosion through an 'outside-in' mechanism.
authors.affiliation: Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand.|Department of Medicine, Naresuan University, Phitsanulok, Thailand.
authors.forename: Patapong
authors.initials: P
authors.lastname: Towiwat
authors.affiliation: Department of Anatomy and Medical Imaging, University of Auckland, Auckland, New Zealand.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Doyle
authors.affiliation: Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand.
authors.forename: Gregory D
authors.initials: GD
authors.lastname: Gamble
authors.affiliation: Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand.
authors.forename: Paul
authors.initials: P
authors.lastname: Tan
authors.affiliation: Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand.
authors.forename: Opetaia
authors.initials: O
authors.lastname: Aati
authors.affiliation: Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand.
authors.forename: Anne
authors.initials: A
authors.lastname: Horne
authors.affiliation: Department of Medicine, University of Otago, Christchurch, New Zealand.
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Stamp
authors.affiliation: Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand. n.dalbeth@auckland.ac.nz.
authors.forename: Nicola
authors.initials: N
authors.lastname: Dalbeth
mesh_terms: D000328:Adult; D000368:Aged; D001842:Bone and Bones / Q000000981:diagnostic imaging* / Q000473:pathology*; D005260:Female; D006073:Gout / Q000000981:diagnostic imaging* / Q000473:pathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D014057:Tomography, X-Ray Computed; D014527:Uric Acid
chemical_list: D014527:Uric Acid
keywords: Bone; Dual-energy computed tomography; Erosion; Gout; Tophus; Urate