title: Lifestyle factors may modify the effect of disease activity on radiographic progression in patients with ankylosing spondylitis: a longitudinal analysis.
abstract: OBJECTIVE
  To investigate the complex relationship between inflammation, mechanical stress and radiographic progression in patients with ankylosing spondylitis (AS), using job type as a proxy for continuous mechanical stress.
  
  
  METHODS
  Patients from the Outcome in Ankylosing Spondylitis International Study were followed up for 12 years, with 2-yearly assessments. Two readers independently scored the X-rays according to the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS). Disease activity was assessed by the AS Disease Activity Score C reactive protein (ASDAS-CRP). The relationship between ASDAS and spinal radiographic progression was investigated with longitudinal analysis, with job type at baseline (physically demanding ('blue-collar') versus sedentary ('white-collar') labour) as a potential factor influencing this relationship. The effects of smoking status and socioeconomic factors were also investigated.
  
  
  RESULTS
  In total, 184 patients were included in the analyses (70% males, 83% human leucocyte antigen-B27 positive, 39% smokers, 48% blue-collar workers (65/136 patients in whom data on job type were available)). The relationship between disease activity and radiographic progression was significantly and independently modified by job type: In 'blue-collar' workers versus 'white-collar' workers, every additional unit of ASDAS resulted in an increase of 1.2 versus 0.2 mSASSS-units/2-years (p=0.014 for the difference between blue-collar and white-collar workers). In smokers versus non-smokers, every additional unit of ASDAS resulted in an increase of 1.9 versus 0.4 mSASSS-units/2-years.
  
  
  CONCLUSIONS
  Physically demanding jobs may amplify the potentiating effects of inflammation on bone formation in AS. Smoking and socioeconomic factors most likely confound this relationship and may have separate effects on bone formation.
authors.affiliation: Department of Clinical Immunology & Rheumatology , Amsterdam Rheumatology Center, University of Amsterdam , Amsterdam , The Netherlands ; Department of Rheumatology , Hospital Garcia de Orta , Almada , Portugal.
authors.forename: Sofia
authors.initials: S
authors.lastname: Ramiro
authors.affiliation: Department of Clinical Immunology & Rheumatology , Amsterdam Rheumatology Center, University of Amsterdam , Amsterdam , The Netherlands ; Department of Rheumatology , Atrium Medical Center , Heerlen , The Netherlands.
authors.forename: Robert
authors.initials: R
authors.lastname: Landewé
authors.affiliation: Department of Medicine, Division of Rheumatology , Maastricht University Medical Center , Maastricht , The Netherlands ; School for Public Health and Primary Care (CAPHRI), University of Maastricht , Maastricht , The Netherlands.
authors.forename: Astrid
authors.initials: A
authors.lastname: van Tubergen
authors.affiliation: Department of Medicine, Division of Rheumatology , Maastricht University Medical Center , Maastricht , The Netherlands ; School for Public Health and Primary Care (CAPHRI), University of Maastricht , Maastricht , The Netherlands.
authors.forename: Annelies
authors.initials: A
authors.lastname: Boonen
authors.affiliation: Department of Medicine, Division of Rheumatology , Maastricht University Medical Center , Maastricht , The Netherlands ; School for Public Health and Primary Care (CAPHRI), University of Maastricht , Maastricht , The Netherlands.
authors.forename: Carmen
authors.initials: C
authors.lastname: Stolwijk
authors.affiliation: Department of Rheumatology, Hôpital Cochin , Paris-Descartes University , Paris , France ; Clinical Epidemiology and Biostatistics, PRES Sorbonne Paris-Cité , Paris , France.
authors.forename: Maxime
authors.initials: M
authors.lastname: Dougados
authors.affiliation: Department of Rheumatology , University of Ghent , Ghent , Belgium.
authors.forename: Filip
authors.initials: F
authors.lastname: van den Bosch
authors.affiliation: Department of Rheumatology , Leiden University Medical Center , Leiden , The Netherlands.
authors.forename: Désirée
authors.initials: D
authors.lastname: van der Heijde
keywords: Ankylosing Spondylitis; Disease Activity; Outcomes research; Spondyloarthritis