Objectives: 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.