Background/Purpose:  We have recently shown that disease activity is longitudinally associated with radiographic progression in AS. In animal models, it has recently been shown that mice that were tail suspended, in order to prevent mechanical loading on paws, had less new bone formation, thus providing a proof-of-concept that mechanical strain drives new bone formation in spondyloarthritis. Our aim was 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 OASIS were followed-up for 12 years, with 2-yearly assessments. Two readers independently scored the x-rays according to the mSASSS. Disease activity was assessed by the ASDAS-CRP. The relationship between ASDAS and spinal radiographic progression was investigated with longitudinal analysis, with job type at baseline (physically demanding (‘blue collar’) vs sedentary (‘white collar’) labor as a potential factor influencing this relationship. The effects of smoking status and socio-economic factors were also investigated.

Results: In total, 184 patients were included in the analyses (70% males, 83% HLA-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 vs ‘white collar’ workers every additional unit of ASDAS resulted in an increase of 1.2 vs 0.2 mSASSS-units/2-years (p=0.014 for the difference between blue collar and white collar workers). In smokers vs non-smokers every additional unit of ASDAS resulted in an increase of 1.9 vs 0.4 mSASSS-units/2-years. Personal income also significantly modified the relationship ASDAS-mSASSS, but education and family income did not (Table).

 Conclusion: Physically demanding jobs may amplify the driving effects of inflammation on radiographic progression, thus supporting the theory that mechanical stress leads to bone formation in AS.  Smoking and personal income are likely classic confounders of this relationship but a separate detrimental effect of smoking on radiographic progression could not be excluded. If confirmed, these findings may have implications for our commonly given advice to patients with SpA to strenuously exercise.

Table: Effects of disease activity (one ASDAS-unit increase) on radiographic progression in subgroups

	2-year increase in mSASSS [units, (95% CI)]
A: Occupation: ‘Blue collar’ (n = 65)	1.19 (0.58; 1.79)
A: Occupation: ‘White collar’ (n = 71)	0.20 (-0.23; 0.64)
B: Smoking: Smokers  (n = 49)	1.94 (1.00, 2.87)
B: Smoking: Non-smokers (n = 78)	0.35 (0.04; 0.65)
C: Education: ‘non-university’ (n = 167)	0.74 (0.41; 1.07)
C: Education: ‘University’    (n = 14)	-0.18 (-1.91, 1.55)
D: Personal income: <€1588 (n = 105)	0.93 (0.45, 1.41)
D: Personal income: ≥€1588 (n = 56)	0.14 (-0.21, 0.50)
E: Family income: <€3176   (n = 90)	0.49 (0.09, 0.89)
E: Family income: ≥€3176  (n = 21)	0.15 (-0.35, 0.65)

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-physically-demanding-job-may-amplify-the-effect-of-disease-activity-on-the-development-of-syndesmophytes-in-patients-with-ankylosing-spondylitis/