ObjectivesInvestigating the association between different definitions of axial involvement and syndesmophytes development over 2 years in patients with psoriatic arthritis (PsA).MethodsPatients from a prospective multicentre cohort (Belgian Epidemiological Psoriatic Arthritis Study) involving 17 Belgian rheumatology practices were recruited between December 2012 and July 2014 and included when fulfilling the Classification Criteria for Psoriatic Arthritis. Axial involvement included six clinical and two radiographic oriented definitions.Two calibrated central readers evaluated radiographic damage by assessing the modified Stoke Ankylosing Spondylitis Spinal Score and modified New York criteria. New syndesmophytes after 2 years were described conditional on axial involvement at baseline. Logistic regression analyses were used to investigate the association between syndesmophyte development and axial involvement. All definitions of axial involvement were evaluated separately.ResultsFrom 150 patients, a 2-year follow-up of spinal radiographs was obtained. There are 11 patients with new syndesmophytes after 2 years. For the clinical definitions of axial involvement ‘global assessment’, ‘detailed assessment’, ‘back pain (BP)’ and ‘inflammatory BP (IBP)’ the probabilities of developing syndesmophytes ranged between 0.06 and 0.08 and were similar for the presence or absence of the definition. When including elevated C reactive protein (CRP) to the definitions the probability of developing syndesmophytes over 2 years increased two times for CBP and seven times for IBP.With radiographic axial involvement a similar trend was seen; radiographic sacroiliitis as definition showed a probability three times higher. When combined with elevated CRP there would be a 14 times higher chance to develop syndesmophytes in 2 years. The ORs varied from 0.83 to 13.80, though none of them were statistically significant.ConclusionsThe likelihood of syndesmophyte formation in PsA is low. The probability of developing syndesmophytes is much higher when axial involvement is determined radiographically rather than clinically, particularly in the context of high CRP.