Fig. 2

Evolution of axial disease prevalence. a Axial involvement (defined as: 1) inflammatory low back pain or inflammatory dorsal pain lasting for more than one month; 2) limited spine mobility; 3) sacroiliac pain at examination or intermittent buttock pain; or 4) presence of axial disease by imagery.). b Sacroiliitis Axial involvement was rare in the first years of the disease with a progressive linear increase in its prevalence up to 60 to 70% after five years