Progression of radiographic damage in patients with ankylosing spondylitis: defining the central role of syndesmophytes

Background: Structural changes such as erosions, syndesmophytes and ankylosis are characteristic of ankylosing spondylitis ( AS). These can be quantified by the modified Stokes Anklylosing Spondylitis Spinal Score ( mSASSS). It is unknown which radiographic feature is most relevant for the assessment of change and the prediction of future damage in AS. Objectives: To analyse radiographic progression in AS by using different assessments to define the most important changes. Methods: Spinal radiographs of 116 patients with AS were scored by the mSASSS at baseline ( BL) and after 2 years. Radiographic progression was assessed by differentiating ( 1) any change; ( 2) progression to syndesmophytes/ ankylosis ( definite change); and ( 3) changes exceeding the smallest detectable change ( SDC) as predefined. A growth angle of 45 was used to differentiate syndesmophytes from spondylophytes. Results: Some radiographic progression after 2 years was detected in 42% of patients, novel syndesmophytes in 31% of patients, and, using the SDC ( calculated at 2 mSASSS units) as cut- off, progression was seen in 28% of patients. Thus, in 74% of the patients changes were because of syndesmophytes and/ or ankylosis. Using the predefined cut- off, only 12% of all syndesmophytes were spondylophytes. Patients with such changes were of older age. Definite radiographic progression was found in 44% of the patients with syndesmophytes/ ankylosis at BL ( n = 57) versus 19% ( p = 0.03) of the patients without such changes ( n = 59). Conclusions: Syndesmophytes and ankylosis are the most relevant structural changes in AS, and also in the mSASSS. Development of just one syndesmophyte within 2 years indicates progression of structural changes in AS; this is relevant for clinical practice. Syndesmophytes are the best predictors of radiographic progression.