SYNDESMOPHYTES ARE ASSOCIATED WITH HIGHER BONE TURNOVER IN ANKYLOSING SPONDYLITIS PATIENTS WITH ACTIVE DISEASE

Background Bone formation and bone loss are both present in ankylosing spondylitis (AS). New bone formation can lead to the formation of syndesmophytes, ankylosis of the spine and sacroiliac joints, and bone formations on enthesal sites. Objectives To investigate the relation between the presence of syndesmophytes and bone turnover markers (BTM) in AS patients with active disease. Methods 55 consecutive outpatients with AS based on the modified New York criteria (n=54) or the ASAS criteria for axial spondyloarthritis (n=1) and active disease defined by Bath AS Disease Activity Index (BASDAI) ≥4 (range 0-10) or based on expert opinion were included. Patients with recent fractures, use of bisphosphonates, and/or inflammatory bowel disease were excluded. The lateral view of radiographs of the cervical and lumbar spine were blinded and scored for the presence of syndesmophytes at the anterior corners of the vertebrae by two independent observers. In case of discrepancy between the two observers, there was a consensus meeting afterwards. Markers of bone formation procollagen type 1 N-terminal peptide (PINP) and bone-specific alkaline phosphatase (BALP), and marker of bone resorption serum collagen-telopeptide (sCTX) were measured. Z-scores of BTM were calculated using matched 10-years-cohorts of a Dutch reference group (200 men or 350 women) to correct for the normal influence that age and gender have on bone turnover. Results Mean age of the 55 AS patients was 40.4 years (SD ± 11.2), median duration of symptoms was 15 years (range 3-46), median time since diagnosis was 5 years (range 0-27), mean BASDAI was 6.1 (SD ± 1.8), and 58% were male. Syndesmophytes were present in 33 of the 55 (60%) patients. Patients with syndesmophytes had significantly higher levels of PINP (median Z-score: 0.41 vs. -0.17, p=0.005) and sCTX (median Z-score: -0.02 vs. -0.58, p=0.025) than patients without syndesmophytes. No significant differences were found in BALP (median Z-score: 1.56 vs. 0.67, p=0.213) and BASDAI (mean: 6.2 vs. 5.9, p=0.581) between patients with and without syndesmophytes. Conclusions This cross-sectional analysis in AS patients with active disease shows that markers of both bone formation (PINP) and bone resorption (sCTX) are significantly higher in patients with syndesmophytes compared to patients without syndesmophytes. Further longitudinal studies are needed to investigate whether BTM can serve as potential biomarkers for radiographic damage in AS. Disclosure of Interest None Declared