Background Ankylosing spondylitis (AS) is characterized by the combination of inflammation, bone formation, and bone loss. New bone formation can lead to the formation of syndesmophytes, ankylosis of the spine and sacroiliac joints, and bone formations on enthesal sites, whereas bone loss can result in osteoporosis and vertebral fractures.
Objectives To investigate the relation between bone turnover markers (BTM) and the presence of (bridging) syndesmophytes and/or low bone mineral density (BMD) in AS patients with active disease.
Methods 149 consecutive outpatients with AS based on the modified New York criteria and active disease (Bath AS disease activity index (BASDAI) ≥4 and/or based on expert opinion) were included. Patients with recent fractures ore use of bisphosphonates were excluded.
The lateral view of radiographs of the cervical and lumbar spine were scored for the presence of (bridging) syndesmophytes at the anterior corners of the vertebrae by two independent observers. In case of discrepancy between the observers, consensus was reached afterwards. BMD of the lumbar spine (anterior-posterior projection at L1-L4) and hip (total proximal femur) were measured using DXA. Low BMD was defined as a lumbar spine and/or hip BMD Z-score ≤1. 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 was 42 years (SD±11), median symptom duration 15 years (range: 1-53), mean BASDAI 6.2 (SD±1.6), and 70% were male. Syndesmophytes, bridging syndesmophytes, and low BMD were present in 68%, 33%, and 42% of the patients, respectively. No significant differences in BTM were found between patients with and without syndesmophytes. Patients with bridging syndesmophytes had significantly higher PINP Z-scores (median: 0.55 vs. -0.01, p=0.002) and sCTX Z-scores (median: 0.44 vs. -0.52, p=0.000) than patients without bridging syndesmophytes. Patients with low BMD had significantly higher sCTX Z-scores (median: -0.03 vs. -0.61, p=0.010) than patients with normal BMD. The differences in sCTX Z-score remained statistically significantly after correcting for gender, age, disease duration, and disease activity.
Conclusions This cross-sectional analysis in AS patients with active and relatively long-standing disease indicates that higher serum levels of sCTX are significantly associated with the presence of bridging syndesmophytes and/or low BMD. Longitudinal studies are needed to investigate whether BTM can serve as potential biomarkers or predictors for radiographic progression and/or osteoporosis in AS.
Disclosure of Interest None Declared