Bone Abstracts

Osteoporosis and vertebral fractures are recognised complications of ankylosing spondylitis (AS) but the underlying causes are incompletely understood. Osteoprotegerin (OPG) is a decoy receptor for RANK-L and inhibits osteoclastogenesis. We have previously demonstrated that antibodies to OPG (OPG-Ab) are associated with osteoporosis and increased bone turnover in patients with autoimmune diseases. The aim of this study was to determine whether OPG-Abs were detectable in AS patients and relate these to bone health.

Patients with AS were recruited from outpatient clinics at two centres in the UK between 2011 and 2015. Patient demographics, disease characteristics and fracture history were recorded. BMD was assessed by antero-posterior DEXA. Serum levels of OPG-Ab were measured using an in-house ELISA and considered to be positive if values were above three S.D.s above mean in healthy controls. Associations between OPG-Ab and BMD and fractures were assessed using logistic regression, adjusted for age, gender, duration since diagnosis, BMI and study centre.

We studied 134 patients, of whom 75% were male. The mean age was 47 (S.D.±15) years and median disease duration 6.5 years. 16 patients were tested positive for OPG-Ab (11.9%). The presence of OPG-Ab was associated with lower hip BMD (P=0.018), and an increased number of fractures (P=0.007). There was no association between OPG-Ab and patient demographics, disease characteristics or activity. Logistic regression revealed an association between OPG-Ab and disease duration (OR 1.04; 95% CI 1.00, 1.07; P=0.045) and a strong independent association with hip T-score (OR_adj 0.43; 95% CI 0.22, 0.85; P=0.015) and history of fractures (OR_adj 4.78; 95% CI 1.37, 16.7; P=0.014).

In conclusion, this cross-sectional study demonstrates that OPG-Ab were present in 11.9% of AS patients. OPG-Ab was strongly and independently associated with hip BMD and history of fractures. This raises the possibility that OPG-Ab may play an important role in accelerated bone loss and increased fracture risk in AS.