Levels of osteocalcin and N-terminal telopeptide of type I collagen in men with ankylosing spondylitis: associations with disease course and structural-functional status of bone tissue.

Introduction: The aim was to assess osteocalcin (OC) and N-terminal telopeptide of type I collagen (NTx) levels in men with ankylosing spondylitis (AS) and evaluate their relationship with the course of the disease and the structural and functional state of bone tissue.

Material and methods: The study was conducted on 83 male patients with AS and 29 healthy individuals constituting the control group. Disease activity and functional limitations were assessed using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Ankylosing Spondyloarthritis Disease Activity Score with C-reactive protein (ASDAS-CRP), and Bath Ankylosing Spondylitis Functional Index (BASFI) functional indices. Laboratory testing included CRP and markers of bone synthesis and resorption. Bone mineral density (BMD) of the lumbar spine and femoral neck was determined using dual-energy X-ray absorptiometry.

Results: The mean OC levels did not differ significantly between AS patients and the control group and showed no significant correlation with ASDAS, BASDAI, BASFI and CRP indices. On the other hand, NTx values were significantly higher in AS patients than in the control group (105.8 ±3.4 ng/ml vs. 92.6 ±5.1 ng/ml) and were closely related to the activity of the inflammatory process and low functional capacity. The structural and functional state of the bone is impaired by increased bone resorption. Thus, the proportion of patients with low BMD and fractures (68.2% and 27.3%) in the group of patients with high NTx content was 4-12 times higher than with optimal levels of this marker (17.7% and 2.2%). On the other hand, osteoproliferative changes were not associated with NTx levels but were dependent on serum OC levels. In particular, in the group of patients with syndesmophytes, serum OC values were higher (by 12.4%) than in the group without syndesmophytes.

Conclusions: Elevated NTx levels are associated with high inflammatory activity and low BMD. On the other hand, OC concentration is not associated with disease progression but is increased in individuals with syndesmophytes.