High sacral slope, lumbar lordosis, and sacral slope-to-pelvic incidence ratio are associated with new bone formation in ankylosing spondylitis.

Background/aim: Syndesmophyte formation appears to be site-specific in ankylosing spondylitis (AS) and new bone formation seems to occur in regions of microtrauma that are prone to tensile forces. Pelvic and spinal parameters are unique for each individual. Pelvic tilt and sacral slope are important anatomical features that compensate in harmony in keeping the sagittal balance. After puberty, the sacral slope shapes the lumbar lordosis, whereas the pelvic incidence has an individual constant value. This study aimed to analyze the properties of pelvic parameters in AS patients with and without syndesmophyte formation in the spine after 15 years of disease duration. Materials and methods: Whole-spine radiographs and clinical data of 104 AS patients were analyzed according to radiographic damage in the spine. AS patients were grouped as those with and without syndesmophytes. Patients with complete bridging in at least one vertebral unit were excluded. Sacral slope, pelvic tilt, pelvic incidence, and lumbar lordosis were measured. Results: The mean disease duration was 14.5 years and 60% of the AS patients were male. The groups were similar in terms of age, sex distribution, and disease duration. Although numerically higher in patients with syndesmophytes, the mean pelvic incidence of AS patients was not significantly different between groups (55.2 ± 13.6 vs. 57.2 ± 15.4). The sacral slope was higher in patients with lumbar syndesmophytes (p < 0.005). Conclusion: The sacral slope was significantly higher in patients with syndesmophytes, which in turn resulted in increased lumbar lordosis. Our results imply that the individual shape of the spine affects the distribution of weight and tensile forces in AS, and some patients are possibly more prone to new bone formation due to altered repetitive microtrauma in the general genetic background of AS. Prospective studies addressing this cross-sectional observation may contribute to the development of new treatment strategies addressing mechanical load and may aid in decreasing the management costs of AS with the present biological therapies targeting new bone formation.