Bridging Plays an Important Role in Modifying Vertebral Bone Loss in Ankylosing Spondylitis

Using quantitative CT, researchers studied the associations between vertebral bone mineral density, strength, and syndesmophytes in ankylosing spondylitis.

Among patients with ankylosing spondylitis (AS), associations between syndesmophytes and vertebral density and strength differ between bridged and nonbridged vertebrae, with extensive bridging more likely to cause trabecular bone mineral density (tBMD) loss, according to study results published in Arthritis & Rheumatology.

Divergent processes of bone formation and loss in the spine can occur among patients with AS, with syndesmophyte formation leading to bridging and reduced spinal flexibility. In addition, vertebral osteopenia can weaken the bones and increase risk for fractures.

In a prospective study, researchers explored the relationship between syndesmophyte formation and bone loss in the presence and absence of vertebral bridging.

The researchers used quantitative computed tomography (QCT) to measure tBMD and syndesmophyte height in 5 vertebrae (T11 to L3) in patients with AS. Vertebral strength (total, trabecular, and cortical) was calculated using finite-element analysis. Cross-sectional analyses were used to test the associations between tBMD and vertebral strength and syndesmophyte height of each vertebra and evaluate whether these associations were modified by the presence of bridging.

A longitudinal cohort was followed up with over a period of 2 years to study the associations between baseline tBMD, vertebral strength, and syndesmophyte growth.

Among 61 patients enrolled in the study, 25 were included in the finite-element analysis; 33 patients participated in the longitudinal study. Syndesmophyte height was calculated for 286 of the 305 vertebrae, of which 126 (44%) had bridging syndesmophytes, 77 (26.9%) had nonbridging syndesmophytes, and 83 (29%) had no syndesmophytes.

Among nonbridged vertebrae only, low tBMD and low total bone strength had more syndesmophyte growth over time (adjusted beta, -0.01; 95% CI, -0.019 to -0.0012 and -0.0003; 95% CI, -0.0004 to -0.0002, respectively). Among bridged vertebrae, strong inverse associations were observed between trabecular bone mass density, total strength, and trabecular strength and syndesmophyte height in the cross-sectional analysis. In the longitudinal analysis, bridged vertebrae at baseline showed a loss in percent tBMD over time (adjusted beta, -0.001; 95% CI, -0.0017 to -0.0004).

Limitations of the study included the small number of patients in the longitudinal cohort and the inclusion of a narrow region of the spine.

The researchers concluded, “Although previously suspected, clear evidence of the unique role of bridging in mediating low tBMD has been lacking. Low tBMD in AS should therefore be considered in the context of whether bridging syndesmophytes are present or not, as the primary mechanisms underlying low bone density are likely different. Treatments targeting inflammation may be more effective in improving tBMD in non-bridged vertebrae. Conversely, among bridged vertebrae, where mechanical off-loading may be primarily responsible, antiresorptive or anabolic medications may be more effective treatments to improve tBMD.”

Disclosure: Some study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of disclosures.


Tan S, Bagheri H, Lee D, et al. Vertebral bone mineral density, vertebral strength, and syndesmophyte growth in ankylosing spondylitis: the importance of bridging. Arthritis Rheumatol. Published online March 21, 2022. doi:10.1002/art.42120