Association of bone morphogenetic proteins with spinal fusion in ankylosing spondylitis

To measure serum concentrations of bone morphogenetic proteins (BMP) in patients with ankylosing spondylitis (AS), and to investigate the relationship between BMP and clinical manifestations and radiographic changes.

We studied 60 consecutive AS patients with and 60 patients without spinal fusion. Spinal radiographs were assessed using the Bath Ankylosing Spondylitis Radiology Index (BASRI) and the modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS). Spinal fusion was defined as the presence of total bony bridging between 2 adjacent vertebral bodies in either the lumbar or cervical spine. Serum levels of BMP were determined by enzyme-linked immunosorbent assay.

Patients with spinal fusion had higher serum levels of BMP-2 and BMP-4 than either the healthy controls or patients without spinal fusion (p < 0.001), but there was no difference between the latter 2 groups. Serum BMP-7, erythrocyte sedimentation rate, and C-reactive protein (CRP) levels were elevated in patients with spinal fusion compared with those without (p < 0.05). Serum BMP-4 and BMP-7 levels were higher in patients with hip involvement than in those without (p < 0.05). BMP-2 and BMP-4 levels had a significant correlation with spinal radiograph scores, especially for BASRI of the lumbar spine (r = 0.356 and 0.348, respectively, p < 0.001). CRP showed a significant correlation with spine BASRI and mSASSS scores (r = 0.261 and 0.260, respectively, p < 0.05).

Rising levels of BMP in AS patients with spinal fusion and the positive correlation between BMP and spinal radiograph scores indicate that BMP may play a role in the process of spinal ankylosis. Serum levels of BMP may reflect radiographic progression of the spine and hip joints.