A higher body mass index and increased syndesmophytes volume are associated with facet joints ankylosis on thoracic spine in patients with ankylosing spondylitis: a retrospective cohort study

To investigate the association between syndesmophytes and facet joint (FJ) lesions in patients with ankylosing spondylitis (AS), and to identify clinical factors associated with FJ ankylosis (FJA) in thoracic segment. Ninety-seven patients with AS who underwent thoracic spine computed tomography (CT) or chest CT and without completely thoracic spine fusion were included. FJ lesions were analyzed for the numbers and distribution of normal, ankylosis, erosions, joint-space narrowing, osteophytes, and subchondral sclerosis. The volume of vertebral syndesmophtes unit (VSU) and total thoracic syndesmophtes volume were separately calculated by Mimics software. Clinical factors associated with FJA were investigated using generalized estimation equation (GEE). The association between syndesmophtes volume and numbers of FJ structural lesions was analyzed using generalized additive mixed model (GAMM). 2328 FJ and 1164 VSUs in thoracic spine were assessed. The majority FJ structural lesions were ankylosis (32.39%). FJA was more frequently seen in vertebrae with syndesmophytes formation (p < 0.001). GEE showed that patients with normal BMI (18.5–24.9 kg/m2) and high BMI (> 24.9 kg/m2) were more likely to have FJA in thoracic spine (odds rations [95% confidence interval]: 0.27(0.12–0.59), 1.45(1.03–8.57), respectively). GAMM showed that syndesmophytes volume increase the numbers of FJA (standard β = 0.009, p < 0.05) and decreased the numbers of normal FJ (standard β = -0.07, p < 0.01). FJA was the most common FJ structural lesion in thoracic spine, and it increases linearly with syndesmophytes before the bridging syndesmophytes formed. A higher BMI (especially > 24.9 kg/m2) and increased syndesmophytes volume are associated with FJA in thoracic spine.