The Effects of Clinical Factors and Retro-Odontoid Soft Tissue Thickness on Atlantoaxial Instability in Patients with Rheumatoid Arthritis.

BACKGROUND: Rheumatoid arthritis (RA) is an autoimmune disease that often occurs in the atlantoaxial segment of the cervical spine and results in instability that can cause severe pain and neurologic symptoms. Thickening of soft tissue posterior to the odontoid process of the atlantoaxial segment is referred to as retro-odontoid soft tissue (ROST) thickness or pseudotumor. The mechanism of ROST thickness is still unknown.

METHODS: Among patients diagnosed with RA, those with cervical radiographs and magnetic resonance imaging records were selected for investigation of their clinical symptoms, laboratory findings, radiologic evaluation, and RA medication history.

RESULTS: A total of 199 patients were selected and divided into lower and upper median groups according to the ROST thickness value. In patients with RA, the median ROST thickness value was 2.3 mm (interquartile range, 1.4-3.4). The median value of the anterior atlanto-dens interval (ADI) was 2.4 mm, with a significant difference being observed between the lower and upper median ROST thickness groups (P = 0.001). Multivariable linear regression analysis revealed a correlation between Steinbrocker stage and the positivity of rheumatoid factor. For each unit increase in the Steinbrocker stage, the ROST thickness increased by 0.35 mm (β, -0.349; 95% confidence interval -0.643 to -0.055; P = 0.020). For every 1-mm increase in the ADI, ROST thickness decreased by 0.16 mm (β, -0.163; 95% confidence interval -0.264 to -0.062; P = 0.002).

CONCLUSIONS: This study showed a statistical correlation between ROST thickness and ADI related to biomechanical changes in the dynamic atlantoaxial segment, thereby suggesting the value of a prospective study.