OBJECTIVE Increasing soft-tissue mass posterior to the odontoid process causes spinal cord compression. Retro-odontoid pseudotumors are considered to be associated with atlantoaxial instability in patients with rheumatoid arthritis (RA), but the exact mechanism by which these lesions develop has not been elucidated. The purpose of this study was to identify the relationships between retro-odontoid soft-tissue (ROST) thickness and radiological findings or clinical data in patients with RA. METHODS A total of 201 patients with RA who had been followed up at the outpatient clinic of the authors' institution were enrolled in this study. ROST thickness was evaluated on midsagittal T1-weighted MRI. The correlations between ROST thickness and radiographic findings or clinical data on RA were examined. The independent factors related to ROST thickness were analyzed using stepwise multiple regression analysis. RESULTS The average thickness of ROST was 3.0 ± 1.4 mm. ROST thickness showed an inverse correlation with disease duration (r = -0.329, p < 0.01), Steinbrocker stage (r = -0.284, p < 0.01), the atlantodental interval (ADI) in the neutral position (r = -0.326, p < 0.01), the ADI in the flexion position (r = -0.383, p < 0.01), and the ADI in the extension position (r = -0.240, p < 0.01). On stepwise multiple regression analysis, ADI in the flexion position and Steinbrocker stage were independent factors associated with ROST thickness. CONCLUSIONS Although the correlations were not strong, ROST thickness in patients with RA was inversely correlated with ADI and Steinbrocker stage. In other words, ROST thickness tends to be smaller as atlantoaxial instability and peripheral joint destruction worsen. Clinical trial registration no.: UMIN000000980 (UMIN Clinical Trials Registry).
Keywords: ADI = atlantodental interval; ICC = intraclass correlation coefficient; RA = rheumatoid arthritis; ROST = retro-odontoid soft tissue; atlantoaxial instability; cervical spine; mHAQ = modified Health Assessment Questionnaire; magnetic resonance imaging; odontoid process; retro-odontoid pseudotumor; rheumatoid arthritis; transverse ligament.