Risk factors for the development of retro-odontoid pseudotumor in non-rheumatoid arthritis patients with atlantoaxial subluxation: a multicenter cross-sectional study

Purpose: Retro-odontoid pseudotumor (ROP) is a condition characterized by benign soft tissue proliferation at the odontoid process due to mechanical stress in the atlantoaxial joint, often represented by atlantoaxial subluxation (AAS). However, the pathogenesis of ROP remains unclear, as AAS can also occur without ROP. The purpose of this study is to clarify the pathogenesis of ROP in patients with AAS by analyzing imaging findings in patients without rheumatoid arthritis who underwent surgery for AAS and/or ROP. Methods: A multicenter cross-sectional study was conducted. Patients without rheumatoid arthritis who underwent surgery for AAS and/or ROP at three university hospitals between 2010 and 2022 were enrolled in this study. Patients were divided into two groups: the ROP group, comprising patients with AAS and ROP, and the non-ROP group, comprising those with AAS without ROP. Patient demographics and preoperative imaging findings, including plain radiography, multidetector computed tomography, and magnetic resonance imaging, were compared between the two groups. Results: The ROP group included 32 patients (age: 75.6 ± 7.7 years; 23 male and 9 female patients), while the non-ROP group comprised 18 patients (age: 70.5 ± 14.6 years; 9 male and 9 female patients). No significant differences were observed between the two groups in terms of age, sex, or history of cervical spine surgery. However, the O–C2 angle in the extension position and C2–C7 sagittal vertical axis were significantly larger in the ROP group than in the non-ROP group. Additionally, degeneration of the facet joints and intervertebral discs in the subaxial cervical spine was significantly more advanced in the ROP group than in the non-ROP group. Conclusion: In patients with AAS, the anterior translation of the cervical spine, as well as the progressive degeneration of the subaxial cervical facet joints and intervertebral discs, may increase mechanical stress on the atlantoaxial joint, contributing to the development of ROP.