Stiffness of the Middle and Lower Cervical Spine Is Associated with Idiopathic Atlantoaxial Joint Disorder: A Retrospective Case-Control Study

Background Atlantoaxial joint disorders (AAD), including instability and retro-odontoid pseudotumors, can cause serious neurological deficits; however, some patients lack an identifiable primary disease. We hypothesized that stiffness of the middle and lower cervical spines contributes to idiopathic AAD. This study aimed to compare the range of motion (ROM) of the middle and lower cervical spines and extent of ossification of the anterior longitudinal ligament (ALL) in patients with cervical spondylotic myelopathy (CSM) due to idiopathic AAD and age- and sex-matched patients with CSM without AAD. Methods We enrolled 56 patients who underwent surgery for myelopathy due to AAD between 2007 and 2020. Among them, 27 (48.2%) patients without a primary disease were classified into the idiopathic AAD group. The CSM group included patients who underwent surgery at the same time and included two age- (± 1 year) and sex-matched patients for each patient in the idiopathic AAD group. The ROM (C2–7) and extent of ALL ossification were compared between the groups, and the correlation between the extent of ossification and ROM was assessed. Results Patients with idiopathic AAD demonstrated a significantly reduced ROM and more severe ligament ossification than the matched patients without AAD. A significant negative correlation was found between ROM and ALL ossification. Conclusion The ROM of the middle and lower cervical spines were significantly lower in patients with idiopathic AAD. The extent of ossification of the ALL, which negatively correlated with the ROM of the cervical spine, was also greater in patients with CSM with idiopathic AAD.