Evaluation of lateral instability of the atlanto-axial joint in rheumatoid arthritis using dynamic open-mouth view radiographs.

Upper cervical involvement is common in patients with rheumatoid arthritis (RA). Anterior atlanto-axial subluxation (aAAS) sometimes occurs at an early stage of the disease. We hypothesized that not only antero-posterior instability but lateral instability may occur with atlanto-axial involvement in RA. To prove this hypothesis, we evaluated the lateral instability of the atlanto-axial joint in RA, using dynamic open-mouth view radiographs. Thirty RA patients and a control group of 22 non-RA outpatients were enrolled in this study. The patients underwent lateral view radiographs of the cervical spine during flexion and extension, and antero-posterior (AP) open-mouth views during maximum right and left bending of the neck. The anterior atlanto-dental interval (AADI) was measured to evaluate antero-posterior instability of the atlanto-axial joint, and atlanto-dental lateral shift (ADLS) was defined to evaluate dynamic lateral instability. In the RA group, AADI averaged 3.2 mm in flexion, and in eight patients, it exceeded 3 mm in flexion (aAAS). In the control group, the AADI averaged 1.0 mm in flexion. The ADLS in the RA group averaged 14.8%, and this was significantly greater than in the control group, in which it averaged 6.1%. The ADLS averaged 20.6% in the RA subgroup with aAAS, and 12.7% in the RA subgroup without aAAS. In both subgroups, the ADLS was significantly greater than that of the control group. In this study, dynamic lateral instability of the atlanto-axial joint in RA was demonstrated. The results suggest that an evaluation of the dynamic lateral instability of the atlanto-axial joint can be useful for early diagnosis of atlanto-axial lesions in RA.