Fig. 1

Cervical lesions associated with RA and surgical procedures for individual cervical lesions. A Atlantoaxial subluxation (AAS) was defined as an expansion of the atlantodental interval over 3 mm (between the white lines) at the flexed position on X-ray lateral view. B Vertical subluxation (VS) was defined as a shortening of the Ranawat C1–C2 index to less than 15 mm for men and 13 mm for women. The measurement of the Ranawat index was made from the center of the pedicles of the axis (white circle and dot) to a line connecting the midpoint of the anterior and posterior arches of the atlas (white line). C Subaxial subluxation (SAS) was diagnosed as migration of the superior vertebra compared to the inferior vertebra over 3 mm. Migration distance was measured between posterior walls of adjacent vertebrae (between the lines). D–G Four types of surgical procedures were performed for cervical spine lesions in patients with RA. Laminoplasty was performed for myelopathy in the stable cervical spine (D). Fixative procedures were chosen depending on how unstable the cervical spine was: atlantoaxial fusion for unstable AAS (E), occipital-cervical fusion for VS with or without AAS (F), and occipital/thoracic fusion for SAS that included AAS and/or VS (G)