To The Editor: We read with interest “Diagnosing Basilar Invagination in the Rheumatoid Patient. The Reliability of Radiographic Criteria” (2001;83:194-200), by Riew et al. Their study, which was well conducted, demonstrates the difficulty in establishing this diagnosis. According to the authors’ criteria, true basilar invagination was diagnosed if the odontoid tip extended above the foramen magnum when visualized with use of magnetic resonance imaging, computerized tomography, or conventional tomography. This definition was used as a “gold standard” when the sensitivity and the specificity of several diagnostic methods (based on plain radiography) were tested. Unfortunately, the authors did not differentiate between basilar invagination and atlantoaxial impaction and did not mention the pathogenetic mechanism of the phenomenon1-2. Basilar invagination represents the late stage of atlantoaxial impaction that is caused by chronic arthritis1. The difference between these two pathological entities should be taken into account when treating rheumatoid patients1-10. Atlantoaxial subluxation may develop rather early because of injury to the stabilizing ligaments resulting from rheumatoid arthritis1-4. If the inflammation continues in the lateral atlantoaxial facet joints, the layers of cartilage and, later, the subchondral bone structures will erode. The facets may even collapse because of erosion and osteoporosis. The gradual destruction of the atlantoaxial facets lets the atlas fall down around the axis, followed by the skull, and atlantoaxial impaction develops1-2. The term “atlantoaxial impaction” describes the process from its beginning. In cases of severe atlantoaxial impaction, the tip of the odontoid process may penetrate into the foramen …
