Basilar invagination can be difficult to diagnose with plain radiography in patients with rheumatoid arthritis. Although numerous radiographic criteria have been described, few studies have addressed the reliability of these parameters in the rheumatoid population. The purpose of the present study was to validate and compare the most widely accepted plain radiographic criteria for basilar invagination in this patient population.
Cervical radiographs of 131 rheumatoid patients were examined. Of these patients, sixty-seven (twenty-nine with basilar invagination and thirty-eight without it) were also evaluated with tomograms, magnetic resonance imaging, and/or sagittally reconstructed computed tomography scans to detect the presence of basilar invagination. Three observers who were blinded with regard to the diagnosis independently scored each radiograph as positive, negative, or indeterminate according to the established criteria for invagination proposed by Clark et al., McRae and Barnum, Chamberlain, McGregor, Redlund-Johnell and Pettersson, Ranawat et al., Fischgold and Metzger, and Wackenheim. Interobserver and intraobserver variability, sensitivity, specificity, total percentage of correct results, and negative and positive predictive values were determined for each criterion as well as for various combinations of the criteria.
No single test had a sensitivity and a negative predictive value of greater than 90% as well as a reasonable specificity and a reasonable positive predictive value. The combination of the Clark station, the Redlund-Johnell criterion, and the Ranawat criterion, scored as positive for basilar invagination if any of the three were positive, proved to be better than any single criterion; the sensitivity of the combined criteria was 94%, and the negative predictive value was 91%.
A screening test for basilar invagination should have a high sensitivity and a high negative predictive value, so that the disease will not be missed, and yet be specific, so that the disease will not be overdiagnosed. Our data suggest that none of the widely utilized plain radiographic criteria meet these goals. We recommend that measurements be made according to the methods described by Clark et al., Redlund-Johnell et al., and Ranawat et al. and, if any of these suggests basilar invagination, tomography or magnetic resonance imaging should be performed. Since approximately 6% of the cases of basilar invagination in rheumatoid patients would still be missed with this approach, tomography or magnetic resonance imaging should be performed on a rheumatoid patient whenever plain radiographs leave any doubt about the diagnosis of basilar invagination.