[Value and limits of tomodensitometry applied to the diagnosis of sacroiliitis in young adults: study of 200 cases].

Diagnosis of sacroiliitis may be difficult to establish in patients under 25 years of age since growth is not yet completed and joint damage is often still minor. A prospective study of 200 subjects with a median age of 22 years was carried out to compare the value of CT scan and conventional radiology. The study population included 32 healthy subjects and 168 consecutive patients with presumptive spondylarthropathy including 36 with bilateral sacroiliitis and 8 with unilateral sacroiliitis. Conventional roentgenograms and CT scans were performed in every patient. Blind reading of roentgenograms and CT scans was carried out by two pairs of observers with differing experience. CT scan provided no additional information when reading was done by experienced observers: rates of mistaken and doubtful results were similar with both investigations (10%); specificity of both tests was comparable (90%) but sensitivity was significantly greater for CT scan (91.2%) than for conventional roentgenograms (71.6%), reflecting improved detection of roentgenographically occult sacroiliitis. Less experienced observers obtained better results with CT scans, illustrating the ease of interpretation of CT scan images. Analysis of false-positive CT scans revealed that normal variations and, above all, features due to as yet uncompleted growth were the main sources of mistakes. These mistakes cancelled the advantage of increased sensitivity of CT scan studies and explained why CT scan failed to improve diagnosis.