A new method for the radiological assessment of vertebral squaring in ankylosing spondylitis.

A method has been developed for the objective assessment of vertebral 'squaring' based on quantitative morphometric analysis of vertebral 'concavity' in lateral radiographs of the lumbar spine. The reference range for vertebral concavity was defined as greater than 1.0-4.0 mm from measurements of 255 radiologically normal lumbar vertebrae in 51 patients with non-specific back pain. Evidence of vertebral squaring, as defined by concavity measurements of 1 mm or less, was found in 28% of vertebrae from 103 patients with ankylosing spondylitis and 8% of vertebrae from 10 patients with Reiter's syndrome. Assessment of vertebral squaring by the concavity measurement was more reproducible than subjective analysis in the diagnosis of vertebral squaring; in a subgroup of 30 patients with seronegative spondyloarthropathy, the interobserver agreement on the presence or absence of vertebral squaring as assessed by two independent clinicians was 84%, compared with 94% using the concavity measurement. Corresponding values for intraobserver agreement were 79% for subjective analysis and 84% for the concavity measurement. The vertebral concavity measurement is performed simply, rapidly (about three minutes for five lumbar vertebrae), and requires no special experience or equipment. As a result of this, the technique may be of value in the radiological diagnosis of ankylosing spondylitis and in assessing changes in vertebral squaring in individual patients with time.