Interobserver agreement using Schlapbach graded scale for diffuse idiopathic skeletal hyperostosis (DISH): can we reduce the cut-off point of vertebral affection?

Resnick-Niwayama criteria for diagnosing DISH depict an advanced stage, and a new reduced cut-off point with three contiguous vertebrae affected (two bone bridges) has been proposed. The aim has been to know the interobserver agreement by using a graded scale of DISH in which grade II matches with the new proposed cut-off point and grade III matches with the first criterion of Resnick-Niwayama. Males ≥ 50 years and postmenopausal women included in a population-based prospective study (the Camargo Cohort) were analyzed. Sample size was obtained according to an expected kappa of 0.95 and an accuracy of ± 8%. Three physicians applied independently Schlapbach graded scale (ranged from grade 0, no ossification, to grade III, ≥ 3 consecutive bone bridges) on the lateral radiographs of thoracic and lumbar spine of participants. We calculated inter- and intra-observer agreement and correlation. One hundred and fifty eight radiographs (79 patients, 68 ± 9 years) were assessed. Kappa values (95% confidence interval) for grades 0, I, II, and III were 0.63 (0.50-0.77), 0.49 (0.37-0.62), 0.32 (0.17-0.47), and 0.69 (0.60-0.77), respectively. Weighted kappa for the three pairs of raters were 0.87 (0.82-0.93), 0.84 (0.77-0.91), and 0.81 (0.72-0.90). Grade III was the image that generated greater agreement, while a significant decrease was noted in grade II, the new proposed criterion. The simultaneous presence of an incomplete DISH and osteoarthritis, in a thoracic spinal segment with peculiar anatomical characteristics (reduced disk spaces, kyphotic curve), is thought to be a major cause of variability in the results.