Key points to differentiate diffuse idiopathic skeletal hyperostosis (DISH) from ankylosing spondylitis (AS) or ossification of the posterior longitudinal ligament (OPLL):

1. dense ossification of anterior longitudinal ligament with or without osteophytes (versus fine calcification of the annulus fibrosis in AS)
2. at least four contiguous vertebral bodies involved
3. intervertebral disc spaces maintained
4. ossification of the superior portion of sacroiliac joints, pelvic ligaments and posterior longitudinal ligament (versus posterior longitudinal ligament ossification with bulky anterior osteophyte formation in OPLL)

Special thanks to Dr Tamer Elholiby.