Ankylosing spondylitis – dagger sign

Ankylosing spondylitis. This patient illustrates a nice example of the ‘dagger’ sign of AS – ossification of the ligaments between the spinous processes of the vertebral bodies has resulted in a thick line of sclerosis (orange arrows), which looks a little like a blade. There is also ossification along the edges of the disk spaces (white arrows), and fusion of both sacroiliac joints (yellow arrows).