Single radiograph of the lumbar spine with linear vertical increased density along the spinous processes. This single central radiodense line on frontal radiographs is related to ossification of supraspinous and interspinous ligaments. In addition, there are two more lateral vertical linear lines, which represent ossification of the apophyseal joint capsules. The three vertical ossified lines together make up the trolley-track sign.
Reference
Resnick D, Niwayama G.: Ankylosing Spondylitis. Resnick D. Bone and Joint Imaging, 1st Edition, Phyladelphia, Resnick D. W.B. Saunders 1989:299-319.
