Which radiographic feature is characteristic of advanced Ankylosing Spondylitis?

• A. Ossification of posterior longitudinal ligament
• B. Calcified annulus fibrosus
• C. Facet joint fusion only in cervical spine
• D. Bamboo spine

Answer: (D)

In advanced Ankylosing Spondylitis the spine undergoes inflammatory-driven new bone formation along the entheses, especially at the margins of the intervertebral discs. These bridging bony spurs, called syndesmophytes, progressively fuse adjacent vertebrae. As this process continues across the entire spine, the vertebral bodies become united into a single, rigid column. This gives the characteristic bamboo spine appearance on radiographs—a hallmark of advanced disease because it reflects the extensive fusion and loss of spinal mobility.

Other findings listed don’t match the typical pattern of advanced AS. Ossification of the posterior longitudinal ligament, calcified annulus fibrosus, or fusion limited to a particular region like the cervical spine do not describe the diffuse, bridging fusion across the spine that creates the bamboo spine image.