A 24-year-old man with chronic low back pain and morning stiffness has pelvic and lumbar spine radiographs showing bilateral symmetric sacroiliac joint erosion and sclerosis, thin vertical syndesmophytes bridging multiple vertebral bodies, and a central vertical dense line over the spinous processes. The central dense line represents:

• A Ossification of the interspinous and supraspinous ligaments (dagger sign) ✓
• B Ossification of the anterior longitudinal ligament
• C Calcification of the nucleus pulposus
• D Baastrup disease with interspinous bursitis

Explanation

In advanced ankylosing spondylitis, ossification of the interspinous and supraspinous ligaments produces a single central radiodense line on the frontal radiograph called the dagger sign. Bilateral symmetric sacroiliitis plus marginal syndesmophytes forming a bamboo spine confirm the diagnosis. Flowing ossification of the anterior longitudinal ligament with preserved disc spaces defines DISH, which spares the sacroiliac joints, and Baastrup disease causes kissing spines posteriorly without a continuous central line.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.