Chalk-Stick Fracture in Ankylosing Spondylitis

Clinical Background

A 77-year-old patient with Ankylosing Spondylitis presented after a low-energy fall with acute thoracic pain.

Imaging Findings

• Extensive bridging syndesmophytes throughout the spine
• Complete spinal ankylosis (“bamboo spine” morphology)
• At the T3–T4 level, a transdiscal fracture is observed
• Fracture line extends:
  • From anterior fused syndesmophyte
  • Through the intervertebral disc
  • Into the posterior elements (facet joints)
• Associated marrow edema and soft tissue changes on MRI

This represents a classic “chalk-stick fracture”, a transverse fracture through a rigid, ankylosed spine.

 

 

 

 

   

Diagnosis

Chalk-stick fracture of the ankylosed thoracic spine

 

Pathophysiology (Why it happens?)

• Ankylosed spine behaves as a long rigid lever arm
• Loss of segmental mobility > stress concentration at weak points
• The intervertebral disc space becomes the weakest link
• Even minor trauma can result in highly unstable three-column fractures

 

Key Radiological Insight

In ankylosed spines, the fracture is not where you expect—it is where the spine is weakest: the disc level

 

TEACHING PEARLS

 1. Always suspect fracture—even after trivial trauma

• Ankylosed spine = “fracture-prone spine”
• Low-energy trauma can cause catastrophic instability

2. The fracture is typically transdiscal

• Most occur at disco-vertebral junctions
• Especially:
  • Lower cervical
  • Cervicothoracic junction
  • Thoracolumbar junction

3. Look beyond plain radiographs

• X-ray may miss fractures
• CT (especially sagittal reconstructions) is essential
• MRI → evaluates:
  • Bone marrow edema
  • Ligamentous injury
  • Spinal cord involvement

4. Follow the syndesmophytes

• Continuity disruption = fracture
• Carefully track the anterior longitudinal ossification line

5. Think “three-column injury”

• These fractures are usually:
  • Highly unstable
  • Extend through anterior + middle + posterior columns
• High risk of neurological deterioration

6. Don’t forget the mimickers / associations

Also seen in:

• Diffuse Idiopathic Skeletal Hyperostosis
• Surgical spinal fusion
• Ossified ligaments (OPLL / OLF)

“An ankylosed spine is not strong—it is brittle.”
 “In trauma, systematically check every disc level.”
 “Missed fracture = delayed neurological catastrophe.”