Chalk stick fracture

Overview

• Definition: A catastrophic, usually 3-column fracture occurring in an ankylosed (fused) spine.
• Biomechanics: The "long lever arm" effect.
  • The fused spine cannot dissipate energy via segmental motion.
  • Force is concentrated at a single focal point, causing a shear or hyperextension injury.
• Etiology:
  1. Ankylosing Spondylitis (Most common due to concurrent osteoporosis).
  2. DISH (Frequent, usually trans-discal).
  3. Surgical fusion (long segment).

Clinical presentation

• Mechanism: Often trivial trauma (e.g., fall from standing/sitting).
• Neurology: High rate of neurological deficit (SCI) because the fracture is highly unstable.
• The "Fatal pause": Patients may present with only "back pain" and no neuro deficit initially. If missed, the spine shifts, shearing the cord later.

Imaging features

Plain radiograph (The "miss" zone)

• Sensitivity: Poor. The fracture line is often horizontal and obscures easily.
• Signs:
  • Discontinuity in the "bamboo" or "candle wax" line.
  • Widening of a disc space (Transdiscal fracture).
  • Look specifically at the cervicothoracic junction (C7/T1) and thoracolumbar junction (T10-L2).

CT (Mandatory)

• Rule: Any patient with AS or DISH + back pain + history of trauma = CT whole spine.
• Findings:
  • "Through-and-through" fracture line involving anterior and posterior elements.
  • "Open book" appearance of the disc space.

MRI

• Role: Assess cord injury and soft tissue.
• Key Finding: Epidural Haematoma.
  • The rigid spine does not tamponade bleeding well. Venous plexus bleeding can compress the cord rapidly even if the bone alignment looks okay.

Complications

1. Spinal Cord Injury (SCI): severe and permanent.
2. Epidural Haematoma: Requires urgent decompression.
3. Pseudoarthrosis (Andersson lesion):
   • If the fracture is missed or not fixed, the constant motion at that single mobile segment creates a "false joint".
   • Leads to destructive endplate changes (mimics infection/discitis).

Management principles

• Stability: The spine is notoriously unstable (like two long sticks held together by a loose hinge).
• Treatment: Almost always surgical fixation (long-segment posterior instrumentation) to re-create the lever arm stability.
• Contraindication: Conservative management (collars/braces) is usually insufficient and dangerous (high rate of non-union and secondary cord injury).

High-yield exam pearls