A patient with ankylosing spondylitis (AS) develops a fracture through a rigid ankylosed cervical spine after a trivial fall. Which description best characterises the biomechanical risk of this fracture pattern?

• A Low energy fractures through osteoporotic vertebral bodies only
• B Stable fractures because the ligamentous fusion provides additional stability
• C Highly unstable fracture acting as a 'long-bone fracture' through the fused spine — high risk of neurological compromise ✓
• D Hyperflexion injury analogous to a distraction fracture in a normal spine

Explanation

In ankylosing spondylitis, the spine is fused into a rigid column (bamboo spine). When a fracture occurs, it typically propagates through all three columns simultaneously (distraction injury), analogous to a transverse fracture through a long bone. This 'chalk-stick' fracture is highly unstable because there is no motion segment to distribute forces. The lever arm effect of the rigid fused spine magnifies displacing forces, conferring high rates of neurological injury (>50% with cervical fractures). Management requires posterior long-segment instrumented fusion with an urgent surgical approach.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.