Ankylosing Spondylitis – Cervical Spine Fracture Biomechanics

A 48-year-old man with a 12-year history of ankylosing spondylitis (AS) presents with acute-onset severe neck pain and bilateral upper limb weakness following a minor fall from standing height. Imaging reveals a hyperextension injury with a fracture through the disc space at C5–C6 with retropulsion of bone into the spinal canal. Which of the following best explains why patients with AS are at dramatically increased risk for unstable cervical spine fractures after minor trauma?