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?

Explanation

## Biomechanics of Cervical Spine Fractures in Ankylosing Spondylitis ### Why AS Patients Are at High Risk **Key Point:** Ankylosing spondylitis causes progressive ossification of the spinal ligaments and disc spaces, leading to fusion of the entire spine into a rigid, inflexible column. This fundamentally alters the biomechanics of spinal injury. ### The Rigid Lever Arm Mechanism In a healthy spine, the cervical vertebrae move individually and absorb energy through segmental motion. The normal cervical lordosis (forward curve) also distributes forces across multiple segments. In advanced AS: 1. **Ossification of the anterior and posterior longitudinal ligaments** converts the spine into a single rigid structure 2. **Loss of normal cervical lordosis** (the spine becomes straight or kyphotic) eliminates the natural shock-absorbing curve 3. **The entire cervical spine becomes a long, rigid lever arm** When a patient with AS falls or experiences even minor trauma: - The rigid spine cannot absorb energy through segmental motion - The entire cervical column acts as a single lever, concentrating force at the weakest point (often the disc space) - A minor fall that would cause only soft-tissue injury in a normal spine becomes equivalent to a high-energy hyperextension injury - Fractures are often **unstable** and occur through the disc space ("bamboo spine" fracture) **Clinical Pearl:** These fractures are notoriously dangerous because they may appear deceptively simple on imaging but are biomechanically unstable. Neurological injury is common even with minor displacement. ### Why This Patient's Injury Pattern Fits The fracture through the disc space at C5–C6 with retropulsion is classic for AS. The minor fall provided minimal energy, but the rigid spine converted it into a severe hyperextension injury. **High-Yield:** AS patients with cervical spine fractures have high rates of neurological deficit (up to 60%) and mortality if not recognized and stabilized urgently. Even "minor" falls warrant imaging. **Mnemonic:** **RIGID = Risk In Gravity's Inflexible Disc** — the rigidity of the AS spine is the key risk factor. [cite:Harrison 21e Ch 324, Robbins 10e Ch 26]