AO Spine classification of upper cervical injuries

The AO Spine classification of upper cervical injuries aims to simplify and universalise the process of classifying upper cervical injuries and improve interobserver and intraobserver reliability.


Although in routine clinical practice, at least in many institutions, injuries will be described using more 'traditional' terminology (e.g. hangman's fracture of C2, type II dens fracture, etc...) rather than by specifying exact AO nomenclature, these classifications highlight important unifying principles and use increasingly universal terminology. As such, familiarity with them is worthwhile.


Injuries are broadly divided into three types based on location-specific patterns and then further subdivided according to injury type and presence of neurological signs and/or modifying factors.

  • type 1: occipital condyle and occipital cervical joint complex injuries
  • type 2: C1 ring and C1-2 joint complex injuries
  • type 3: C2 and C2-3 joint complex injuries
Injury types (A, B or C)
  • A: bony injury only - considered stable injuries
  • B: tension band injuries - considered potentially unstable injuries
  • C: translation injuries - considered unstable injuries
Neurological signs (N)
  • NX: the patient cannot be examined
  • N0: no neurological deficits
  • N1: transient neurological injury
  • N2: nerve root injury
  • N3: incomplete spinal cord injury
  • N4: complete spinal cord injury
  • +: continued spinal cord compression
Modifiers (M)
  • M1: injury at high risk of non-union with nonoperative treatment
  • M2: injury with significant potential for instability
  • M3: patient-specific factors affecting treatment (e.g. ankylosing spondylitis)
  • M4: vascular injury or abnormality affecting treatment 

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