AO classification of subaxial injuries

The AO Spine classification of subaxial injuries aims to simplify and universalise the classification of subaxial cervical spine fractures and improve interobserver and intraobserver reliability.


Although its existence is widely known among the relevant subspecialty groups, its day-to-day use varies greatly from institution to institution and it is not safe to assume that clinicians reading a report are familiar with it.

The terminology/descriptive terms used in the classification are, however, widely known. Additionally, these classification capture important high-level grouping of injuries and it is, therefore, worth being familiar with them.


The AO Spine subaxial cervical injury classification aims to categorize injuries into 9 main groups (A0-A4, B1-B3, and C) based on the morphology of the injury. Additional features such as facet joint injury, neurologic status and case-specific modifiers are also captured.

Injury type (A, B or C)

First, injuries are categorized into three groups based on the main morphological feature:

  • A: compression fracture of the vertebrae, without tension band (ligamentous) injury
  • B: disruption of anterior or posterior tension band
  • C: displaced fracture, with translocation of one vertebral body relative to another in any direction
A: compression injuries

Type A injuries involve compression of the anterior structures or fractures of the spinal process that are mechanically insignificant

  • A0: no injury or an isolated injury such as a lamina or spinous process fracture
  • A1: compression fracture that only involves one endplate, but does not involve the vertebral body posterior wall
  • A2: split fracture that is oriented either coronal or pincer that includes both endplates, but does not involve the posterior wall
  • A3: incomplete burst fracture that only involves one endplate, any involvement of the posterior vertebral wall results in retropulsion of fragments
  • A4: complete burst fracture that involves both endplates
B: tension band injuries

Type B injuries affect the tension bands anterior or posterior to the cervical spine. It should be noted that any type B injuries that also have translation are automatically type C injuries.

The anterior tension band includes the anterior longitudinal ligament, while the posterior tension band refers to a combination of osseous and ligamentous structures, including the supraspinous ligaments, interspinous ligaments, articular facet capsules, and ligamenta flava, collectively also known as the posterior ligamentous complex.

  • B1: posterior tension band injury (bony) with physical separation between fractured bony structures, anterior structures may also be included
  • B2: posterior tension band injury (bony, capsuloligamentous, ligamentous) with complete separation of the capsuloligamentous or bony capsuloligamentous structures of the posterior aspect. Again this can include anterior structures
  • B3: anterior tension band injury with physical separation between anterior structures with a persistent connection (tethering) of posterior structures
C: translational injury in any axis

Type C injuries are translational injuries with displacement/translation in any direction from one vertebral body relative to another. Specific additional injuries (either type A or facet) should be categorized as a subtype, e.g. ‘type C, subtype A2’.

Facet injuries

Type F injuries are to describe a range of facet joint injuries. In the context of multiple ipsilateral facet injuries, the highest class is used for classification. The "bilateral" modifier is used if both facets on the same level have the same level of injury. If the injury category on either side is different, the right side is described first.

  • F1: non-displaced facet fracture (fragment size <1 cm; <40% lateral mass involvement)
  • F2: facet fracture that can become unstable (fragment size >1 cm; >40% lateral mass involvement or displacement)
  • F3: floating lateral mass due to disruption of pedicle and lamina
  • F4: subluxation that is pathologic or perched/dislocated facet
Neurological signs (N)
  • NX: undetermined
  • N0: neurological intact
  • N1: transient neurological injury: resolved on presentation or <24 hours after injury
  • N2: radiculopathy
  • N3: incomplete spinal cord injury
  • N4: complete spinal cord injury
Modifiers (M)
  • M1: posterior capsuloligamentous injury without associated complete disruption, from a bony perspective the injury may seem stable, however, often seen on MRI the posterior ligaments are damaged
  • M2: critical disk herniation where the nucleus pulposus will be seen protruding posterior to a vertical line along the posterior border of the lowest injured vertebra
  • M3: stiffening/metabolic bone disease, e.g. ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis (DISH)
  • M4: signs of vertebral artery injury

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