AO spine classification of thoracolumbar injuries

The AO Spine classification of thoracolumbar injuries is one of the more commonly used thoracolumbar spinal fracture classification systems and aims to simplify and universalise the process of classifying spinal injuries and improve interobserver and intraobserver reliability .

Unlike the other widely used system, the thoracolumbar injury classification and severity score (TLICS) , the AO Spine classification system does not aim to determine treatment.


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. It is therefore not wise to simply describe an injury as "B2".

The terminology/descriptive terms used in the classification are, however, widely used and also are an excellent systematic overview of how to think about and describe these injuries. As such, familiarity with them is worthwhile.

Classification system

The AO Spine thoracolumbar classification system consists of only three classes of thoracolumbar injuries.

Three separate components to every fracture are considered, with only the first fully assessable on imaging alone .

  • morphology of the fracture
  • presence of neurological signs
  • presence of ligamentous injuries or co-morbid conditions (referred to as modifiers)
  • Morphology (A, B or C)

    Injuries are broadly categorized into three groups:

    • A: compression injuries
    • B: distraction injuries
    • C: displacement or dislocation
    A: compression injuries

    Type A injuries involve the anterior portion of the vertebral column with an intact posterior tension band (the group of muscles, ligaments and processes/pedicles that maintain the integrity of the vertebral column).

    • A0: no or clinically insignificant fractures of the spinous or transverse processes
    • A1: also known as wedge compression injuries; they involve a single anterior or middle endplate of the vertebral body without the involvement of the posterior aspect of the posterior vertebral wall
    • A2: also known as split or pincer type injuries; they involve both endplates without the involvement of the posterior wall
    • A3: also known as incomplete burst injuries; they involve a single end plate along with the posterior vertebral wall; a vertical laminar fracture is usually also present (insufficient to qualify as a tension band failure)
    • A4: also known as complete burst injuries; they involve both end plates along with the posterior vertebral wall and are also often associated with a laminar fracture (insufficient to qualify as a tension band failure)
    B: distraction injuries

    Type B injuries involve the anterior or posterior tension band.

    • B1: also known as Chance fractures or pure transosseous tension band disruption; they disrupt the pedicles and spinous process in a single vertebral level; a distracted horizontal fracture through the vertebral body is often but not necessarily present
    • B2: also known as osseoligamentous posterior tension band disruption; they involve an intervertebral body level with disruption to the posterior tension band ligaments with or without involving the posterior bones; a type A fracture is often present and should be specified separately
    • B3: also known as hyperextension injuries; they disrupt the anterior tension band and extend through the intervertebral disk or vertebral body
    C: translation injuries

    Type C injuries involve displacement in any direction. No subtypes are present as there are numerous possibilities of dislocating fractures.

    Neurological signs (N)
    • N0: no focal neurological signs present
    • N1: a history of transient neurological signs
    • N2: current symptoms of radiculopathy
    • N3: an incomplete spinal cord or cauda equina injury
    • N4: complete spinal cord injury (complete absence of motor and sensory function)
    • NX: cannot be assessed
    Modifiers (M)
    • M1: the presence of tension band injury is indeterminate based on spinal imaging (whether or not it is MRI); applies only to vertebral compression (type A) injuries
    • M2: the presence of co-morbid conditions such as ankylosing spondylitis, osteopenia, osteoporosis, overlying burns, etc.

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