AO Spine classification of thoracolumbar injuries

Last revised by Ciléin Kearns on 3 Feb 2023

The AO Spine classification of thoracolumbar injuries is a commonly used thoracolumbar spinal fracture classification system.

The current AO Spine system (2013) supercedes the more complex and less reproducible AO Magerl classification 1. Unlike the other widely used system, the thoracolumbar injury classification and severity score (TLICS), the AO Spine classification system is primarily descriptive rather than aiming to determine treatment, has more options for describing fracture morphology, and is designed for primary evaluation by CT with or without MRI 1. Moreover, separate alphanumeric codes are given for the injury morphology and neurological status in the AO Spine system, whereas TLICS combines these assessments in a numeric score.

The AO Spine system appears suitable for pediatric populations, correlating highly with TLICS (which has been validated for children), intraoperative evaluation of the posterior ligamentous complex 6, and having good intra- and inter-observer reliability 7.

Although the existence of the AO Spine classification is known among international subspecialty groups and most spine surgeons could apply it as or more reliably than prior systems 2, 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 components and descriptive terms used in the classification are, however, an excellent systematic overview of how to assess and describe these injuries. Combining the colloquial descriptor along with the AO Spine alphanumeric classification, even though redundant, may increase acceptance among practitioners who are not familiar ​1.

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

Three separate components to every injury are coded:

  1. injury morphology

  2. neurological status

  3. indeterminate status of ligamentous integrity or presence of comorbid conditions (referred to as modifiers)

Injuries are categorized into three groups, in order of increasing severity: 

  • A: compression injuries

  • B: distraction injuries

  • C: displacement/translational injuries

Injuries are coded according to the vertebral level involved (e.g., T12: A4) except for injuries that involve the discs, facets, or ligaments between vertebrae, which are coded by the motion segment (e.g., T12-L1: B2).

Multiple injury types can be present at the same level, but the more severe injury (type B or C) is the primary coding and the less severe type (type A or B) is specified as a secondary descriptor.

When multiple levels are injured, each injury is classified separately and should be reported in order of declining severity and, in case of ties, cranial to caudal 1.

Type A injuries involve the vertebral body, with the exception of A0.

  • A0: no fracture or clinically insignificant fracture of the spinous or transverse processes

  • A1: wedge compression or impaction fracture, which involves a single endplate of the vertebral body without involvement of the posterior vertebral wall

  • A2: split or pincer type fracture, which involves both endplates without the involvement of the posterior wall

  • A3: incomplete burst fracture, which involves a single endplate along with the posterior vertebral wall

  • A4: complete burst fracture, which involves both endplates along with the posterior vertebral wall; split fractures that involve the posterior vertebral wall are also included

With burst (A3 or A4) fractures, vertical fractures of the lamina may also be present. However, there is no horizontal fracture through the posterior elements or other posterior tension band injury that would qualify the primary injury as B1 or B2 type.

Type B injuries involve either the anterior or posterior tension band and are often combined with type A vertebral body fractures.

  • B1: Chance fracture or pure transosseous tension band disruption, which involves a single vertebra with fracture through the pedicles and out the pars interarticularis or spinous process

  • B2: osseoligamentous posterior tension band disruption, which involves a motion segment with disruption of the posterior ligamentous complex with or without involving the bony posterior elements

  • B3: hyperextension injuries, which disrupt the anterior tension band by tearing the anterior longitudinal ligament and extending either through the intervertebral disc or vertebral body; the injury may extend into the posterior tension band but an at least partially intact posterior hinge prevents complete displacement

Type C injuries involve displacement in any direction. No subtypes are present as there are numerous possibilities of dislocating fractures. However, they should be specified along with relevant vertebral body (A-type) or tension band (B-type) injuries to better describe the morphology.

Distraction (B-type) fractures with clear and complete disruption of both anterior and posterior vertebral elements/tension bands should be described as a type C injury with secondary B descriptor even if there is not displacement at the time of imaging 1.

  • N0: no focal neurological signs present

  • N1: a history of transient neurological deficit

  • N2: current signs or symptoms of radiculopathy

  • N3: an incomplete spinal cord or cauda equina injury

  • N4: complete spinal cord injury (complete absence of motor and sensory function; ASIA A)

  • NX: cannot be assessed (e.g., due to head injury, intoxication, sedation)

  • M1: the presence of tension band injury is indeterminate (whether or not MRI was performed); applies to injuries that seem stable from a bony standpoint (type A) but the possibility of ligamentous insufficiency (type B) remains, which would guide consideration of operative stabilization

  • M2: the presence of co-morbid conditions such as ankylosing spondylitisdiffuse idiopathic skeletal hyperostosisosteopenia, osteoporosis, overlying burns, etc.

The thoracolumbar AOSpine injury score (TL AOSIS) was devised and validated to guide surgical treatment 5. The injury categories correspond to points:

Injury type:

  • A0: 0 points

  • A1: 1 point

  • A2: 2 points

  • A3: 3 points

  • A4: 5 points

  • B1: 5 points

  • B2: 6 points

  • B3: 7 points

  • C: 8 points

Neurologic status:

  • N0: 0 points

  • N1: 1 point

  • N2: 2 points

  • N3: 4 points

  • N4: 4 points

  • NX: 3 points

Patient-specific modifiers:

  • M1: 1 point

  • M2: 0 points

The points from the three categories are added together. Based on a survey of practitioners, the following treatment algorithm was suggested:

  • 0-3 points: conservative treatment

  • 4-5 points: operative or non-operative treatment

  • >5 points: surgical intervention

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Cases and figures

  • Figure 1
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  • Figure 2
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  • L1: A0
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  • L3: A1
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  • L2: A1 (M1)
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  • L3: A2
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  • T12: A3
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  • L5: A4
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  • T12: A4
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  • T11: B1 (A1)
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  • T12: B1 (A3)
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  • L1-2: B2 (L2: A4)
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  • T11-12: B2 (T12: A1)
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  • L1-2: B3 (M2)
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  • L4-5: C (B3; M2)
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  • T5-6: C
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  • T8-9: C
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