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 ³.

Unlike the other widely used system, the thoracolumbar injury classification and severity score (TLICS) ¹, the AO Spine classification system does not aim to determine treatment.

Usage

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. Combining the colloquial descriptor along with the AO Spine alphanumeric classification, even though redundant, may increase acceptance among practitioners who are not familiar ^​1.

Classification system

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

Three separate components to every injury are coded ⁴:

1. injury morphology
2. neurological status
3. indeterminate status of ligamentous integrity or presence of comorbid conditions (referred to as modifiers)

Morphology (A, B or C)

Injuries are categorised 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.

A: compression injuries

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.

B: distraction injuries

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 (flaval, interspinous, and supraspinous ligaments and facet joint capsules) with 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

C: displacement/translational injuries

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.

Neurological signs (N)

• 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) ²
• NX: cannot be assessed (e.g., due to head injury, intoxication, sedation)

Modifiers (M)

• M1: the presence of tension band injury is indeterminate based on spinal imaging (whether or not MRI was performed); applies to injuries that seem stable from a bony standpoint (type A) but ligamentous insufficiency would help determine consideration of operative stabilization
• M2: the presence of co-morbid conditions such as ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, osteopenia, osteoporosis, overlying burns, etc.

Scoring

The thoracolumbar AOSpine injury score (TL AOSIS) was devised and validated to guide surgical treatment ⁶. 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

See also

• AO Spine classification systems
  • AO Spine: upper cervical classification system
  • AO Spine: subaxial cervical classification system
  • AO Spine: sacral classification system