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 only three classes of thoracolumbar injuries. 

Three separate components to every fractureinjury are considered, with only the first fully assessable on imaging alonecoded ⁴:

1. injury morphology of the fracture
2. presence of neurological signsstatus
3. presenceindeterminate status of ligamentous injuriesintegrity or co-morbidpresence of comorbid conditions (referred to as modifiers)

Morphology (A, B or C)

Injuries are broadly 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 dislocationligaments 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 anterior portionvertebral body, with the exception 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.

• A0: no fracture or clinically insignificant fracturesfracture of the spinous or transverse processes
• A1: also known as wedge compression or impaction fracture, which involves 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 involvefracture, which involves both endplates without the involvement of the posterior wall
• A3: also known as incomplete incomplete burst fracture, which involves a single end plateendplate along with the posterior vertebral wall
• A4: complete burst fracture, which involves both endplates 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; theysplit fractures that involve both end plates along with the posterior vertebral wall and are also often associated with a laminarincluded

With burst (A3 or A4) fractures, vertical fractures of the lamina may also be present. However, there is no horizontal fracture (insufficient to qualify as athrough the posterior elements or other posterior tension band failure)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: also known as Chance fracturesfracture or pure transosseous tension band disruption; they disrupt, which involves a single vertebra with fracture through the pedicles and out the pars interarticularis or 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, which involves a motion segment with disruption toof the posterior tension bandligamentous complex (flaval, interspinous, and supraspinous ligaments and facet joint capsules) with or without involving the bony posterior bones; a type A fracture is often present and should be specified separatelyelements
• B3: also known as hyperextension injuries; they, which disrupt the anterior tension band by tearing the anterior longitudinal ligament and extendextending either through the intervertebral diskdisc or vertebral body; the injury may extend into the posterior tension band but an at least partially intact posterior hinge prevents complete displacement

C: translationdisplacement/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 signsdeficit
• 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 it is MRIMRI was performed); applies only to vertebral compressioninjuries that seem stable from a bony standpoint (type A) injuriesbut 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