AO Spine Classification¶

Overview¶

The AO Spine Classification system is a comprehensive, internationally validated framework for classifying thoracolumbar and subaxial cervical spine injuries. Developed by the AO Spine Knowledge Forum, it provides a standardized language for describing fracture morphology, neurological status, and clinical modifiers that together guide treatment decisions.

Thoracolumbar Classification¶

The AO Spine Thoracolumbar Injury Classification uses three primary parameters:

Morphology (Type A, B, C)¶

Type A — Compression Injuries

Compression injuries involve failure of the vertebral body under axial loading. The posterior tension band (posterior ligamentous complex) remains intact.

• A0 — Minor, non-structural fracture (spinous process or transverse process fracture)
• A1 — Compression fracture involving a single endplate with no posterior wall involvement
• A2 — Split (pincer) fracture involving both endplates without posterior wall involvement
• A3 — Incomplete burst fracture — posterior wall involvement with a single endplate fracture
• A4 — Complete burst fracture — posterior wall and both endplates involved

Type B — Distraction Injuries

Distraction injuries involve failure of the anterior or posterior tension band, indicating instability.

• B1 — Transosseous distraction of the posterior tension band (Chance fracture — purely bony injury through the posterior elements)
• B2 — Posterior tension band disruption (ligamentous or combined bony-ligamentous failure of the posterior elements with hyperflexion)
• B3 — Anterior tension band disruption (hyperextension injury — common in ankylosing spondylitis and DISH)

Type C — Translation/Rotation Injuries

Translation injuries involve displacement or dislocation of one vertebral segment relative to another in any plane. These are the most severe injuries and are inherently unstable.

Neurological Status¶

• N0 — Neurologically intact
• N1 — Transient neurological deficit (resolved)
• N2 — Radiculopathy
• N3 — Incomplete spinal cord injury or cauda equina injury
• N4 — Complete spinal cord injury
• NX — Cannot be assessed (obtunded patient)

Modifiers¶

• M1 — Posterior ligamentous complex integrity indeterminate (on MRI)
• M2 — Patient-specific comorbidities affecting treatment (e.g., ankylosing spondylitis, DISH, osteoporosis)

AO Spine Injury Severity Score¶

The AO Spine Thoracolumbar Injury Score assigns points based on morphology, neurology, and modifiers to produce a total score that guides management:

• Score ≤3: Generally conservative management
• Score 4: Either conservative or operative management (surgeon judgment)
• Score ≥5: Generally operative management

Subaxial Cervical Spine Classification¶

A parallel AO Spine system exists for subaxial cervical injuries (C3–C7), using the same Type A/B/C morphological framework with facet injury modifiers (F1–F4):

• F1 — Non-displaced facet fracture
• F2 — Facet fracture with potential for instability
• F3 — Floating lateral mass (ipsilateral pedicle and lamina fracture)
• F4 — Pathological subluxation or perched/dislocated facet

Upper Cervical Spine Classification¶

The AO Spine group has also developed specific classification systems for the craniocervical junction (C0–C2), addressing:

• Occipital condyle fractures
• Atlanto-occipital dislocation
• Atlas (C1) fractures
• Odontoid fractures
• Hangman fractures (C2 pars interarticularis)

Imaging Considerations¶

• CT with multiplanar reformats is essential for accurate morphological classification — sagittal and coronal reformats are critical for identifying posterior wall involvement, endplate fractures, and translation
• MRI is necessary for assessing the posterior ligamentous complex (modifier M1) and neurological status
• Vertical lamina fractures frequently accompany burst fractures (Type A3/A4) and do not by themselves constitute posterior tension band failure

Key Points¶

• The AO Spine Classification uses fracture morphology (A/B/C), neurological status (N0–N4), and modifiers (M1/M2) to classify spinal injuries
• Type A injuries are compression, Type B are distraction, and Type C are translation — reflecting increasing severity and instability
• The injury severity score guides management: ≤3 conservative, ≥5 operative
• CT is essential for morphological classification; MRI is required for PLC assessment
• Specific AO Spine classification systems exist for the upper cervical spine (C0–C2) and subaxial cervical spine (C3–C7)

References¶

1. Vaccaro AR, Oner C, Kepler CK, et al. AOSpine thoracolumbar spine injury classification system: fracture description, neurological status, and key modifiers. Spine (Phila Pa 1976). 2013;38(23):2028-37. Available from: https://pubmed.ncbi.nlm.nih.gov/23970107/
2. Vaccaro AR, Koerner JD, Radcliff KE, et al. AOSpine subaxial cervical spine injury classification system. Eur Spine J. 2016;25(7):2173-84. Available from: https://pubmed.ncbi.nlm.nih.gov/25716661/
3. Kepler CK, Vaccaro AR, Schroeder GD, et al. The Thoracolumbar AOSpine Injury Score. Global Spine J. 2016;6(4):329-34. Available from: https://pubmed.ncbi.nlm.nih.gov/27190734/
4. Vaccaro AR, Schroeder GD, Kepler CK, et al. The surgical algorithm for the AOSpine thoracolumbar spine injury classification system. Eur Spine J. 2016;25(4):1087-94. Available from: https://pubmed.ncbi.nlm.nih.gov/25953527/
5. Vaccaro AR, Lambrechts MJ, Karamian BA, et al. AO Spine upper cervical injury classification system: a description and reliability study. Spine J. 2022;22(12):2042-9. Available from: https://pubmed.ncbi.nlm.nih.gov/35964830/
6. AO Spine classification of thoracolumbar injuries. Radiopaedia. Available from: https://radiopaedia.org/articles/ao-spine-classification-of-thoracolumbar-injuries-1

Related Articles¶

• Denis Three-Column Model
• TLICS
• SLIC
• Thoracolumbar Burst Fracture
• Chance Fracture