AO Spine classification of upper cervical injuries

Last revised by Dr Francis Deng on 17 Jan 2022

The AO Spine classification of upper cervical injuries is a concise and comprehensive system for categorizing the spectrum of traumatic bony and ligamentous injuries involving the occipital condyles, atlas (C1), or axis (C2).

The AO Spine upper cervical classification system is relatively new, having been described in 2018 and shown to have reproducibility in a pilot study published in 2020 1,2.  Adoption is not yet widespread and at many institutions, injuries will still be described using more traditional terminology or site-specific classification systems. Nonetheless, the AO Spine classification highlights important unifying principles and uses terminology that is common with the more popular AO Spine classifications of injuries in the subaxial cervical and thoracolumbar spine. As such, familiarity with the system is worthwhile.

Injuries are divided into three sites and then further subdivided according to injury type. Additional modifiers indicate the presence of neurological signs and other potentially relevant clinical factors.

Injuries are first categorized anatomically based on the condyle/vertebra involved or its caudal joint.

  • I: occipital condyle and craniocervical junction (occipital cervical joint complex)
  • II: C1 ring and C1-2 joint
  • III: C2 and C2-3 joint

The injury subtypes are hierarchical descriptors that progress from stable to unstable. The highest severity of injury at a given level determines the designation.

  • type A: bony injury only
    • without significant ligamentous, tension band, or disc injury
    • considered stable
  • type B: tension band/ligamentous injuries
    • with or without bony injury
    • without complete separation of anatomic integrity
    • considered stable or unstable depending on injury specifics
  • type C: translation injuries
    • significant vertebral translation in any plane and separation of anatomic integrity through a joint or disc
    • considered unstable

Combining the site and injury type together indicates the primary injury. The nomenclature can either use the I/II/III site category or the actual name of the site 1,3, although it seems inconsistent whether type B and C injuries are designated by the motion segment as in other AO Spine systems or by one level.

  • IA (OC type A): occipital condyle fracture without craniocervical dissociation
  • IB (OC type B): nondisplaced occipital cervical joint complex (craniocervical junction) ligamentous injury (e.g. alar ligament, often requiring MRI for diagnosis)
  • IC (OC type C): occipital cervical joint complex (craniocervical junction) separation/displacement (atlantooccipital dislocation/subluxation)
  • IIA (C1 type A): C1 fracture with intact transverse atlantal ligament (involving bony arch, lateral mass, or transverse process) 
  • IIB (C1 type B): C1 fracture with transverse atlantal ligament injury resulting in lateral mass overhang
  • IIC (C1-2 type C): C1-2 (atlantoaxial) joint pathologic rotational or translational displacement (atlantoaxial subluxation)
  • IIIA (C2 type A): C2 fracture (involving dens, body, pedicle, or posterior arch) without ligamentous or discal injury
  • IIIB (C2 type B): C2-3 tension band/ligamentous injury (e.g., intervertebral disc and posterior longitudinal ligament disruption resulting in angular instability)
  • IIIC (C2-3 type C): C2-3 joint pathologic translation with disc injury
  • NX: the patient cannot be examined
  • N0: neurologically intact (no neurological deficits)
  • N1: transient neurological injury
  • N2: radicular symptoms
  • N3: incomplete spinal cord injury
  • N4: complete spinal cord injury
  • +: continued spinal cord compression

These optional modifiers are patient-specific variables that may affect clinical outcome and therefore inform management 3.

  • M1: injury at high risk of non-union with nonoperative treatment
    • e.g., fracture through the odontoid waist with significant displacement or angulation
  • M2: injury with significant potential for instability
    • e.g., midsubstance tear of the transverse atlantal ligament or significant displacement of the C1 lateral masses >6.9 mm in the coronal plane
  • M3: patient-specific factors affecting treatment
    • e.g., age, comorbidities, smoking status, bone diseases
  • M4: vascular injury or abnormality affecting treatment
    • e.g., vertebral artery dissection, pseudoaneurysm, transection, or arteriovenous fistula

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Cases and figures

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