The AO Spine classification of sacral injuries aims to simplify and universalise the process of classifying sacral injuries and improve interobserver and intraobserver reliability.

The AO Spine sacral classification is broken into three subsections that follow a hierarchical structure similar to the AO classifications of the thoracolumbar spine.

Injury types (A, B or C)

Injuries are broadly categorised into three groups based on the region of injury: 

• A: lower sacrococcygeal injuries

• B: posterior pelvic injuries

• C: spinopelvic injuries

A: lower sacrococcygeal injuries

Type A injuries involve the lower portion of the sacrococcygeal region. The stability of both the pelvis and the spinopelvic region are unaffected by type A injuries, however higher A-type injuries can have associated neurological impact.

• A1: injury to the coccygeal region or compression injuries including that of ligamentous avulsion fractures

• A2: fractures below the level of the sacroiliac joint that are non-displaced and transverse in orientation

• A3: fractures below the level of the sacroiliac joint that are displaced and transverse in orientation

B: posterior pelvic injuries

Type B injuries are categorised as fractures of the sacrum which are unilateral and longitudinal in orientation whereby the ipsilateral superior S1 facet is no longer discontinuous with the medial aspect of the sacrum. Unlike type A, type B injuries have an impact on the stability of the pelvis, however a minimal impact on the spinopelvic region.

• B1: a fracture that is longitudinal in orientation, medial to the foramen and involves the spinal canal

• B2: a fracture that is longitudinal in orientation, lateral to the foramen and does not involve the spinal canal nor the foramen

• B3: a fracture that is longitudinal in orientation involving the foramen but not the spinal canal

C: spinopelvic injuries

Type C injuries always result in instability of the spinopelvic region 

• C0: nondisplaced sacral U-type variant fracture

• C1: a sacral U-type variant without any posterior pelvic instability

• C2: bilateral complete B-type fracture without transverse component

• C3: displaced sacral U-type variant fracture

Neurological signs (N)

• Nx: the patient cannot be examined

• N0: no neurological deficits

• N1: transient neurological injury

• N2: nerve root injury

• N3: cauda equina syndrome/incomplete spinal cord injury

• N4: complete spinal cord injury

Modifiers (M)

• M1: soft tissue injury

• M2: metabolic bone disease

• M3: anterior pelvic ring injury

• M4: sacroiliac joint injury

See also

• AO Spine classification systems

  • AO Spine: upper cervical classification system

  • AO Spine: subaxial cervical classification system

  • AO Spine: thoracolumbar classification system