Denis Three-Column Concept

Disruption of ≥2 columns indicates mechanical instability:

Anterior column: Anterior longitudinal ligament (ALL) + anterior half of vertebral body and disc
Middle column: Posterior half of vertebral body and disc + posterior longitudinal ligament (PLL)
Posterior column: Posterior elements (pedicles, facets, laminae, spinous processes, PLC)

AO Spine Classification

Three injury types reflecting progressive instability. Neurologic modifier (N0–N4/NX) appended; "+" added if continued spinal cord compression.

Modifiers:

M1 = indeterminate tension band injury (may determine need for surgery)
M2 = patient-specific comorbidity affecting surgical decision (e.g., ankylosing spondylitis, burns)

Type A — Compression

Subtype	Description
A0	Minor, nonstructural — transverse process or spinous process fracture; no vertebral body involvement
A1	Wedge-compression — single endplate; posterior wall intact
A2	Split / pincer — both endplates; posterior wall intact
A3	Incomplete burst — one endplate + posterior wall involvement; retropulsion present
A4	Complete burst — both endplates + posterior wall; highest compression severity

AO Type A1 wedge compression fracture — diagram and CT showing single endplate fracture with anterior wedging — Type A1 — wedge compression fracture

AO Type A2 split pincer fracture — diagram, radiograph, and CT showing biconcave endplate fractures with intact posterior wall — Type A2 — split / pincer fracture (both endplates, posterior wall intact)

AO Type A3/A4 burst fracture — diagram and CT showing posterior wall retropulsion and vertebral body comminution — Type A3/A4 — burst fracture with posterior wall retropulsion and canal compromise

Type B — Distraction

Subtype	Description
B1	Transosseous tension band disruption — monosegmental, pure bony; Chance fracture equivalent
B2	Posterior tension band disruption — osseoligamentous; posterior soft tissue + bone involvement
B3	Hyperextension — anterior tension band disruption through disc; anterior column distraction

AO Type B1/B2 flexion-distraction Chance fracture — diagram and CT showing posterior tension band disruption with anterior distraction — Type B1/B2 — flexion-distraction (Chance fracture equivalent); posterior tension band failure

AO Type B3 hyperextension injury — diagram and CT showing anterior tension band disruption through disc with anterior column distraction — Type B3 — hyperextension injury; anterior tension band disruption

Type C — Translation / Dislocation

Subtype	Description
C	Displacement or dislocation in any plane; complete column disruption; always surgical

AO Type C fracture-dislocation — CT sagittal and coronal showing complete translational displacement of the spinal column — Type C — fracture-dislocation; complete translational instability

Posterior Ligamentous Complex (PLC)

The PLC = supraspinous ligament + interspinous ligaments + facet capsules + ligamentum flavum. It serves as the posterior tension band — disruption leads to instability, kyphotic progression, and collapse. Poor healing potential usually requires surgical stabilization.

Posterior ligamentous complex anatomy — supraspinous ligament, interspinous ligament, ligamentum flavum, facet capsules, ALL, PLL, and anterior disc annulus labeled on a spinal motion segment — Spinal column anatomy — anterior tension band (ALL, disc), posterior tension band (PLC: supraspinous, interspinous, ligamentum flavum, facet capsules)

CT signs of PLC disruption (indirect):

Interspinous distance widening (spinous process splaying)
Facet joint widening
Empty / "naked" facet joints
Perched or dislocated facets
Spinous process avulsion fracture
Vertebral body subluxation or dislocation

Posterior endplate avulsion fracture should raise suspicion for flexion-distraction + PLC injury even with minimal kyphosis or height loss. MRI is mandatory before conservative management of burst fractures — osseous retropulsion alone does not indicate PLC injury.

MRI — direct PLC assessment:

PLC Component	Best Sequence	Intact Appearance	Disruption Finding
Supraspinous ligament	Sagittal T1 or T2	Continuous dark stripe between spinous process tips	Loss of stripe; T2 hyperintensity replacing dark line
Ligamentum flavum	Sagittal T1 or T2	Continuous dark stripe between laminae	Absence, T2 signal, or fluid at expected location
Interspinous ligaments	STIR or fat-sat T2	Thin hypointense band between spinous processes	T2/STIR hyperintensity (edema); fluid gap = disrupted
Facet capsules	Axial fat-sat T2	Thin hypointense capsular rim; no joint fluid	Capsular fluid/edema; capsular disruption

Reporting Checklist — Thoracolumbar Trauma

AO Spine type: A0–A4 / B1–B3 / C; degree of comminution
Vertebral body height loss: estimate % anterior compression relative to adjacent levels
Retropulsion: distance (mm); % canal compromise = (1 − x/y) × 100 (x = midsagittal canal at injury; y = mean of levels above and below)
Degree of kyphosis at injury level
PLC predictors on CT: interspinous widening / facet widening / naked facets / perched or dislocated facets / spinous process avulsion / vertebral subluxation
Contiguous and noncontiguous injuries: document all levels
MRI PLC status (intact / indeterminate / disrupted): supraspinous ligament, ligamentum flavum, interspinous ligaments, facet capsules, ALL, PLL, intervertebral disc
MRI neurologic injuries: cord/conus signal change / cauda equina compression / nerve root injury / epidural hematoma
Neurologic modifier (N): N0 intact / N1 transient / N2 radiculopathy / N3 incomplete SCI or cauda equina / N4 complete SCI / NX cannot be examined / + continued compression
Modifiers: M1 indeterminate tension band / M2 patient-specific comorbidity

Reference

Khurana B, Sheehan SE, Sodickson A, Bono CM, Harris MB. Traumatic Thoracolumbar Spine Injuries: What the Spine Surgeon Wants to Know. RadioGraphics. 2013;33(7):2031–2046.

Thoracolumbar Spine Trauma — AO Spine Classification and PLC Assessment