AO Spine Thoracolumbar Injury Classification System

A fracture in a fused, rigid spine (ankylosing spondylitis or DISH) is a different and far more dangerous entity — exactly what the M2 modifier flags:

• It behaves like a long-bone fracture. The brittle, fused column has long lever arms, so even a low-energy injury (a ground-level fall) produces a transverse fracture through all three columns, typically an extension (B3) pattern through the ossified disc or across bone — and it is highly unstable.
• It is easily MISSED and prone to deterioration. The fracture is hard to see against the ankylosed bone and the pain is often blamed on the underlying disease; there is a high rate of epidural haematoma and delayed/secondary neurological deterioration. Mortality is high.
• Image the WHOLE spine. Plain films are unreliable → CT of the entire spine (look for non-contiguous fractures) plus MRI (epidural haematoma, occult fracture, PLC). Have a very low threshold in any ankylosed patient with new back pain after even trivial trauma.
• Management: usually long-segment posterior instrumented fixation (several levels above and below — short constructs fail in this stiff bone). Positioning is critical — transfer and position the patient maintaining their pre-existing fixed kyphotic deformity; forcing them supine/flat or into extension can displace the fracture and injure the cord. Avoid over-reduction.

The thresholds are useless without the inputs. Add the points from the three axes:

• Morphology: A0 = 0, A1 = 1, A2 = 2, A3 = 3, A4 = 5; B1 = 5, B2 = 6, B3 = 7; C = 8.
• Neurology: N0 = 0, N1 = 1, N2 = 2, N3 = 4, N4 = 4, NX = 3 (not assessable scores like an incomplete deficit, so it errs toward surgery).
• Modifiers: M1 (indeterminate PLC) = 1; M2 (comorbidity) = 0.

Sum, then map: total 3 or less → trial of conservative care, 4 or 5 → grey zone (either acceptable), 6 or more → surgery.

Worked examples: an A1 wedge, neurology intact = 1 + 0 = 1 → brace; an A3 incomplete burst, intact = 3 → conservative, but add an indeterminate PLC (M1, +1) = 4 → grey zone; an A4 complete burst with an incomplete cord injury (N3) = 5 + 4 = 9 → surgery; any Type C starts at 8, so it is always surgical. Notice that any single tension-band (B) or translation (C) morphology, or an N3/N4 deficit, already pushes the total to 5 or more — which is why "B/C or a deficit means operate" is the shortcut.