AO fracture classification made easy

It is an alpha-numeric system of classification developed by Muller and colleagues.

AO classification system

Step 1: 1st digit specifies bone

  • 1 = Humerus and Shoulder bones
  • 2r/u = Radius/Ulna
  • 3 = Femur
  • 4t/f = Tibia/Fibula
  • 5 = Spine
  • 6 = Pelvis/Acetabulum
  • 7 = Hand
  • 8 = Foot
  • 9 = Craniomaxillofacial bones

Step 2: 2nd digit specifies segment of bone

  • 1 = Proximal end/Pelvic ring
  • 2 = Diaphyseal/Acetabulum
  • 3 = Distal end
  • 4 = Scapula/Patella/Malleolus (14, 34, 44 respectively)
  • 5 = Clavicle (15)

Square definition: Proximal and distal segments are defined by a square whose sides = length of widest part of epiphysis

Exceptions of square definition:

  • Proximal femur = above a line that passes transversely through the inferior edge of the lesser trochanter

Doesn’t separate metaphysis and epiphysis in proximal and distal segments

Step 3: Alphabet and numbers represents fracture pattern

1. Diaphysis:

  • A = Simple
    • 1 = Spiral
    • 2 = Oblique (>30 degrees)
    • 3 = Transverse (<30 degrees)
  • B = Wedge
    • 1 = Spiral wedge
    • 2 = Bending wedge
    • 3 = Fragmented wedge
  • C = Complex
    • 1 = Complex, spiral
    • 2 = Complex, segmental
    • 3 = Complex, irregular

2. Proximal and Distal ends:

  • A = Extra-articular
    • 1 = Metaphyseal simple
    • 2 = Metaphyseal wedge
    • 3 = Metaphyseal complex
  • B = Partial articular
    • 1 = Lateral condyle, sagittal
    • 2 = Medial condyle, sagittal
    • 3 = Frontal plane
  • C = Complete articular
    • 1 = Articular and metaphyseal simple
    • 2 = Articular simple, metaphyseal multifragmentary
    • 3 = Articular and metaphyseal multifragmentary

Exceptions:

a. Proximal humerus (11):

  • A = Extra-articular unifocal
  • B = Extra-articular bifocal
  • C = Intra-articular

b. Proximal femur (31):

  • A = Trochanteric
  • B = Neck
  • C = Head

c. Malleolar (44):

  • A = Infra-syndesmotic
  • B = Trans-syndesmotic
  • C = Supra-syndesmotic

Source and further reading: https://classification.aoeducation.org/files/download/AOOTA_Classification_2018_Compendium.pdf


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