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AO classification of distal femur fractures

Last revised by Andrew Murphy on 17 Oct 2021

The AO classification of distal femoral fractures is one of the commonly used fracture classification systems in orthopedics.

Each long bone has a single number with the parts of the bone denoted numerically, the proximal end is 1, diaphysis is 2, and the distal end is 3. 

The distal femur system's prefix is 33, indicating it is the femur (3) and the distal portion of the femur (3). 

The AO classification divides the distal femoral fractures into three groups, A, B and C with complexity and severity increasing 1.

  • type A: extraarticular 
    • A1: avulsion
    • A2: simple 
    • A3: wedge or multifragmentary
      • A3.1 intact wedge
      • A3.2 fragmentary wedge
  • type B: partial articular, the fracture is involving one part of the articular surface, yet the rest of the joint is still attached to the metaphysis and diaphysis
    • B1: lateral sagittal
      • B1.1 through the notch 
      • B1.2 through load-bearing lateral condyle
      • B1.3 sagittal fragmentary
    • B2: medial sagittal
      • B2.1 through the notch
      • B2.2 through load-bearing medial condyle
      • B2.3 sagittal fragmentary
    • B3: frontal/coronal 
      • B3.1 anterior and lateral flake fracture
      • B3.2 unicondylar posterior 
      • B3.3 bicondylar posterior
  • type C: complete articular, the fracture is distributing the joint surface and separated from the diaphysis
    • C1: simple articular
      • C1.1 above transcondylar axis
      • C1.3 through or below transcondylar axis
    • C2: simple articular with a fragmentary metaphyseal component 
      • C2.1 intact metaphyseal wedge
      • C2.1  fragmentary metaphyseal wedge
    • C3: multifragmentary articular
      • C3.1 simple metaphyseal 
      • C3.2 wedge metaphyseal
      • C3.3 metaphysio-diaphyseal multifragmentary

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Cases and figures

  • Figure 1: classification of extra articular fractures
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  • Figure 2: classification of partial articular fractures
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  • Figure 3: classification of complete articular fractures
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