Anderson and Montesano classification of occipital condyle fractures

Last revised by Dr Francis Deng on 18 Jan 2022

The Anderson and Montesano classification is a widely used system for describing occipital condyle fractures. It divides injuries into three types based on morphology and mechanism of injury 1-5.

  • type I: impacted type occipital condyle fracture
    • morphology: comminution of the condyle with minimal or no displacement of fragments into the foramen magnum
    • mechanism: axial loading of the skull onto the atlas
    • stability: stable because the tectorial membrane and contralateral alar ligament are intact (the ipsilateral alar ligament may be functionally inadequate)
  • type II: basilar skull type occipital condyle fracture
    • morphology: condyle fracture that extends outside of the condyle to elsewhere in the posterior base of skull
    • mechanism: direct blow to the lower skull
    • stability: stable because the tectorial membrane and alar ligaments are intact
  • type III: avulsion type occipital condyle fracture
    • morphology: small inferomedial occipital condyle fracture fragment displaced toward the odontoid tip
    • mechanism: rotation and/or contralateral bending
    • stability: potentially unstable due to loading of the contralateral alar ligament and tectorial membrane

A range of incidences of the Anderson and Montesano types amongst all occipital condyle fractures has been reported 2,4,6:

  • type I: 3-13%
  • type II: 22-54%
  • type III: 33-75%

The classification was described in 1988 by American orthopedic surgeons Paul A. Anderson and Pasquale X. Montesano on the basis of 6 patients diagnosed by conventional anterior posterior polytomography or CT with coronal reconstructions 1.

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Cases and figures

  • Figure 1: type 1
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  • Case 1: type 1
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  • Case 2: type 1
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  • Case 3: type 1
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  • Figure 2: type 2
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  • Case 4: type 2
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  • Case 5: bilateral type 2
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  • Figure 3: type 3
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  • Case 6: type 3
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  • Case 7: type 3
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  • Case 8: bilateral type 3
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