Anderson and Montesano classified fractures of the occipital condyles at the foramen magnum of the skull. The authors are from Harborview Medical Center in Seattle and the University of California Davis in Sacramento.


Fractures may be easily overlooked on routine X-rays. CT scan or MRI may be necessary to define the extent of these fractures.



(1) kind of fracture

(2) mechanism

(3) spine stability

Kind of Fracture


Spine Stability


impacted fracture of the condyle; comminution with minimal or no displacement of fragments into the foramen magnum

axial loading onto the atlas

stable; ipsilateral alar ligament functionally inadequate but tectorial membrane and contralateral alar ligament intact


basilar skull fracture; the fracture exits at the occipital condyle into the foramen magnum

direct blow to the base of the skull

stable; tectorial membrane and both alar ligaments are intact


avulsion fracture by the alar ligament; bone fragment displaced towards the odontoid tip

rotation and/or lateral bending of the skull

potentially unstable; tentorial membrane and contralateral alar ligament are loaded




• The avulsed bone fragment in a Type III fracture can injure the brain stem.


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