The Anderson and Montesano classification system divides injuries into three types based on morphology and mechanism of injury:

• type I - comminuted (3-15%)
  • impaction fracture of occipital condyle
  • associated with axial compression injury
  • stable injury
• type II - basilar skull (25-50%)
  • basilar skull fracture extending into the occipital condyle
  • associated with a direct blow to the lower skull
  • stable injury
• type III - avulsion (35-75%)
  • condylar avulsion injury at the alar ligament attachment
  • associated with forced contralateral bending and rotation
  • potentially unstable injury