Os Odontoideum

  • Etiology: Congenital lesion due to failure of center of ossification of dens to fuse with body of C2 versus unremembered and-or unrecognised fracture through growth plate of dens before age of 5 or 6
  • Imaging:
    — Smooth well-corticated ossicle at superior component of hypoplastic dens
    — Around half the size of normal dens
    — Associated with hypertrophied and rounded anterior arch of atlas
    — Can be orthotopic with os odontoideum in normal position with wide gap between C2 and os odontoideum or dystopic with the os odontoideum being displaced
  • DDX: Persistent ossiculum terminale, Type 2 odontoid fracture
  • Complications: Atlantoaxial instability
  • Treatment: Surgical fusion
  • Clinical:

Radiology Cases of Os Odontoideum

CT of os odontoideum
Sagittal CT without contrast of the cervical spine (left) shows a large well-corticated ossicle posterior and superior to the anterior arch of C1 and superior to the dens, hypertrophy of the anterior arch of C1, and a large atlanto-dens interval due to atlanto-axial dislocation. Sagittal T2 MRI without contrast of the cervical spine (right) shows a narrow spinal canal secondary to the atlanto axial dislocation with spinal cord compression and abnormal bright signal in the spinal cord at C1-C2.