Separation of the odontoid process from rest of the body of the axis was first described in a post mortem specimen In 1863.
Giacomini coined the term os odontoideum for this condition in 1886.
The entity is clinically important because a mobile or insufficient odontoid process renders the transverse ligament ineffective at restraining atlantoaxial motion.
Many cases detected incidentally, others are diagnosed when become symptomatic.
There is continuing controversy over its etiology. Initially, os odontoideum was thought to represent a congenital failure of fusion of the dens to the remainder of the axis. Now its clear that failure of the secondary ossification center of the dens to fuse with the base of the odontoid represents a separate entity known as persistent ossiculum terminale and the Os odontoideum actually represents a previous fracture of the odontoid synchondrosis before its closure at age 5-6 years. There is high incidence of Oo in Morquio syndrome, Multiple epiphyseal dysplasia, Down’s Syndrome.
Size/shape vary with smooth cortical borders (the differentiating point from fracture)
Orthotopic – In normal position at tip of dens.
Dystopic – Displaced towards base of occiput where it may fuse with clivus or anterior ring of C1.
Xray: Open-mouth, anterior-posterior, and flexion-extension lateral radiograph may demonstrate a Gap separating the OO and axis proper. An associated hypertrophy of anterior arch of C1
CT with sagittal CT reconstruction give more detail into the atlanto-axial junction
MRI to evaluate spinal cord for any compression.
Fluoroscopy to show instability
Differential Diagnosis include Persistent ossiculum terminale, True hypoplasia of odontoid peg, Neurocentral synchondrosis, Odontoid fracture nonunion.
Oo predisposes to increased risk of cranio-vertebral junction trauma.
Pt may present with acute neurological dysfunction. Torticollis, localized pain, neurovascular compromise signs.
If cord compression may require neurosurgery.