Atul Gupta
Neuroradiology
Overview
Os odontoideum (OO) is an uncommon
craniovertebral junction (CVJ) abnormality
characterized by a separate ossicle superior to
the dens.
 Location:

 Orthotopic – In normal position at tip of dens
 Dystopic – Displaced towards base of occiput where it
may fuse w/clivus or anterior ring of C1. Associated
w/hypoplastic dens
 Spinal canal may narrowed in both types

Size/shape vary, smooth cortical borders

Leads to atlanto-axial instability (both types)

Transverse atlantal ligament is ineffective at restraining
atlantoaxial motion.
B
A
C
Dystopic OO. A. Coronal CT shows OO (arrow) fused with clivus. B.
Coronal CT shows incomplete (right) C1. C. Axial view shows clefts
involving C1 anteriorly & posteriorly & a dysplastic C2.
Dystopic OO. Midsagittal T1
WI shows large OO (arrow)
fused with clivus, small
anterior arch of C1, &
narrowed spinal canal.
A
B
Orthotopic OO. A. Sagittal CT shows large OO (arrow) not
fused with clivus but angled slightly anterior. B. Corresponding
MR T1WI shows narrowed spinal canal.
Causes
Trauma
 Congenital:

 Increased incidence in:
○ Morquio syndrome
○ Multiple epiphyseal dysplasia
○ Down’s Syndrome

There is continuing controversy over its etiology
Diagnosis
o
o
Usually incidentally detected or when symptoms
occur
Open-mouth, anterior-posterior, and flexionextension lateral radiographs
o Gap separating the OO and axis proper should be above
level of superior articular facets
o Hypertrophy of anterior arch of C1
o
o
o
1 mm cuts sagittal CT reconstruction give more
detail into the atlanto-axial junction
MRI – can help visualize spinal cord pathology,
show space available for cord and provide ant-post
canal dimensions
Fluoroscopy is recommended to show instability
A
B
Orthotopic OO. Flexion (A) & extension (B) radiographs
show widening of atlantodental interval compatible with
subluxation & instability.
Differential Diagnosis
Persistent ossiculum terminale
 True hypoplasia of odontoid peg
 Neurocentral synchondrosis
 Odontoid fracture nonunion

Symptoms
Predisposes to increased risk of craniovertebral junction trauma
 Acute neurological dysfunction with an insidious
onset and:

 Torticollis
 Localized pain
 Neurovascular compromise signs

Cervicomedullary compromise may require
neurosurgery in irreducible cranio-cervical
stenosis.
Treatment









Monitor diagnosed patient for:
 Motor dynamics – look for increase in multidirectional movement at
cranio-vertabral junction indicating increased laxity of secondary
ligaments
 Monitor for neurological signs
Dorsal arthrodesis
Posterior atlantoaxial onlay fusion
Posterior atlantoaxial wiring and fusion
Posterior occipitocervical wiring and fusion
Posterior Magerl screw fixation and fusion
Harms technique of C1-2 fusion
Anterior resection of the os fragment
Posterior transarticular screw fixation