1. Atul Gupta Neuroradiology
Os odontoideum

2. 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.

3. 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.

4. Dystopic OO. Midsagittal T1 WI shows large OO (arrow) fused with clivus, small anterior arch of C1, & narrowed spinal canal.

5. 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.

6. Causes Trauma Congenital:
Increased incidence in:
Morquio syndrome
Multiple epiphyseal dysplasia
Down’s Syndrome
There is continuing controversy over its etiology

7. Diagnosis Usually incidentally detected or when symptoms occur
Open-mouth, anterior-posterior, and flexion-extension lateral radiographs
Gap separating the OO and axis proper should be above level of superior articular facets
 Hypertrophy of anterior arch of C1
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

8. A B
Orthotopic OO. Flexion (A) & extension (B) radiographs show widening of atlantodental interval compatible with subluxation & instability.

9. Differential Diagnosis
Persistent ossiculum terminale
True hypoplasia of odontoid peg
Neurocentral synchondrosis
Odontoid fracture nonunion
The ossicle of ossiculum terminale is much smaller than that of os odontoideum. More important, that ossicle lies at the level of the atlantal ring above the transverse atlantal ligament. In this cranial location, ossiculum terminale, unlike os odontoideum, is not associated with significant instability.

10. Symptoms
Predisposes to increased risk of cranio-vertebral 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.

11. 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