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 densDystopic – 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.

A

B

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.

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.

A B

Causes

Trauma Congenital:

Increased incidence in:○ Morquio syndrome○ Multiple epiphyseal dysplasia○ Down’s Syndrome

There is continuing controversy over its etiology

Diagnosis

o Usually incidentally detected or when symptoms occur

o Open-mouth, anterior-posterior, and flexion-extension lateral radiographso Gap separating the OO and axis proper should be above

level of superior articular facetso  Hypertrophy of anterior arch of C1

o 1 mm cuts sagittal CT reconstruction give more detail into the atlanto-axial junction

o MRI – can help visualize spinal cord pathology, show space available for cord and provide ant-post canal dimensions

o 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 cranio-

vertebral junction trauma Acute neurological dysfunction with an insidious

onset and:TorticollisLocalized painNeurovascular 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