Os%20odontoideum

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Os odontoideum

• Atul Gupta
• Neuroradiology

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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.

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

• Trauma
• Congenital
• Increased incidence in
• Morquio syndrome
• Multiple epiphyseal dysplasia
• Downs Syndrome
• There is continuing controversy over its etiology

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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

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

• Persistent ossiculum terminale
• True hypoplasia of odontoid peg
• Neurocentral synchondrosis
• Odontoid fracture nonunion

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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.

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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