2OverviewOs 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 densSpinal canal may narrowed in both typesSize/shape vary, smooth cortical bordersLeads to atlanto-axial instability (both types)Transverse atlantal ligament is ineffective at restraining atlantoaxial motion.
3BACDystopic 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.
4Dystopic OO. Midsagittal T1 WI shows large OO (arrow) fused with clivus, small anterior arch of C1, & narrowed spinal canal.
5ABOrthotopic OO. A. Sagittal CT shows large OO (arrow) not fused with clivus but angled slightly anterior. B. Corresponding MR T1WI shows narrowed spinal canal.
6Causes Trauma Congenital: Increased incidence in:Morquio syndromeMultiple epiphyseal dysplasiaDown’s SyndromeThere is continuing controversy over its etiology
7Diagnosis Usually incidentally detected or when symptoms occur Open-mouth, anterior-posterior, and flexion-extension lateral radiographsGap separating the OO and axis proper should be above level of superior articular facets Hypertrophy of anterior arch of C11 mm cuts sagittal CT reconstruction give more detail into the atlanto-axial junctionMRI – can help visualize spinal cord pathology, show space available for cord and provide ant-post canal dimensionsFluoroscopy is recommended to show instability
8ABOrthotopic OO. Flexion (A) & extension (B) radiographs show widening of atlantodental interval compatible with subluxation & instability.
9Differential Diagnosis Persistent ossiculum terminaleTrue hypoplasia of odontoid pegNeurocentral synchondrosisOdontoid fracture nonunionThe 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.
10SymptomsPredisposes to increased risk of cranio-vertebral junction traumaAcute neurological dysfunction with an insidious onset and:TorticollisLocalized painNeurovascular compromise signsCervicomedullary compromise may require neurosurgery in irreducible cranio-cervical stenosis.
11Treatment Monitor diagnosed patient for: Motor dynamics – look for increase in multidirectional movement at cranio-vertabral junction indicating increased laxity of secondary ligamentsMonitor for neurological signsDorsal arthrodesisPosterior atlantoaxial onlay fusionPosterior atlantoaxial wiring and fusionPosterior occipitocervical wiring and fusionPosterior Magerl screw fixation and fusionHarms technique of C1-2 fusionAnterior resection of the os fragmentPosterior transarticular screw fixation