General
- May mimic Type 1 or 2 odontoid fracture.
Definition
- A separate bone ossicle of variable size with smooth cortical borders separated from a foreshortened odontoid peg, occasionally may fuse with the clivus.
Aetiology
- Is debated with evidence to support both of the following (diagnosis & treatment do not depend on which etiologic theory is correct)
- Congenital
- Developmental anomaly (Nonunion of dens to body of axis).
- See central pillar condition for pathophysiology
- True os odontoideum is rare.
- Acquired
- Postulated to represent an old nonunion fracture or injury to vascular supply of developing odontoid
Clinical features
- Asymptomatic
- Most patients are neurologically intact
- Incidental finding
- Atlantoaxial instability, which may be discovered incidentally.
- Symptomatic: 3 groups of patients
- Those with occipito-cervical pain alone
- Those with myelopathy
- Subcategorized further into
- Those with transient myelopathy (commonly after trauma)
- Those with static myelopathy
- Those with progressive myelopathy
- Those with intracranial symptoms or signs from vertebrobasilar ischemia.
- Some may have spinal cord injury after seemingly minor trauma have been reported.
- Many symptomatic and asymptomatic patients have been reported with no new problems over many years of follow-up.
Investigation
- Critical to R/O C1–2 instability.
- However, myelopathy does not correlate with the degree of C1–2 instability.
- An AP canal diameter < 13mm does correlate with the presence of myelopathy.
- Appears like a type II odontoid # on Xray
- C-spine X-rays: AP, open-mouth odontoid, lateral (static & flexion-extension)
- Plain dynamic radiographs (flexion and extension)
- Use to show
- Degree of abnormal motion between C1 and C2
- Narrowest canal diameter.
- Have two types of motion
- Orthotopic
- Orthotopic means that a structure is located in its normal or usual anatomical position
- Ossicle moves with the anterior arch of C1
- No discernible instability or “posterior instability” with the os odontoideum moving posteriorly into the spinal canal during neck extension.
- Dystopic
- Dystopic means a faulty, abnormal, or malpositioned location of a structure. i.e. it is not in its usual site.
- Ossicle is functionally fused to the basion.
- May sublux anterior to the C1 arch
- Anterior instability os odontoideum translating forward in relation to the body of C2.
- The degree of C1-C2 instability identified on cervical X-rays does not correlate with the presence of myelopathy.
- A sagittal diameter of the spinal canal at the C1-C2 level of 13 mm does correlate with myelopathy detected on clinical examination.
Dystopic os odontieum | os Avis | |
The fused portion to the basion is | Proatlas sclerotome + C1 resegmented sclerotome | Proatlas sclerotome |
The intervertebral boundary zone (IBZ) that is missing | Lower dental synchondrosis | Upper dental synchondrosis |
- MRI of craniocervical junction
- MRI can depict spinal cord compression and signal changes within the cord that correlate with the presence of myelopathy.
Management
Conservative
- Indication
- No Neurological deficit
- No signs of instability on flexion and extension X-rays
- Clinical and x ray surveillance
- Fibrous non union is acceptable results
- Regardless of whether Os odontoideum is congenital or an old nonunion fracture, immobilization is unlikely to result in fusion.
- Therefore, when treatment is elected, surgery—usually atlantoaxial arthrodesis is required.
Surgery
- Indication
- Neurological deficit
- C0-C2 fixation and fusion + C1 laminectomy + Ventral decompression (Transoral odontoidectomy)
- For pt with irreducible Cervicomedullary compression
- Clinical or radiological C1/2 instability
- As minor trauma can lead to spinal cord injury
- Technique
- Posterior C1/2 fixation and fusion
- Sublaminar wiring techniques (Gallie or Brooks)+ post op Halo
- Odontoid screw fixation has no role in the treatment of this disorder.