Os Odontoideum



- See: Development and Anomalies of the Axis:

- Discussion:
    - dens may be completely absent, hypoplastic, or incompletely fused to body of C2 (lesion called os odontoideum)
    - the os odontoideum is smaller than the normal dens but size may vary;
           - the base of the dense is almost always hypoplastic;
    - it is located usually in position of the normal odontoid tip (orthotopic) or near basiocciptial bone in the area of the foramen magnum 
           where it may fuse with the clivus (dystopic);
           - often, it is fixed to the anterior ring of atlas, and the two move as a unit;
    - subluxation and instability are common;
    - etiology:
           - some believe it is caused by failure of fusion of the base of odontoid;
           - others believe it results from traumatic process;
           - may result from frx of odontoid synchondrosis before closure at age 5-6 yr;
                  - w/ growth it is postulated, the alar ligaments carry the fragment away from its base;
    - associated syndromes:
           - Morquio's syndrome
           - multiple epiphyseal dysplasia;

- Clinical Presentation:
    - often asymptomatic and discovered incidentally;
    - pts may present w/ no symptoms, local neck symptoms (neck pain, torticollis, or headache);
    - transitory episodes of paresis following trauma;
    - myelopathy (cord compression) or cervical & brain stem ischemia due to vertebral artery compression (seizures, syncope, vertigo,
            visual disturbances);

- Diff Dx:
    - odontoid hypoplasia & os odontoideum:
          - have instability, with displacement of atlas on the axis;
    - non union:
          - os odontoideum may be difficult to differentiate from dens frx non union;
          - w/ non union following a frx'ed dens, narrow line of separation occurs at base of the dens;
          - preservation of the normal shape and size of the dens on AP view is an important distinguishing feature;
          - w/ os odontoideum, gap between os & hypoplastic dens is wide & it usuallly lies well above the level of superior articular facets 
                of axis;
          - os generally does not preserve normal shape or size of dens usually being half size, rounded or oval, and having a smooth 
                uniform cortex;
          - if os is in area of foramen magnum, there is little dx problem;

- Radiographs:
    - os odontoideum may be overlooked without tomograms;
    - appears as a radiolucent oval or round ossicle with a smooth, dense border of bone;
    - free ossicle of os odontoideum usually appears fixed to the anterior arch of the Atlas and moves with it in flexion and extension;
    - if posterior ring of C1 is narrowed and there is abnormal anterior displacement of C1, less space is available for the cord;
    - specific characteristics:
          - w/ os odontoideum, gap between os & hypoplastic dens is wide & it usuallly lies well above the level of superior articular facets 
                 of axis;
          - os generally does not preserve normal shape or size of dens usually being half size, rounded or oval, and having a smooth 
                  uniform cortex;
           - if os is in area of foramen magnum, there is little dx problem;

- Indications for C1-C2 fusion:
    - ADI > 10 mm or SAC < 13 mm;
    - neurological involvement (even if transient);
    - persistent neck discomfort assoc w/ atlantoaxial instability;
    -  transient syncope or neck extension;
    - isolated local symptoms (pain, Torticollis) or brain stem symptoms (diplopia, nystagmus vertigo, dysarthria, dysphagia) are Not indictions;
    - instability w/ vertebral > 3 mm translation on Flexion and Extension Views


Os Odontoideum.

Os odontoideum in children: neurological manifestations and surgical management.                                                             

Natural history of os odontoideum.



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Thursday, December 22, 2011 3:51 pm