- 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 in Children. Treatment Outcomes and Neurological Risk Factors
Os odontoideum in children: neurological manifestations and surgical management.