- 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
x
management.
x
Natural history of os odontoideum.
