Os odontoideum is a condition characterized by a smooth, well-corticated, round or oval ossicle separated from the body of the axis by a gap, representing a detached odontoid process. It is considered either a congenital segmentation anomaly or an acquired post-traumatic non-union of the odontoid synchondrosis.

 

Embryology and Pathogenesis:

 

The odontoid process develops from multiple ossification centers; one for the basal (body) portion and another for the apical portion (ossiculum terminale). Fusion with the C2 body occurs through the dentocentral synchondrosis (also known as the subdental synchondrosis) during early childhood.

At birth, this synchondrosis separates the dens from the C2 body. It normally begins to fuse around 3 years of age and is completely fused by 5–6 years. Persistence beyond 7 years is considered abnormal.

Failure of this fusion (congenital) or post-traumatic separation before fusion (acquired) results in os odontoideum. Hence, os odontoideum represents either a developmental anomaly of the dens or the sequela of early childhood fracture before synchondrosis closure.

 

Classification:

 

1. Orthotopic type – The ossicle lies in the normal anatomic position, in line with the anterior arch of the atlas.
2. Dystopic type – The ossicle is displaced upward toward the foramen magnum, sometimes even articulating with the clivus.
Both forms may be associated with atlantoaxial instability.

 

Associated Conditions:

 

Os odontoideum frequently coexists with congenital and craniovertebral anomalies such as Chiari I malformation, basilar invagination, platybasia, occipitalization of the atlas, and Klippel–Feil syndrome. These associations support the congenital or developmental theory in many cases.

 

Clinical Presentation:

 

Patients may be asymptomatic or present with symptoms caused by instability at the atlantoaxial junction or direct cord compression, including neck pain or stiffness, restricted cervical motion, torticollis, transient quadriparesis after minor trauma, or progressive myelopathy. Sudden neurological deterioration after trivial trauma is a known presentation.

 

Radiologic Features:

 

Plain radiographs show a well-corticated, oval or round ossicle separated from the axis body by a distinct gap. Dynamic flexion–extension X-rays demonstrate atlantoaxial instability. CT defines the corticated ossicle, the degree of separation, and bone remodeling at the base of C2. MRI shows spinal cord compression, myelomalacia, or associated anomalies such as Chiari I malformation or basilar impression.

 

Management:

 

Asymptomatic and stable lesions are managed conservatively with follow-up. Symptomatic or unstable cases require surgical stabilization. 

 

The standard treatment is posterior C1–C2 fixation (Goel–Harms or transarticular screws). Occipitocervical fusion is preferred if posterior elements are dysplastic or instability extends to the occiput.

 

Complications of C1–C2 Fixation in a Young Patient:

 

Intraoperative and immediate complications: vertebral artery injury, dural tear and CSF leak, neural injury (C2 root or spinal cord), hardware malposition or loosening, and wound infection or hematoma.

Delayed complications: pseudoarthrosis or nonunion, loss of fixation, adjacent segment degeneration, growth disturbance, and loss of neck mobility due to fusion.

 

Degrees of Movement Lost After C1–C2 Fusion:

 

The atlantoaxial joint contributes approximately 50–60% of cervical rotation and a smaller component of flexion–extension.
 

Rotation: each side provides around 40°–45°, so total rotation (~80–90°) is reduced by about half after fusion.
 

Flexion–Extension: contributes 10°–15°, so mild reduction in nodding movement occurs.
 

Lateral bending: minimal (<5°).

After fixation, about 50% of rotational mobility is lost but flexion, extension, and lateral bending are largely preserved, allowing normal function through compensation by subaxial cervical segments.

 

Differentiation Between Os Odontoideum and Odontoid Fracture (Type II):

 

Os odontoideum shows smooth, corticated margins, a smaller rounded ossicle, and chronic bone remodeling, while type II odontoid fracture shows sharp, irregular non-corticated edges, normal-sized dens, and soft tissue swelling.

Radiologic differences:
Os odontoideum: wide, uniform gap (2–4 mm), hypertrophied anterior arch of atlas, smooth concave upper C2 surface, no marrow edema.
Fracture: irregular gap, sharp surfaces, prevertebral swelling, marrow edema on MRI.

Associated anomalies such as Chiari I malformation or basilar invagination further support os odontoideum, while fracture is purely traumatic.

 

Recalls:

 

Dentocentral fusion normally completes by 5–6 years.
Os odontoideum: detached corticated ossicle, chronic pseudoarthrosis.
Two types: orthotopic and dystopic.
Associations: Chiari I, basilar invagination, Klippel–Feil.
Management: stabilize if unstable or symptomatic.
C1–C2 fusion results in ~50% loss of rotation with minimal effect on flexion/extension.
Differentiation from acute fracture: corticated margins, absence of marrow edema, remodeling of C2 body.