Os Odontoideum¶

Definition¶

Os odontoideum is a well-corticated ossicle separated from the body of C2 by a wide gap, representing either a congenital failure of fusion of the odontoid with the C2 body or a post-traumatic nonunion of an odontoid fracture sustained in early childhood. The distinction is debated, but the current consensus favors an acquired (post-traumatic) etiology in most cases.

Imaging Findings¶

Radiography/CT¶

A well-corticated, round or oval ossicle separated from the hypoplastic dens by a smooth gap
The ossicle may be in the orthotopic position (at the normal dens location) or dystopic position (fused to the basion or anterior arch of C1)
Smooth, sclerotic margins (unlike the irregular margins of an acute fracture)
The C2 body is hypoplastic with a smooth, rounded superior surface
Hypertrophy of the anterior arch of C1 (compensatory)

MRI¶

Evaluates for atlantoaxial instability — anterior translation of C1 on C2
Pannus may form around the os, similar to RA
Cord compression or signal change (myelopathy)
No bone marrow edema (unlike acute fracture, which shows STIR hyperintensity)

Flexion-Extension Radiographs¶

Dynamic instability — the ADI increases in flexion, demonstrating C1–C2 instability

Clinical Pearl

Os odontoideum must be distinguished from an acute Type II odontoid fracture — os odontoideum has smooth, sclerotic cortical margins and no surrounding edema, while an acute fracture has irregular margins and shows bone marrow edema on STIR. This distinction is critical because os odontoideum may require elective surgical stabilization if unstable, while an acute fracture is managed urgently.

Management¶

Stable, asymptomatic — Observation with serial flexion-extension radiographs
Unstable or symptomatic — C1–C2 posterior fusion
Associated with Down syndrome, Morquio syndrome, and spondyloepiphyseal dysplasia

Key Points¶

Well-corticated ossicle separated from the C2 body — likely acquired (post-traumatic)
Smooth sclerotic margins distinguish from acute odontoid fracture
May cause atlantoaxial instability — flexion-extension views are essential
MRI evaluates cord compression and pannus formation
C1–C2 fusion for unstable or symptomatic cases

References¶

Pereira Duarte M, Das JM, Weisbrod LJ, Camino Willhuber GO. Os Odontoideum. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542297/
Jumah F, Alkhdour S, Mansour S, et al. Os Odontoideum: A Comprehensive Clinical and Surgical Review. Cureus. 2017;9(8):e1551. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5630463/
Cho S, Shlobin NA, Dahdaleh NS. Os odontoideum: A comprehensive review. J Craniovertebr Junction Spine. 2022;13(3):256-264. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9574117/
Sankar WN, Wills BPD, Dormans JP, Drummond DS. Os odontoideum revisited: the case for a multifactorial etiology. Spine (Phila Pa 1976). 2006;31(9):979-984. Available from: https://pubmed.ncbi.nlm.nih.gov/16641773/
Holt RG, Helms CA, Munk PL, Gillespy T 3rd. Hypertrophy of C-1 anterior arch: useful sign to distinguish os odontoideum from acute dens fracture. Radiology. 1989;173(1):207-209. Available from: https://pubmed.ncbi.nlm.nih.gov/2781009/
Jain N, Verma R, Garga UC, Baruah BP, Jain SK, Bhaskar SN. CT and MR imaging of odontoid abnormalities: A pictorial review. Indian J Radiol Imaging. 2016;26(1):108-119. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC4813060/
Os odontoideum. Radiopaedia.org. Available from: https://radiopaedia.org/articles/os-odontoideum