Os Odontoideum
Adolescent with Chronic Neck Pain and Suboccipital Discomfort: Diagnosis? • Xray of the Week
Figure 1. CT images of the cervical spine (A) sagittal view and (B) coronal view. What is the important finding on this CT scan?
Figure 2. A. Sagittal CT shows a small, round, well-corticated ossicle (yellow arrow) representing the os odontoideum separated from the underlying body of C2 (green arrow). Note the smooth, radiolucent gap between the body of C2 and the os odontoideum (red arrow). Also note the thickening and abnormal rounding of the anterior arch of C1 (blue arrow).
B. Coronal CT section again demonstrates the well-corticated ossicle representing the os odontoideum (yellow arrow) positioned symmetrically between the lateral masses of C1 (blue arrows) and separated from the underlying body of C2 (green arrow).
C. Axial CT through the ring of C1 shows the os odontoideum (yellow arrow) situated just posterior to the anterior arch of C1 (blue arrow). Note the thickening and abnormal rounding of the anterior arch of C1 (blue arrow).
Feature | Os Odontoideum | Chronic Fracture Dens (Nonunion) |
Ossicle Margins | Smooth, distinct, and completely well-corticated. | Often irregular, sclerotic, but lack a uniform smooth outer cortex of an independent ossicle. |
Gap / Interface | Wide, smooth, radiolucent gap with corticated margins on both opposing surfaces. | Jagged or sharply angulated lucent cleft matching the original fracture line (typically Type II base). |
Morphology of Dens | Hypoplastic, foreshortened, or round/oval ossicle located superiorly. | Normal height/shape of the residual dens base, matching the displaced fragment size. |
C1 (Atlas) Remodeling | Frequently shows hypertrophy and rounding of the anterior arch. | Anterior arch of C1 is usually normal in morphology unless secondary degenerative changes exist. |
Location of Gap | Typically well above the superior articulating facets of C2. | Typically at the base of the dens (classic Type II location). |
Clinical Context | May be an unrecognized childhood injury or congenital variant; highly prone to atlantoaxial instability. | Known prior acute trauma history with documented failure of bony union. |
Table 1. Differentiating Os odontoideum from a chronic nonunited Type 2 dens fracture on a CT scan relies on margin characteristics, gap appearance, and associated bony remodeling as summarized here.
Epidemiology
Os odontoideum is a rare anomaly of the craniocervical junction. Its true prevalence and incidence are unknown, since most cases are discovered incidentally or only after symptoms develop; reported prevalence on adult imaging series is well under 1%. Diagnosis spans a wide age range, from early childhood through late adulthood, although many patients present within the first three decades of life. An increased frequency has been reported in patients with Down syndrome, spondyloepiphyseal dysplasia, Morquio syndrome, and other skeletal dysplasias, supporting a multifactorial rather than a single unifying etiology (1,2).
Clinical Findings
Many cases are asymptomatic and are identified incidentally on imaging obtained for unrelated indications. When symptomatic, neck pain is the most common complaint, followed by mechanical or intermittent neurologic symptoms related to atlantoaxial instability, including myelopathy, transient quadriparesis after minor trauma, and vertebrobasilar symptoms such as vertigo or syncope from vertebral artery compromise. Because an unstable ossicle may be tolerated for years before a seemingly trivial injury precipitates catastrophic cord compression, even an incidentally detected os odontoideum warrants further clinical and radiographic evaluation (3,4).
Pathology The etiology of os odontoideum remains debated. The congenital theory attributes the anomaly to failure of fusion between the odontoid process and the body of the axis at the subdental synchondrosis. Growing evidence instead favors an acquired origin: an unrecognized fracture through the odontoid synchondrosis before ossification is complete (roughly before 5-6 years of age), with subsequent avascular remodeling of the separated fragment into a smooth, independently corticated ossicle (5). Both mechanisms likely contribute in different patients. Because the transverse atlantal ligament inserts on the mobile ossicle rather than on a fixed dens, it is rendered ineffective at restraining atlantoaxial translation.
Classification Os odontoideum is divided into two types based on the position of the ossicle relative to the atlas and clivus. In the orthotopic type, the more common pattern, the ossicle lies in the normal anatomic position of the odontoid tip and moves together with the anterior arch of C1. In the dystopic type, the ossicle is displaced, most often superiorly, and may fuse to the basion/clivus, moving with the occiput rather than the atlas; this pattern is more frequently associated with instability and neurologic injury (6).
Radiographic Features
CT best depicts the bony morphology of os odontoideum and is the modality of choice for surgical planning. The ossicle appears round-to-oval, roughly half the size of a normal dens, with a smooth and completely corticated margin. It is separated from the hypoplastic dens/axis body by a wide, smooth, radiolucent gap located above the level of the superior articular facets of C2 (Figures 1-2). Compensatory hypertrophy and rounding of the anterior arch of C1 is a recognized associated finding. Dynamic flexion-extension radiographs or CT, together with MRI, are used to quantify atlantoaxial instability and to assess for cord compression or signal change (6,7).
Differentiating os odontoideum from a chronic nonunited (Type II) dens fracture on CT relies on margin characteristics, gap appearance, and associated bony remodeling (Table 1).
Treatment and Prognosis
Management depends on symptoms and the degree of instability rather than on the mere presence of the ossicle. Asymptomatic patients without instability on flexion-extension imaging can be managed with observation and serial radiographic surveillance. Operative posterior C1-C2 fusion is favored for patients with neurologic deficits, myelopathy, or significant instability, given the risk of delayed neurologic deterioration, which has been documented even after a period of apparent clinical stability (7). Fusion rates and symptomatic improvement after surgery are high, although the timing of surgery in asymptomatic patients remains controversial.
Conclusion
Os odontoideum is an important, often incidental, cause of atlantoaxial instability that must be reliably distinguished from a chronic ununited dens fracture, as the two entities carry different implications for surveillance and treatment. Recognizing the smooth, corticated ossicle, the wide suprafacetal gap, and the hypertrophied anterior arch of C1 on CT allows a confident diagnosis and appropriate management, helping to avert neurologic catastrophe.
References:
Fielding JW, Hensinger RN, Hawkins RJ. Os Odontoideum. J Bone Joint Surg Am. 1980;62(3):376-383. https://pubmed.ncbi.nlm.nih.gov/7364809/
Sankar WN, Wills BP, Dormans JP, Drummond DS. Os odontoideum revisited: the case for a multifactorial etiology. Spine (Phila Pa 1976). 2006;31(9):979-984. DOI: 10.1097/01.brs.0000214935.70868.1c
Fagan AB, Askin GN, Earwaker JW. The jigsaw sign. A reliable indicator of congenital aetiology in os odontoideum. Eur Spine J. 2004;13(4):295-300. DOI: 10.1007/s00586-004-0732-2
Klimo P Jr, Kan P, Rao G, Apfelbaum R, Brockmeyer D. Os odontoideum: presentation, diagnosis, and treatment in a series of 78 patients. J Neurosurg Spine. 2008;9(4):332-342. DOI: 10.3171/SPI.2008.9.10.332
Cho S, Shlobin NA, Dahdaleh NS. Os odontoideum: A comprehensive review. J Craniovertebr Junction Spine. 2022;13(3):256-264.DOI: 10.4103/jcvjs.jcvjs_71_22
Nelson C, Mujahed T, Tedford NJ. Adolescent male with neck pain. J Am Coll Emerg Physicians Open. 2024;5(3):e13222. Published 2024 Jun 16.DOI: 10.1002/emp2.13222
Menezes AH. Os odontoideum: database analysis of 260 patients regarding etiology, associated abnormalities, and literature review. Front Surg. 2023;10:1291056. Published 2023 Dec 5. DOI: 10.3389/fsurg.2023.12910561
Umaiza Ihsan, MD is a medical graduate of King Edward Medical University,
Pakistan. She completed her internship at Mayo Hospital, Lahore, including a three-
month rotation in Radiology, which further strengthened her interest in diagnostic
imaging. She has gained additional clinical exposure in Radiology through observerships at Mon Health Morgantown and the University of Maryland, Baltimore. She has a strong interest in Radiology, particularly diagnostic imaging and its role in clinical decision-making, and is actively interested in clinical research, with experience in systematic reviews and medical research.
Kevin M. Rice, MD is the president of Global Radiology CME and is a radiologist with Cape Radiology Group. He has held several leadership positions including Board Member and Chief of Staff at Valley Presbyterian Hospital in Los Angeles, California. Dr. Rice has made several media appearances as part of his ongoing commitment to public education. Dr. Rice's passion for state of the art radiology and teaching includes acting as a guest lecturer at UCLA. In 2015, Dr. Rice and Natalie Rice founded Global Radiology CME to provide innovative radiology education at exciting international destinations, with the world's foremost authorities in their field. In 2016, Dr. Rice was nominated and became a semifinalist for a "Minnie" Award for the Most Effective Radiology Educator. He was once again a semifinalist for a "Minnie" for 2021's Most Effective Radiology Educator by AuntMinnie.com. He has continued to teach by mentoring medical students interested in radiology. Everyone who he has mentored has been accepted into top programs across the country including Harvard, UC San Diego, Northwestern, Vanderbilt, and Thomas Jefferson.
Follow Dr. Rice on Twitter @KevinRiceMD
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