eMedicine Specialties > Orthopedic Surgery > Spine

Os Odontoideum: Workup

Author: Eeric Truumees, MD, Consulting Surgeon, Department of Orthopedic Surgery, William Beaumont HospitalOrthopaedic Director, Gehring Biomechanics LaboratoryAdjunct Faculty, Bio-Engineering Center, Wayne State University
Contributor Information and Disclosures

Updated: Sep 12, 2008

Workup

Imaging Studies

Radiologic evaluation is utilized to confirm the diagnosis and estimate the degree of spinal instability. Initial evaluation includes open-mouth, anterior-posterior, and flexion-extension lateral radiographs. Os odontoideum appears as a round or oval ossicle with a smooth, uniform cortex separated from the base of the axis by a wide gap. The ossicle border does not directly match up with the axis body. The gap separating the os and the axis proper should lie above the level of the superior articular facets. Orthotopic os odontoideum (see Image 2) may appear free and in a relatively anatomic position.28,29,30

An orthotopic os odontoideum may be difficult to differentiate from an unfused neurocentral synchondrosis, odontoid hypoplasia, or odontoid fracture nonunion. In children younger than 5 years, the neurocentral synchondrosis often has not fused. Dynamic lateral radiographs of those with an unfused synchondrosis do not demonstrate motion, whereas radiographs of individuals with an os odontoideum may.

A dystopic ossicle may be fixed to the clivus or to the anterior ring of the atlas. The remaining axis is hypoplastic as well. With a dystopic os odontoideum (see Image 3), the radiographic diagnosis is clear. A dens fracture nonunion (see Image 4) typically exhibits a narrow gap between the axis base and dens. The normal shape and size of the dens are preserved on the open-mouth view.

With an os odontoideum, hypertrophy of the anterior arch of the atlas may be seen.31 This hypertrophy is believed to represent osseous reaction to chronic atlantoaxial instability and is unlikely with an acute dens fracture. Criteria for stability or instability include the following:

  • Flexion-extension lateral radiographs (see Images 5-6)
  • Most symptomatic patients demonstrate radiographic instability.
  • In one series, the average translatory motion was 1 cm.
    • Instability was mainly in the anterior-posterior plane.
    • Some patients were unstable in all directions.
  • Important prognostic indices:
    • Anterior atlantoaxial translation
    • Posterior atlantodens interval (PADI)
    • Instability index
    • Sagittal plane angulation
  • In forms of atlantoaxial subluxation, the anterior atlantodens interval (AADI) is used to measure instability.
    • However, with os odontoideum, the os fragment often moves with the atlas.
    • The AADI does not reflect the abnormal motion of the segment.
    • Direct measurement of the motion of C1 on the body of C2 is more useful.
    • The space between a line projected superiorly from the anterior border of the body of the axis and a line projected inferiorly from the posterior border of the anterior arch of the atlas are measured.
    • More than 3 mm of separation is pathologic. Critical evaluation of the bony anatomy of the upper cervical spine is often difficult with plain radiographs alone. Lateral tomography or, more commonly, fine 1-mm cut, sagittally reconstructed computer tomography (CT) scans allow more detailed depictions of the atlantoaxial articulation.
  • Previously, the distance between the posterior border of the dens and the anterior border of the posterior ring of the atlas on plain radiographs was termed the space available for the cord (SAC).
    • More recently, this distance is called the PADI.
    • These terms occasionally are used synonymously. However, with MRI or CT myelography, the actual space for the cord can be measured readily, and the SAC can be used to refer to the actual anterior-posterior canal dimension.
  • Space available for the cord
    • SAC refers to the PADI minus additional compression from soft tissue; soft tissue structures such as synovial cysts may further diminish the SAC in some cases of os odontoideum.
    • A PADI of less than 13 mm is associated with progressive neurologic decline.
    • The instability index refers to the change in SAC from flexion to extension.
    • The critical measurement is from the superior posterior corner of C2 to the posterior ring of C1.
  • Cord compression and cord signal anomaly
    • In one study, radiographic measurements of translation and PADI did not accurately reflect clinical status. MRI measurement of cord compression was more predictive of symptomatology. Hadley’s literature review was unable to establish a linear relationship between PADI and neurologic status.32
    • MRI also may delineate pathologic changes within the cord. A T2-weighted MRI (see Image 7) sequence may depict myelomalacia as an increased signal in the substance of the cord.33,34,35
  • A number of dynamic imaging modalities have been recommended as means to more completely understand the degree and nature of abnormal motion in patients with os odontoideum. Cine radiographs have been recommended because of their ability to define the relationship of the os to surrounding bony elements.36 Also, dynamic MRI scans may detail the degree and planes of instability in real time but are rarely needed.34,35

More on Os Odontoideum

Overview: Os Odontoideum
Workup: Os Odontoideum
Treatment: Os Odontoideum
Follow-up: Os Odontoideum
Multimedia: Os Odontoideum
References
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Further Reading

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Keywords

os odontoideum, atlanto-axial instability, atlantoaxial instability, cervical instability, atlanto-axial joint, atlantoaxial joint, atlas bone, axis bone

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Contributor Information and Disclosures

Author

Eeric Truumees, MD, Consulting Surgeon, Department of Orthopedic Surgery, William Beaumont HospitalOrthopaedic Director, Gehring Biomechanics LaboratoryAdjunct Faculty, Bio-Engineering Center, Wayne State University
Eeric Truumees, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Orthopaedic Surgeons, American Orthopaedic Association, Cervical Spine Research Society, Michigan State Medical Society, Mid-America Orthopaedic Association, and North American Spine Society
Disclosure: Stryker Spine Consulting fee Consulting; DePuy Spine Consulting fee Consulting; Stryker Spine Royalty Other

Medical Editor

Lee H Riley III, MD, Chief, Division of Orthopedic Spine Surgery, Assistant Professor, Departments of Orthopedic Surgery and Neurosurgery, Johns Hopkins University
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

William O Shaffer, MD, Professor, Vice-Chairman and Residency Program Director, Department of Orthopedic Surgery, University of Kentucky at Lexington
William O Shaffer, MD is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Orthopaedic Association, International Society for the Study of the Lumbar Spine, Kentucky Medical Association, Kentucky Orthopaedic Society, North American Spine Society, Southern Medical Association, and Southern Orthopaedic Association
Disclosure: DePuySpine 1997-2007 (not presently) Royalty Consulting; DePuySpine 2002-2007 (closed) Grant/research funds SacroPelvic Instrumentation Biomechanical Study; DePuyBiologics 2005-2008 (closed) Grant/research funds Healos study just closed; No present Industry grants or funds. None None

CME Editor

Dinesh Patel, MD, FACS, Associate Clinical Professor of Orthopedic Surgery, Harvard Medical School; Chief of Arthroscopic Surgery, Department of Orthopedic Surgery, Massachusetts General Hospital
Dinesh Patel, MD, FACS is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Association of Physicians of Indian Origin, American College of International Physicians, and American College of Surgeons
Disclosure: Nothing to disclose.

Chief Editor

Mary Ann E Keenan, MD, Professor, Vice Chair for Graduate Medical Education, Department of Orthopedic Surgery, University of Pennsylvania School of Medicine; Chief of Neuro-Orthopedics Program, Department of Orthopedic Surgery, Hospital of the University of Pennsylvania
Mary Ann E Keenan, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Orthopaedic Surgeons, American Orthopaedic Association, American Orthopaedic Foot and Ankle Society, American Society for Surgery of the Hand, and Orthopaedic Rehabilitation Association
Disclosure: Nothing to disclose.

 
 
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