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Anomalies of the Odontoid process (also known by its other anatomical name - the "dens") may range from complete absence (aplasia) to partial absence (hypoplasia) to separate Odontoid process (a condition known as Os Odontoideum.) These are all very serious problems that can result in Atlanto-axial instability which, itself, may cause neurological deficits and death.

The precise incidence of these anomalies is unknown. They are probably more common than is recognized since many congenital anomalies remain asymptomatic. Aplasia is extremely rare. Hypoplasia and Os Odontoideum are quite uncommon. Neurological signs and symptoms relating to Atlanto-axial instability are more common in patients with Down's Syndrome, Morquio Syndrome, Klippel-Feil Syndrome. These disease states are often accompanied by ligamentous laxity which results in the failure of the ligaments that normally hold the Odontoid firmly in position.


Patients commonly experience neck pain and stiffness although many are asymptomatic when they are diagnosed after investigation for other unrelated problems. In those patients where the clinical manifestations are limited to neck pain and torticollis without neurological involvement, the prognosis is excellent.

Other symptoms that are more serious include transitory episodes of paresis following trauma, or extremity weakness, spasticity, clumsiness and unsteady gait secondary to Spinal Cord compression.Patients exhibiting these symptoms and signs and those manifesting evidence of Cerebral and Brain Stem ischemia, seizures, and mental retardation are at risk for sudden death due to minor trauma.

An important factor differentiating Os Odontoideum from other anomalies of the Occipito-vertebral junction is that Os Odontoideum patients seldom have symptoms referable to Cranial Nerves, since the area of the Spinal Cord impingement is below the Foramen Magnum.



CT and MRI scans are often the first studies obtained during the investigation of patients with the many conditions that produce symptoms common to all of these entities.

Once any Odontoid Anomaly has been identified it is imperative to obtain CT Scan "reconstruction" films as well as flexion-extension images. Flexion-extension CT scans may be required since plain films do not always show the anomaly or the extent of motion. (NOTE: FLEXION-EXTENSION FILMS SHOULD ONLY BE DONE WHEN SUPERVISED BY A NEURORADIOLOGIST OR OTHER QUALIFIED EXPERT.)

In Os Odontoideum, there is a joint-like articulation between the Odontoid and the body of the Axis (the C2 vertebra). This gap may be confused with a normal finding in patients younger than 5 years. In children, the diagnosis must be confirmed by demonstrating motion between the Odontoid and the body of the Axis.

MRI scan is particularly important to examine the space available for the Spinal Cord.


Surgical stabilization is indicated if neurological involvement is present with more than 10 mm of instability on flexion-extension films, if progressive instability is present, or if persistent neck symptoms are present.

While preoperative correction with traction or positioning is highly desirable, it is not always possible. Operative reduction should be avoided since there is no reliable way to evaluate the patient while they are under anesthesia and intra-operative Neurophysiological monitoring (in the form of "evoked potentials") is not sufficiently reliable.

Posterior Cervical Fusion of C1-C2 with rigid metal fixation and supplemented with a bone graft is the most common method of stabilization. This is not without risk since any intra-operative maneuver that results in compressing the Medulla and/or Spinal Cord can have disastrous consequences.

Prophylactic stabilization for Odontoid Anomalies is controversial. Nevertheless encouraging long term outcomes with low surgical morbidity are common in the hands of experienced surgeons.

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This page last edited on 2/19

All content ©2022 by Neurosurgical Consultants, P.A.
Author, Martin L. Lazar, MD, FACS
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