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
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
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