Os Odontoideum: Floating Bone of the Axis

In order to understand what an os odontoideum is, we have to first appreciate the anatomy of the first two cervical vertebrae.

The first cervical vertebrae is known as the "atlas". It forms joints with the base of the skull and the second cervical vertebrae, which is also known as the axis. It has a an elongated structure on its ventral aspect called the “odontoid”. The odontoid of the axis connects to the atlas via numerous ligaments. This joint provides most of the flexibility that allows you to move your head in various directions.

An os odontoideum is a failure of the tip of the odontoid (ie: the part closest to the atlas) to fuse with its base on the axis.

Exactly why this occurs is still debated. The first theory is that it represents a congenital failure of the odontoid to fuse properly with the axis. The second, and more supported theory is that it may be caused by a previous fracture in early childhood that failed to heal properly. Regardless of the cause, the end result is a floating mass of bone that represents the superior (ie: top) most portion of the odontoid process.

This mass of bone may be fused to the base of the skull. If this is the case, the term "dystopic" os odontoideum is used. Or it may articulate and move with the atlas; if this is the case, the term "orthotopic" os odontoideum is used.

Signs and Symptoms

Many patients with os odontoideum are asymptomatic. However, because the tip of the odontoid is not technically connected to the base of the axis the patient may have an unstable neck. If the instability is severe, damage to the spinal cord can result causing myelopathy.

Myelopathy can manifest with several symptoms. Patients may have numbness and tingling in the upper and lower extremities. If damage to the nervous tissue responsible for motor movements occurs, patients may complain of weakness (and possibly even paralysis in extreme cases!).

On examination, patients may have both upper and lower motor neuron signs. Upper motor neuron signs refer to exaggerated reflexes – Babinski and Hoffmann signs, and clonus are all examples of this. These findings tend to be seen below the level of the actual spinal cord injury. Lower motor neuron findings typically occur at the level of the spinal cord damage, and consist of flaccid weakness with decreased reflexes.


Diagnosis of os odontoideum is made by x-rays or CT of the cervical spine. To assess the degree of instability in the joint, some doctors will get flexion and extension x-rays as well.

The image to the right is a CT of the cervical spine that illustrates the missing portion of the odontoid process (marked by arrows in the image). A normal CT of the cervical spine is shown to the left for comparison.

Os Ondontoideum

Some patients may also get an MRI to assess for spinal cord and ligamentous injury, especially when symptoms or physical examination findings are present.


Treatment depends on whether or not symptoms are present, and whether or not the cervical spine is unstable. Many patients without symptoms may be followed with serial X-rays or CT scans to assess for progression of instability.

If significant instability exists, or the patient has signs and symptoms consistent with spinal cord injury, then surgical stabilization is performed. There are numerous ways to achieve stabilization in this region surgically, which are outside the scope of this article. Regardless of which method is used, the end result is stabilization of the joint between the first and second cervical vertebrae.


Os odontoideum is an absence of part of the odontoid process. It may be due to a congenital malformation, or an early childhood fracture that fails to heal properly. Symptoms, when present, are due to spinal cord injury (ie: myelopathy) and consist of weakness, numbness, tingling, and other signs of spinal cord dysfunction. Imaging with x-rays or CT scan can show the bony defect. MRI is occasionally used to assess the spinal cord itself. Treatment depends on whether or not symptoms or significant instability is present. The best treatment options are surgical stabilization of the joint between C1 and C2 using one of several potential methods.

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Atlas Fractures: The Weight of the World On Its Shoulders

The atlas, or the first cervical vertebra (C1), is a ring shaped structure. It forms joints with the base of the skull above and the axis (ie: the second cervical vertebrae) below. It also has two foramen transversarium, which are holes that allow the passage of the vertebral arteries on either side of the spinal cord.

Fractures of the atlas occur most commonly with forceful axial loading of the head (ie: a downward force applied to the top of the head). Pressure on the top of the head causes the skull to push down on the atlas, which results in a break(s) of its ring-like structure. Specific fracture types such as a break in the front of the ring, the back of the ring, or one side of the ring versus the other, are dependent on additional force vectors at the time of loading (ie: flexion, extension, lateral bending, etc.).

Fractures of the atlas must also include a discussion of biomechanical stability, which is usually determined by the integrity of the transverse ligament. The transverse ligament attaches the dens (odontoid) of the axis to the anterior ring of the atlas.

Fractures of the atlas with co-existent rupture of the transverse ligament lead to instability of the joint between C1 and C2. In other words, the ring of C1 may be able to move forward relative to the dens of C2. Transverse ligament injury is more common when axial loading is combined with extension of the head.

Not surprisingly, fractures of the atlas often co-exist with fractures of other cervical spine vertebrae. The most common combination is with a fracture of the axis, occurring in up to 40% of cases.

Signs and Symptoms

Patient’s with isolated atlas fractures usually have neck pain and muscle spasms. Frequently they have no injury to the spinal cord because the ring splays outwards as it fractures.

It is important to rule out injuries to the vertebral arteries, which run in bony holes (ie: foramen transversarium) on the sides of the atlas. When injured, the vertebral arteries can cause strokes in the brainstem and cerebellum, which can be life threatening.

Since the atlas is so close to the brainstem, patients may have co-existent injury to the lower cranial nerves. Specifically, injury to the 12th nerve can cause problems with tongue movements, injury to the 11th nerve can cause weakness with shoulder shrug and the ability to turn the head to the side, and injuries to the 9th and 10th cranial nerves can cause problems with swallowing and paralysis of the larynx leading to difficulty with speech.

Co-existent head and brain trauma, which can cause a constellation of different signs and symptoms depending on severity can also occcur.


Diagnosis of an atlas fracture is made using x-rays, CT scans, and MRIs. X-rays should include anterior-posterior views, open mouth odontoid views, and lateral views of the cervical spine. If there is no evidence of neurological injury, flexion-extension x-rays may also be obtained to assess for stability of the C1-C2 joint.

The bony injury associated with atlas fractures is categorized according to the Jefferson or Landell and Van Peteghem classification systems. The Landells classification has three types, whereas the Jefferson classification has four types:

Landell and Van Peteghem Classification
Type 1 Fracture of either the anterior or posterior ring, but not both (posterior ring fractures are most common type)
Type 2 Fractures of both the anterior and posterior ring
Type 3 Fracture of the lateral mass(es)

Jefferson Classification
Type 1 Fracture of the posterior ring only
Type 2 Fracture of the anterior ring only
Type 3 Fracture of the anterior and posterior rings on both sides; this is the classic "burst", or traditional “Jefferson” fracture
Type 4 Fracture of the lateral mass(es)

Atlas fracture

An important part of diagnosing atlas fractures involves assessing the integrity of the transverse ligament, which is best done using MRI. However, if an MRI cannot be performed then open mouth odontoid, flexion-extension x-rays, and CT scans can provide some information regarding transverse ligament injury.

The rule of Spence is one way of assessing the integrity of the transverse ligament on an open mouth odontoid x-ray. The rule states that if the right and left lateral masses of C1 overhang the lateral masses of C2 by greater than a total distance of 6.9mm than the likelihood of co-existent transverse ligament injury is high. The rule of Spence is not fool proof and should be supplemented with MRI and/or flexion-extension films whenever possible.

Atlantodental Interval
Another method for assessing transverse ligament injury is using the "atlantodental" interval (see image to the left). This is the distance between the anterior arch of C1 and the odontoid process (aka: dens) of C2.

This interval is usually quite small, typically less than 3mm in adults and 5mm in children. If the interval is greater than this, then co-existent transverse ligament injury should be suspected.


Treatment of isolated atlas fractures is usually with cervical immobilization. This may be with a halo or with a rigid cervical collar such as a cervical-occipital-mandibular-immobilizer (SOMI).

Atlas fractures that have co-existent transverse ligament rupture often require an operation to stablize the bones of the spine. This is usually in the form of fusing the atlas or occiput (back of the head) to the second cervical vertebrae.

If other injuries (ie: fractures of C2) are present and/or there is significant ligamentous injury then open surgical fusion of the bones may be necessary to re-create stability of the craniocervical junction.


Atlas fractures occur in response to vertical compression of the head on the upper cervical spine. Fractures of the anterior, posterior, or both rings of C1 may be present. Biomechanical stability is typically determined by assessing the integrity of the transverse ligament. Patients with isolated C1 fractures usually complain of neck pain, and rarely have injury to the spinal cord. Diagnosis is based on CT, x-ray, and MRI findings. Treatment is with rigid external immobilization or operative spinal fusion.

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The Hangman’s Fracture

The axis, or second cervical vertebrae (C2), is unique amongst the vertebrae of the spine. It contains a body, which has an elongated structure that extends towards the head known as the dens (aka: odontoid). It also contains a ring-like structure that extends around the spinal cord, which is composed of the pedicles, pars interarticularis, and lamina. It forms joints with the atlas (ie: first cervical vertebrae) above it, and the third cervical vertebrae below it. It also has two foramen transversarium on either side, which are conduits for the vertebral arteries.

A hangman’s fracture is a break in both pedicles and/or pars interarticulari. The terms pedicle and pars interarticularis are not as well defined at C2 as they are for the other vertebrae, and thus have to be more clearly delineated before we discuss the details of a hangman’s fracture.

The pars interarticulari of C2 are the narrow pieces of bone that sit between the superior articulating facets (ie: the portion of bone that allows C2 to form a joint with C1) and the inferior articulating facets (ie: the portion of bone that allows C2 to form a joint with C3) of C2. The pedicles lie more anterior and are defined as the bony "bridges" that lie underneath the superior articulating facet and just medial (ie: closer to the spinal cord) to the transverse foramen, which house the vertebral arteries.

So now let’s get to the good stuff… Why do these fractures happen? Extension of the neck! This is why they are colloquially termed "Hangman’s" fractures; in the old days when a person was hanged the noose would pull the mandible upwards and cause the neck to violently extend. The end result was a tremendous amount of force on the pars interarticulari and pedicles of C2 leading to a fracture.

Since hangings are infrequent in today’s society, a more common cause of hyperextension of the neck is a head hitting the steering wheel or windshield of a car.

The hyperextension also often causes significant anterior ligamentous injury. The anterior longitudinal ligament (ie: the ligament that runs down the front of the spine) and the annulus fibrosis of the C2 disc are often ruptured. These findings are consistent with a hyperextension injury as the ligaments in the front get stretched to the point of rupture.

Signs and Symptoms

Surprisingly, hangman’s fractures rarely cause neurological injury. Most patients are neurologically intact meaning that there is no injury to the spinal cord and/or nerves at the level of the fracture. Typically there is neck pain, which is the most common symptom.

It is important to realize that many patients with hangman’s fractures will also have co-existent head trauma, and roughly a third of patients will have additional spine fractures. So keep a look out for associated injuries!!!


The diagnosis of a hangman’s fracture can be made using x-rays and CT scans. MRI scans are also frequently ordered to determine the extent of co-existent ligamentous and soft tissue injury. A CT angiogram or MR angiogram should also be done to assess for co-existent vertebral artery injury.

Hangman's fracture

There are several different grading systems for hangman’s fractures. They include the Effendi, Francis, and Levine and Edwards classifications.

The Effendi system is based on the orientation of the fracture, as well as the degree of angulation and dislocation between C2 on C3.

The Francis system also takes into account the angulation and displacement between the bodies of C2 and C3, which is measured between the inferior endplate of C2 and the superior endplate of C3.

Perhaps the easiest to implement clinically is the Levine and Edwards classification. A type I Levine fracture is a non-displaced, non-angulated fracture. Type II fractures come in two flavors: type II is a fracture that is significantly angulated (ie: > 11 degrees) and displaced (ie: greater than 3mm) and a type IIa fracture is angulated (ie: greater than 11 degrees), but not significantly displaced. Type III fractures are fracture-dislocations of C2 on C3.

The Levine and Edwards’ Classification System
for Hangman’s Fractures

Type Angulation Displacement Treatment
Type I Minimal Minimal Rigid orthotic
Type II Greater than 11 degrees Greater than 3mm Traction ± rigid orthotic ± surgery
Type IIa Greater than 11 degrees Minimal Traction ± rigid orthotic ± surgery
Type III Minimal to severe Significant (fracture/dislocation) Traction + surgery


Most isolated hangman’s fractures can be treated with external immobilization in a rigid cervical collar (ie: Miami J or Philadelphia collar) or in a halo immobilization device.

However, if there is significant ligamentous disruption, severe angulation and/or dislocation, or the inability to obtain adequate alignment of the spine in an immobilization device (ie: rigid collar or halo) then internal surgical fixation and fusion should be performed.

The need for surgery depends on the severity of the fracture and/or the integrity of the associated ligaments and discs.

A surgical approach from the front (aka: an "anterior" approach) may be performed to fuse the C2 and C3 vertebrae by removing the disc material between them. This approach is most often done in the presence of anterior longitudinal ligament rupture and/or intervertebral disc protrusion (ie: a "traumatic" disc).

Surgery from behind may also be used (aka: a "posterior" approach). Usually, the 1st through 3rd cervical vertebrae are incorporated into the fusion process, but some surgeons may opt to fuse to the base of the skull in cases of more severe injuries.


Hangman’s fractures occur after violent extension of the neck. The pedicles or pars interarticulari are fractured on both sides of the C2 ring. Neurological injury is rare in isolated hangman’s fractures, but frequently there are associated injuries to other bones in the cervical spine, as well as injuries to the brain and face. Diagnosis is made with x-rays, CT scans, and MRI. Treatment is with rigid immobilization of the cervical spine and/or surgical fixation depending on the extent of injury.

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