axis

Os Odontoideum: Floating Bone of the Axis

virtualmedstudent

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

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

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.

Overview

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.

Related Articles

References and Resources

The Hangman’s Fracture

virtualmedstudent

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!!!

Diagnosis

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

Treatment

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.

Overview

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.

Related Articles

References and Resources