Atlanto-Occipital Dislocation (Internal Decapitation)

Atlanto-occipital dislocations (AODs) occur when there is severe damage to the ligaments that connect the skull to the cervical spine. The most common cause of an AOD is trauma.

There are several classifications of atlanto-occipital dislocations. In type 1 dislocations the base of the skull moves anteriorly (ie: forward) relative to the dens of the second cervical vertebrae. In type 2 dislocations the skull moves superiorly (ie: upwards) relative to the cervical spine. In type 3 dislocations the skull moves posteriorly (ie: backwards) relative to the cervical spine.

Often times more than one classification may exist in a single patient. For example, a patient may have both a type 1 and type 2 AOD if the patient’s skull has moved forward and upwards relative to the cervical spine.

Signs and Symptoms

Most cases of complete atlanto-occipital dislocation are fatal. Severe damage to the spinal cord around the cranio-cervical junction leads to respiratory paralysis and death if mechanical ventilation is not started rapidly. Patients are often quadriplegic (ie: unable to move their upper and lower extremities) because of damage to the descending motor axons of the corticospinal tract. Incomplete injuries can also occur with varying degrees of cervical spine injury.

The lower cranial nerves are sometimes involved in the injured segment. The most commonly injured cranial nerves are the abducens (VI), vagus (X), and hypoglossal (XII) nerves. Abducens nerve injury causes an inability to deviate the eye outwards. Damage to the hypoglossal nerve causes deviation of the tongue towards the side of the injured nerve (“lick your wound”).

Surprisingly, some patients with atlanto-occipital dislocation may have no neurologic symptoms! It is therefore important to rule out ligamentous injury, especially in patients who have had severe head and neck trauma.

Diagnosis and Classification

Power's Ratio
Diagnosis is made after appropriate imaging studies are obtained. Plain films and CT scans of the cervical spine are the most commonly obtained initial imaging studies.

There are several methods for determining if atlanto-occipital dislocation has occurred. The first is the “BAI-BDI method”. Two distances are measured. The BDI, or basion-dental interval, measures the distance between the most anterior and inferior portion of the skull (ie: “basion”) and the tip of the dens, which is a superior extension of the second cervical vertebrae. The BDI should normally be less than 12mm. In the image below the BDI is 14mm and is indicative of atlanto-occipital dislocation.

The second distance that is measured is the BAI, or basion-axial interval. The BAI is the distance between the basion and a line drawn straight up from the posterior (ie: back) portion of the dens. The normal interval is less than 12mm. In the image below the BAI is 14.2mm indicating likely atlanto-occipital dislocation.

A second method of assessing atlanto-occipital dislocation is known as Power’s ratio. Power’s ratio is most useful if the dislocation is anterior (ie: the head moves forward relative to the spine). Two distances are measured. The first is the distance between the opisthion and the anterior ring of C1; the second is the distance between the basion and the posterior ring of C1. The second distance is then divided by the first to obtain the ratio. A “normal” ratio is less than 1.0; if the ratio is greater than 1.0 then suspicion for dislocation should be high.

X-rays and CT show bony anatomy very well, but do not demonstrate ligamentous injury. Many patients with suspected AOD may also get an MRI to assess ligamentous damage. MRI will usually show severe injury to the ligaments that connect the first and second cervical vertebrae to the skull base.

There are two commonly used classification systems for describing atlanto-occipital dislocations. The Traynelis system is purely descriptive and is based on which way the head moves relative to the spine.

Traynelis Classification
Type 1 Head moves forward relative to the spine
Type 2 Head moves upwards relative to the spine
Type 3 Head moves backwards relative to the spine

The Bellabarba system is based on CT findings and is used to determine stability; it is also useful because it helps guide treatment decisions. A stable injury is one in which the dislocation is within 2mm of a normal BDI/BAI value and does not get worse with a traction test. Unstable injuries are present when the dislocation gets worse with a traction test; these injuries may be reduced at baseline (ie: within 2mm of a normal BDI/BAI) or grossly abnormal. That being said, it is important to note that very few doctors perform traction tests in patients with suspected AOD as this can worsen the condition!

Bellabarba Classification
Type Description Stability
Type 1 Alignment within 2mm of normal BAI/BDI interval; distracts less than 2mm with traction Stable
Type 2 Reduced at baseline (ie: within 2mm of normal BDI/BAI), but significant distraction with traction test Unstable
Type 3 Severely misaligned relative to normal BDI/BAI values Unstable


Treatment of atlanto-occipital dislocations involves spinal immobilization. This is usually done via a surgical procedure known as an occipital-cervical fusion in which the back of the skull is fused to the cervical spine. Some patients are also put in a halo immobilization device.


Atlanto-occipital dislocation occurs after severe injury to the ligaments that connect the skull to the cervical spine. It most commonly occurs after severe head or neck trauma. Death secondary to ventilatory failure, quadriplegia, and cranial nerve deficits are common symptoms. Diagnosis is based on characteristic findings found on CT, x-rays, and MRI scans. Treatment is with spinal immobilization, either with a halo apparatus, or a surgical procedure known as occipital-cervical fusion.

Some More Useful Learning Material…

References and Resources

  • Garrett M, Consiglieri G, Kakarla UK, et al. Occipitoatlantal dislocation. Neurosurgery. 2010 Mar;66(3 Suppl):48-55.
  • Greenberg MS. Handbook of Neurosurgery. Sixth Edition. New York: Thieme, 2006. Chapter 25.
  • Deliganis AV, Mann FA, Grady MS. Rapid diagnosis and treatment of a traumatic atlantooccipital dissociation. AJR Am J Roentgenol. 1998 Oct;171(4):986.
  • Traynelis VC, Marano GD, Dunker RO, et al. Traumatic atlanto-occipital dislocation. Case report. J Neurosurg. 1986 Dec;65(6):863-70.
  • Bellabarba C, Mirza SK, West GA, et al. Diagnosis and treatment of craniocervical dislocation in a series of 17 consecutive survivors during an 8-year period. J Neurosurg Spine. 2006 Jun;4(6):429-40.
  • Horn EM, Feiz-Erfan I, Lekovic GP, et al. Survivors of occipitoatlantal dislocation injuries: imaging and clinical correlates. J Neurosurg Spine. 2007 Feb;6(2):113-20.

Cervical Facet Dislocation: Houston We Have a Problem…

The spine is composed of thirty-three bony elements referred to as vertebrae. Each vertebrae connects with the vertebrae above and below it. These connection points are referred to as “facet” joints.

Each vertebrae has two bony sections known as superior articulating processes, as well as two bony sections known as inferior articulating processes. The inferior articulating processes of one vertebrae connect with the superior articulating processes of the vertebrae directly below it; the connections are held in place by ligaments.

The combination of the superior articulating process, inferior articulating process, and ligaments that connect them together form the facet joint. This arrangement makes the joints look like shingles on the roof of a house (see image below).

Normal Facet Anatomy

Like any other joint in the body, facet joints can dislocate. This is most likely to occur in the cervical spine where the facet joints are more horizontal in orientation.

Facet joint dislocations occur under conditions of severe flexion and/or rotation. The flexion injury causes disruption of the ligaments that stabilize the joint; this allows the inferior articulating process of the top vertebrae to "jump" over the superior articulating process of the bottom vertebrae. The end result is a dislocated (aka: "jumped" or "locked") facet.

Signs and Symptoms

Roughly a quarter of patients will have no signs or symptoms if one facet joint is dislocated between two vertebrae; another quarter will have incomplete spinal cord injuries. Ten percent will be paralyzed or paretic (ie: unable to move their arms or legs) and the remaining forty percent will have injury to the nerve roots exiting the spinal column. Nerve root injuries can cause paresthesias (ie: abnormal sensations) in a dermatomal pattern, as well as decreased reflexes and muscle weakness in the muscle group that the nerve serves.

The stakes for spinal cord injury increase dramatically if both facet joints are dislocated between two vertebrae. Roughly three quarters of these patients will be paralyzed! Very few people escape bilateral jumped facets without neurological injury.


X-ray jumped facet
Dislocated Facet

Diagnosis can be made with x-ray and CT imaging of the spine. The image will show the inferior articulating surface of the top vertebrae displaced anterior (ie: towards the chest) and inferior (ie: towards the feet) relative to the superior articulating surface of the bottom vertebrae.

The x-ray and CT scan to the right show a jumped facet between the fourth and fifth cervical vertebrae.


Treatment of most facet dislocations begins with external traction. This involves placing tongs on the patient’s head and applying weight to gently distract (ie: pull apart) the spine.

Continuous x-rays are taken until the inferior articulating process of the dislocated joint is perched on top of the superior articulating process of the vertebrae below. At this point the weight is gradually reduced until the joint "pops" back into the proper anatomical position.

Since facet joint dislocations involve severe ligamentous injury, patients often require surgical intervention to stabilize the neck, even after reduction with external traction.


A dislocated facet joint occurs when the inferior articulating process of one vertebrae jumps over the superior articulating process of the vertebrae below it. This usually occurs after severe flexion and rotational injuries. Diagnosis is made on x-ray or CT of the spine. Treatment is usually with a combination of external traction and surgical fusion.

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References and Resources

  • Dvorak MF, Fisher CG, Fehlings MG, et al. The surgical approach to subaxial cervical spine injuries: an evidence-based algorithm based on the SLIC classification system. Spine (Phila Pa 1976). 2007 Nov 1;32(23):2620-9.
  • Patel AA, Dailey A, Brodke DS, et al. Subaxial cervical spine trauma classification: the Subaxial Injury Classification system and case examples. Neurosurg Focus. 2008;25(5):E8.
  • Rabb CH, Lopez J, Beauchamp K, et al. Unilateral cervical facet fractures with subluxation: injury patterns and treatment. J Spinal Disord Tech. 2007 Aug;20(6):416-22.
  • Andreshak JL, Dekutoski MB. Management of unilateral facet dislocations: a review of the literature. Orthopedics. 1997 Oct;20(10):917-26.
  • [No authors listed]. Treatment of subaxial cervical spinal injuries. Neurosurgery. 2002 Mar;50(3 Suppl):S156-65.
  • Handbook of Neurosurgery. Sixth Edition. New York: Thieme, 2006. Chapter 25.
  • Baehr M, Frotscher M. Duus’ Topical Diagnosis in Neurology: Anatomy, Physiology, Signs, Symptoms. Fourth Edition. Stuttgart: Thieme, 2005.

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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References and Resources

Burst Fractures: Axial Loading Leading to Ouch!

Burst fractures are a specific type of spine fracture in which the body of a given vertebrae “bursts” into pieces. By definition a burst fracture involves the entire vertebral body. The image below is an example of a normal lumbar spine with the vertebral bodies outlined.

Burst fractures most commonly occur at the junction between the thoracic and lumbar spine. This junction is an area where the rigid thoracic spine transitions to the more mobile lumbar spine, and hence is an intrinsic point of weakness. This is why most burst fractures occur between the T10 through L2 vertebrae.

CT vertebral body

Axial loading of the spine is what causes burst fractures. They typically occur after a traumatic events like car accidents or falls from significant heights. Elderly individuals, and those with poor bone quality, may suffer burst fractures after minor trauma such as falling from a chair.

Signs and Symptoms

Burst fractures invariably present with back pain at the site of the fracture. Depending on the exact location signs and symptoms of nerve root compression or lower spinal cord injury may occur.

If the nerves that dangle in the lumbar spine (aka: the cauda equina) get compressed by the fragments of bone then weakness, numbness, tingling, and even bowel and bladder problems may occur.

Burst fractures between T10 and L1 can cause damage to the end of the spinal cord (the spinal cord ends at L1 or L2 in most individuals), which can lead to lower extremity weakness, or even paralysis, as well as bowel and bladder dysfunction.


Diagnosis of a burst fracture is made using a combination of x-rays, CT scans, and MRIs. These three imaging modalities serve different functions when evaluating the severity of a burst fracture.

X-rays are usually the first imaging ordered in patients with suspected spine fractures. If the plain x-rays show a burst fracture then CT scanning is usually done to further assess the degree of bony injury (see image below for an example of an L2 burst fracture).

MRI is used to detect ligamentous injury. The degree of ligamentous injury indicates a higher degree of instability; information about ligament integrity helps determine treatment options.

Burst Fracture Lumbar Spine


Treatment of burst fractures is highly dependent on the severity of the burst fracture. Treatment is either conservative with immobilization in a brace (ie: a "TLSO" or thoracolumbar sacral orthotic brace) or surgical fixation.

Burst fracture after instrumentation
As a rough rule of thumb patients with any of the following criteria should be strongly considered for surgical correction:

  • Greater than 50% vertebral body height loss.
  • Greater than 25 to 40 degrees of kyphosis.
  • Greater than 50% spinal canal compromise.
  • Significant posterior ligamentous injury.
  • Any neurological signs or symptoms referable to the injury.
  • If the patient fails conservative therapy with a brace.

Surgical correction can be achieved in a variety of ways and is often related to surgeon preference. Some surgeons will remove a significant portion of the fractured vertebral body and place a “cage” in the area, a procedure known as a “corpectomy”. This, combined with rods and screws from posteriorly provides the greatest stability, but has a higher risk of nerve injury. Not uncommonly, the fractured vertebral body is left alone and rods and screws are placed from behind only. This is especially true if the fractured level shows minimal spinal canal compromise.


Burst fractures of the thoracolumbar spine typically occur after high impact axial loading. They usually occur between T10 and L2, but can be seen anywhere in the spine. Patients will almost invariably have pain at the fracture site and may or may not have neurological signs and symptoms depending on the severity of the fracture. Diagnosis is made with CT, plain x-rays, and MRI. Treatment is highly dependent on the individual fracture and ranges from bracing to surgical fixation.

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