Atlantoaxial Instability (Atlantoaxial Subluxation) 

Updated: May 21, 2021
Author: Chris G Koutures, MD, FAAP; Chief Editor: Craig C Young, MD 


Practice Essentials

Atlantoaxial instability (AAI), also known as atlantoaxial subluxation, is radiologically identified increased mobility or laxity between the body of the first cervical vertebra (atlas) and the odontoid process of the second cervical vertebra (axis) (see the image below).[1, 2, 3, 4, 5]  The subluxation can be anterior, posterior, or lateral, and symptoms occur as a result of cervical cord impingement.

Medial-sagittal cross-sectional view of the atlas Medial-sagittal cross-sectional view of the atlas (C1) and the odontoid process of the axis (C2).


Disability and instability of the unique atlantoaxial joint result in controversies regarding the management of acute trauma and also the screening evaluation of particular at-risk individuals. The purposes of this article are to define AAI; describe the relatively rare symptomatic lesions with significant morbidity and mortality; and, finally, discuss the rationale for and against screening and restricting the activities of at-risk individuals.

For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center. Also, see eMedicineHealth's patient education article Neck Strain.

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Marfan Syndrome

Skeletal Dysplasia

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Resource CenterExercise and Sports Medicine

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United States data

Although traumatic lesions involving the atlantoaxial region are relatively rare, certain disease states and conditions present a higher theoretic risk of instability due to increased atlantoaxial joint laxity.

Surveys indicate 10-25% of patients with trisomy 21 have AAI.[6, 7]  Two thirds of these cases are due to laxity of transverse ligament, whereas one third are due to abnormal odontoid development. Although this association has been depicted on radiographs, the clinical incidence of serious cervical spine injury is not increased in this population compared with other populations.

About 25% of patients with rheumatoid arthritis have atlantoaxial instability, which is thought to be due to chronic inflammation.[8]  Congenital skeletal dysplasias may cause resultant odontoid hypoplasia. Marfan syndrome may involve ligamentous laxity, and acute inflammatory processes can affect the retropharyngeal, neck, or pharyngeal spaces.


Symptomatic AAI is due to acute trauma, usually cervical hyperflexion, hyperextension, or a direct axial load on the head or cervical spine. Although this type of injury can occur in any athlete, certain conditions predispose an individual to AAI dysfunction and disability. These include congenital odontoid anomalies, such as odontoid aplasia, odontoid hypoplasia, and a separate odontoid process; os odontoideum, which is due to nonunion of an early childhood fracture and which creates an unstable lesion or ligamentous laxity; and acute or chronic inflammatory processes.

Related Medscape topic:

Os Odontoideum

Functional Anatomy

The articulation of the odontoid process of C2 (axis) with the anterior arch of C1 (atlas) allows for 50% of cervical lateral rotation. The transverse and alar ligaments maintain joint integrity and limit posterior motion of the odontoid process relative to the C1 anterior arch. Abnormal posterior translation (or subluxation) can cause cervical cord impingement with the potential for significant neurologic compromise and even death.

Sport-Specific Biomechanics

During extremes of cervical flexion or extension, competent transverse and alar ligaments limit posterior translation of the odontoid process. Incompetent ligaments or a damaged odontoid process can allow for significant translation and potential damage in cases of cervical hyperflexion or hyperextension in which axial compression is delivered to the head and cervical spine. Given the potentially serious sequelae of significant atlantoaxial dysfunction, patients with defined instability are restricted from participating in contact sports and in sports requiring significant cervical flexion or extension.[1, 4, 9]




Most patients with atraumatic AAI are asymptomatic.[1, 2, 3, 4, 5] Clinical evidence of the condition is usually not detected until the subluxation is severe enough to cause damage to the spinal cord. Many patients do not present with active disease; rather, these patients are evaluated because of requests for screening radiography or for guidance regarding sports participation. This condition is rarely discovered as an incidental finding during radiologic evaluation for an acute neck injury. During such evaluations, the clinician must obtain a full history to provide adequate management, such as the following:

  • In obtaining the history, a review of any current or past neck trauma, head injury, or fall is essential, especially in children. Previous spine trauma may have resulted in an improperly healed odontoid injury that causes instability and neurologic symptoms years later.

  • A complete review of the patient's medical history is also important because many medical conditions are associated with an increased incidence of AAI.

  • In individuals examined for screening radiography or sport-specific counseling, it is important to obtain a full description of past symptoms as well as their desired sports participation. Certain organizations, such as the Special Olympics, had a mandatory radiographic requirement for participation in a certain event.[6] More recently, this requirement has been changed and is no longer a national requirement, but one which may be required by some state Special Olympic organizations. Screening neurologic examination and review of symptoms are also part of the preparticipation evaluation.

  • Individuals with symptomatic AAI may present with nonspecific symptoms, including neck pain, limited range of motion, and torticollis. A history of worsening symptoms (eg, headache, fatigue, transient upper-extremity paresthesias) with neck flexion is particularly revealing. Other symptoms may include distal muscle weakness and spasticity, gait disturbance, and bowel and/or bladder dysfunction. Quadriplegia due to cord compression is another dramatic presentation.

Physical Examination

In any case of suspected head or cervical spine injury (including cases of unconsciousness or altered mental status) primary on-field management of the patient includes an assessment of the airway, breathing, and circulation (ABCs), with immediate stabilization of the cervical spine in a neutral position.

Great caution must be placed into maintaining the airway without compromising the injured cervical spine. Note: If intubation is needed in the field, use the jaw-thrust maneuver rather than cervical extension.

The injured athlete should not be moved in any fashion until he or she is properly placed on a rigid backboard and the head and neck is immobilized with a rigid cervical collar and head stabilization device.

For an athlete wearing a helmet and/or shoulder pads, special precautions must be taken. For example, the helmet and pads should be removed only if the responders are trained in the proper technique or if the initial screening radiographs are negative.

Arranging for expedient and safe transport is another immediate priority.

A complete neurologic examination can wait until the athlete is in a more controlled environment.

Any athlete who leaves the field with neck pain, limited cervical range of motion, extremity weakness, or paresthesias should be considered at high risk for a cervical spine injury. The athlete should be removed from the activity pending a full evaluation before any considerations of return to play.

High-dose intravenous steroids should be considered in patients with suspected cervical cord injuries to reduce spinal cord swelling.

Formal physical examination of the spine and extremities should be limited until unstable lesions of the cervical spine are ruled out. Once the atlantoaxial joint and cervical spine are deemed stable, further examination can proceed. In the office setting, the clinician must use the history to rule out potentially unstable lesions before performing a full physical examination.

The cervical neck examination includes an assessment of the cervical range of motion, palpation of the cervical spine, and performance of the Spurling maneuver (ie, axial load on head with neck extension and lateral rotation toward each shoulder). Paresthesias radiating past either shoulder signify a positive Spurling maneuver for cervical nerve root impingement.

Full reflex, motor, and sensory examination of the upper extremity is also indicated, with the neck in neutral as well as in a flexed position.

The physical findings are often completely normal in patients with radiographically documented AAI but who have no symptoms.





Imaging Studies

In acute trauma, the initial radiologic evaluation includes 3 views of the cervical spine: anteroposterior (AP), cross-table lateral, and open mouth (odontoid). Significant findings on the AP and lateral views include a soft-tissue space greater than 7 mm anterior to C1-C3; this may represent an occult fracture or instability. The odontoid view may show odontoid anomalies or rotary displacement of the odontoid relative to the atlas (C1) arch.

A formal diagnosis of atlantoaxial instability (AAI) requires lateral cervical spine imaging in flexion and extension. Note: After trauma, the 3 views mentioned above and clinical findings must be cleared before the flexion and extension images are acquired. The patient must also be alert and able to cooperate with the examination.

Instability must be confirmed by looking at the distance between the posterior edge of the anterior arch of C1 and the anterior edge of the odontoid process. This area is referred to as the atlantodens interval (ADI) (also called the atlas-dens interval) (see the image below). On the lateral view, the ADI should be less than 3.5 mm in flexion; this suggests that the transverse ligament is intact. In adults, an ADI of 3-5 mm indicates that the transverse ligament is insufficient. In children, this value may be as large as 4.5 mm and still be considered normal. An ADI of greater than 5 mm is indicative of misalignment of the alar ligaments.

Medial-sagittal cross-sectional view of the atlas Medial-sagittal cross-sectional view of the atlas (C1) and the odontoid process of the axis (C2).

Cervical spine computed tomography (CT) scanning with bone windows can be used to further define injuries to the bone elements of the cervical spine, whereas cervical magnetic resonance imaging (MRI) may be better for evaluating soft-tissue and cervical cord involvement. These studies are not routinely performed and are generally reserved for either extreme cases or situations of diagnostic uncertainty.

In cases of rheumatoid arthritis, MRI is indicated when a neurologic deficit (myelopathy) occurs or when plain radiographs show (1) atlantoaxial subluxation with a posterior ADI less than or equal to 14 mm, (2) any degree of atlantoaxial impaction, or (3) subaxial stenosis with a canal diameter less than or equal to 14 mm.

A screening evaluation for AAI in at-risk populations (ie, those with Down syndrome) is controversial. However, certain governing bodies, such as some Special Olympics state organizations, require screening.[6]

The Special Olympics requires lateral flexion and extension radiography of the cervical spine before any athlete with Down syndrome can participate in activities with a risk of spinal injury.[6] These include gymnastics, soccer, alpine skiing, high jump, equestrian sports, the pentathlon, diving, swimming with the butterfly stroke, and swimming with diving starts. If the ADI is greater than 5 mm, athletes are prohibited from participating in these at-risk sports unless an informed consent and clearance has been obtained from 2 physicians per Special Olympics guidelines.

Some authors advocate screening radiography in all at-risk populations at the age of 5 years. If the findings are abnormal, follow-up imaging every 2-3 years is recommended. When the initial findings are normal, follow-up imaging is controversial. Some authors advocate no formal follow-up unless symptoms evolve, whereas others recommend scheduling repeat radiography when the patient is aged 12 and 18 years, because these are periods of rapid growth.

Arguments against screening include the following: (1) abnormal radiographic findings can revert to normal, and normal results can become abnormal; (2) the risk of asymptomatic AAI becoming symptomatic, with neurologic issues, is uncertain. Symptomatic AAI is better identified by means of history taking and regular neurologic examination, which may have a greater predictive value than screening; and (3) the efficacy of avoiding certain sports to prevent symptomatic AAI has never been formally studied.

The American Academy of Pediatrics (AAP) once supported screening lateral neck radiography in patients with Down syndrome who participate in the Special Olympics.[7] The AAP has retired that recommendation and considers lateral plain radiography of potential but unproven value in identifying patients who are at risk for spinal cord injury during sports participation. Assessing the patient's complaints or physical findings has become a greater priority than obtaining radiographs.

The Faculty of Sport and Exercise Medicine (FSEM), United Kingdom (UK), does not support the use of plain radiography to screen for asymptomatic AAI in patients with Down syndrome before sports participation. Instead, the FSEM, UK, recommends that a preparticipation screening should include a focused history taking, a neurologic examination, and an assessment of neck control.[10]



Acute Phase

Rehabilitation program

Physical therapy

If asymptomatic AAI is detected on screening evaluation of an individual without recent trauma or inflammation, physical therapy may help in teaching patients proper head control and avoidance of extremes of motion or at-risk activities.

Medical issues/complications

The initial management of either traumatic or inflammation-induced AAI depends on the presence and progression of the patient's neurologic symptoms. An experienced neurosurgeon or spinal surgeon should be consulted in most, if not all, cases.

  • Posterior fusion of the upper cervical spine with central cord decompression is indicated in any unstable atlantoaxial joint or in the presence of significant myelopathy.[11] The patient should remain strictly immobilized while awaiting expedient surgical referral and procedures. Surgery is immediately indicated only in cases of irreducible canal compromise or progressive neurologic deterioration. Otherwise, it can be scheduled on a less-emergent basis.Posterior fusion of the upper cervical spine may also be indicated in cases of os odontoideum (nonunion of a previous odontoid fracture) or other forms of odontoid aplasia or hypoplasia.A study by Claybrooks et al reviewed 2 techniques for atlantoaxial fusion. The authors reported that "C1 lateral mass to C2 pedicle (C1LM-C2P) fixation is equivalent to C1 lateral mass to C2 laminar (C1LM-C2L) fixation in flexion/extension and AP translation and superior in lateral bending and axial rotation."[12]

  • Cases of inflammation-related subluxation may require reduction under general anesthesia with subsequent cast fixation or traction under the guidance of an experienced spinal surgeon.

  • Traumatic or inflammation-induced acute AAI that involves stable lesions without any neurologic symptoms may be reduced. The patients may be placed in a halo brace with vest reduction and immobilization for 3 months.[13] Repeat radiographs are necessary after the reduction has been completed; several sources warn that deformity may still be possible, even after reduction.

  • In children with confirmed radiographic evidence of transverse ligament disruption less than 3 weeks old, the likelihood of ligamentous healing increases with halo brace and vest management.

  • In adults, healing of the transverse and alar ligaments is unreliable. Therefore, some authorities do not consider nonoperative management in adults, whereas others favor nonoperative management because of the risk of common surgical complications, the often-incomplete resolution of neurologic symptoms, and the lack of long-term data supporting surgical management.

Surgical intervention

Posterior fusion of asymptomatic individuals with AAI, such as patients with Down syndrome, remains controversial. Although some authorities advocate fusion to reduce the risk of a catastrophic trauma to the spine, others do not recommend fusion if the patient remains asymptomatic.

It is important to note that the incidence of serious cervical spine injury is not increased in patients with Down syndrome and AAI, as compared with other athletic populations. Posterior fusion of the upper cervical spine is mainly indicated in symptomatic individuals.


A spinal surgeon or neurosurgeon should be consulted in all cases of acute AAI. In cases of asymptomatic AAI found on screening examination, referral is indicated to confirm the diagnosis and to evaluate possible activity restrictions.

Recovery Phase

Rehabilitation program

Physical therapy

After prolonged patient immobilization, physical therapy can increase the cervical range of motion and assist the patient in regaining strength deficits due to immobilization. An experienced therapist can also emphasize the need to avoid at-risk activities and extremes of cervical flexion and/or extension.

Surgical intervention

Posterior cervical fusion is indicated in patients who present with a deformity that has been present for longer than 3 months or who present with recurrence after 6 weeks of immobilization. Such fusion can also be considered in the patient with chronic AAI who develops acute symptoms or neurologic compromise.


A study examined in-hospital mortality and postoperative major complications in patients undergoing fusion surgery for atlantoaxial subluxation (AAS) and to examine whether the risk of perioperative complications varies between patients with and without rheumatoid arthritis (RA). The study reported that the in-hospital mortality after fusion surgery for AAS was relatively low. However, patients with RA had an increased risk of postoperative complications and massive blood transfusion compared with patients without RA.[14]



Medication Summary

Some authorities and cervical spine trauma protocols recommend the administration of high-dose intravenous steroids in the initial presentation of acute cervical spine injuries. The reasoning is to potentially reduce cervical cord swelling and subsequent neurologic sequelae. This approach is not a uniform treatment recommendation. Otherwise, no medical therapy is indicated for symptomatic or asymptomatic AAI instability.


Class Summary

These agents have anti-inflammatory properties and cause profound and varied metabolic effects. Corticosteroids modify the body's immune response to diverse stimuli.

Methylprednisolone (Solu-Medrol, Depo-Medrol, Medrol)

Decreases inflammation by suppressing migration of polymorphonuclear leukocytes and reversing increased capillary permeability.



Return to Play

An abnormal ADI on lateral flexion-extension radiographic views is an absolute contraindication to participation in contact sports.[9] Noncontact exercise is permitted, although long-term data about the absolute risk are lacking.

Identified anomalies of the odontoid process represent another absolute contraindication to participation in contact sports, as does a history of cervical fusion or a previous unstable upper-cervical fracture.

The clinician must emphasize to patients with AAI that they need to refrain from activities that place the neck in extremes of flexion and extension.

In the Special Olympics, asymptomatic individuals with AAI may be prohibited from participating in several activities that are thought to have a theoretically increased risk of cervical cord injury.[6] These include gymnastics, diving, the pentathlon, swimming with the butterfly stroke, swimming with a diving start, high jumping, soccer, and performing certain warm-up exercises. Since these restrictions are no longer national, individual state Special Olympic organizations should be contacted for current guidance.

An experienced spinal surgeon or a consultant well versed in the demands of the particular activity should manage all return-to-play considerations.

Related Medscape topic:

Resource CenterExercise and Sports Medicine