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 » Introduction
 » Conclusions
 » Acknowledgment
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Table of Contents    
Year : 2012  |  Volume : 60  |  Issue : 1  |  Page : 9-17

Atlantoaxial dislocation

Department of Neurosurgery, Sir Ganga Ram Hospital, Rajendra Nagar, New Delhi, India

Date of Submission16-Nov-2012
Date of Decision30-Nov-2012
Date of Acceptance11-Dec-2012
Date of Web Publication7-Mar-2012

Correspondence Address:
Vijendra K Jain
Department of Neurosurgery, Sir Ganga Ram Hospital, Rajendra Nagar, New Delhi - 110 060
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0028-3886.93582

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 » Abstract 

Atlanto-axial dislocations (AADs) may be classified into four varieties depending upon the direction and plane of the dislocation i.e. anteroposterior, rotatory, central, and mixed dislocations. However, from the surgical point of view these are divided into two categories i.e. reducible (RAADs) and irreducible (IAADs). Posterior fusion is the treatment of choice for RAAD. Transarticular screw fixation with sub-laminar wiring is the most stable& method of posterior fusion. Often, IAAD is due to inadequate extension in dynamic X-ray study which may also be due to spasm of muscles. If the anatomy at the occipito-atlanto-axial region {O-C1-C2; O: occiput, C1: atlas, C2: axis} is normal on X-ray, the dislocation should be reducible. In case congenital anomalies at O-C1-C2 and IAAD are seen on flexion/extension studies of the cervical spine, the C1-C2 joints should be seen in computerized tomography scan (CT). If the C1-C2 joint facet surfaces are normal, the AAD should be reducible by cervical traction or during surgery by mobilizing the joints. The entity termed "dolichoodontoid" does not exist. It is invariably C2-C3 (C3- third cervical vertebra) fusion which gives an appearance of dolichoodontoid on plain X-ray or on mid-saggital section of magnetic resonance imaging (MRI) or CT scan. The central dislocation and axial invagination should not be confused with basilar invagination. Transoral odontoidectomy alone is never sufficient in cases of congenital IAAD, adequate generous three-dimensional decompression while protecting the underlying neural structures should be achieved. Chronic post-traumatic IAAD are usually Type II odontoid fractures which get malunited or nonunited with pseudoarthrosis in dislocated position. All these dislocations can be reduced by transoral removal of the offending bone, callous and fibrous tissue.

Keywords: Atlanto-axial dislocation, axial invagination, basilar invagination, central, hyper-mobile, irreducible, mobile, post-traumatic, transoral surgery

How to cite this article:
Jain VK. Atlantoaxial dislocation. Neurol India 2012;60:9-17

How to cite this URL:
Jain VK. Atlantoaxial dislocation. Neurol India [serial online] 2012 [cited 2023 Nov 30];60:9-17. Available from:

 » Introduction Top

The opposing facetal surfaces of the normal atlanto-axial joints (AAJs) are almost flat and parallel. These are horizontal in the sagittal plane and mildly sloping downwards mediolaterally in the coronal plane [Figure 1]. Therefore they can have unrestricted movements in various planes. [1],[2] There are 6 degrees of freedom of movements at these joints: (1) angular motion (flexion and extension); (2) rotation (right and left); and (3) linear motion and translation (anterior and posterior, right and left). However, anterior movement of the axis vertebra (C2) is restricted by the anterior arch of  Atlas More Details (C1) when odontoid is normal. Posterior movement is restricted by cruciate ligament (largely by transverse ligament). Rotatory movement is restricted by the alar ligaments. [1],[2] These AAJs form the most mobile segment of the whole spine because of which they have more chances of developing instability. Dislocation due to instability of these joints leads to compression of vital cervicomedullary neural structures that lie underneath and results in neurological disability. A minor trauma may precipitate quadriplegia, respiratory arrest and even death. [3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13]
Figure 1: Normal atlanto-axial joints in both left and central columns, oblique orientation in right column

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Management of atlanto-axial dislocation (AAD) involves careful study of radiological findings to decide the direction and plane of dislocation, and looking for associated skeletal anomalies and Arnold  Chiari malformation More Details (ACM) in this region. These findings help to decide the surgical strategy. [14],[15] As per the direction and plane of dislocation we may have the following types of AAD: (1) Anteroposterior dislocation (mobile and hyper-mobile); (2) Rotatory dislocation; (3) Central dislocation; and (4) Mixed dislocation (any two or three of the above)

Anteroposterior dislocation

  1. Mobile dislocation is in one plane and one direction. This is due to laxity of the transverse ligament. Therefore C2 dislocates posteriorly in flexion and gets aligned (reduces) in extension. Its anterior movement is prevented by the intact anterior arch of atlas [Figure 2]. [16],[17]
  2. Hypermobile dislocation is in one plane and two directions. When there is os odontoideum, the C2 body movements can not be restricted by the transverse ligament and anterior arch of atlas therefore the C2 body dislocates in both directions in the sagittal plane i.e. posteriorly under the transverse ligament in flexion and anteriorly under the arch of atlas in extension [Figure 2]. It is important to identify this hypermobile variety before surgery. During intubation for surgery, anesthetists generally extend the neck of the patient which can result in anterior dislocation of C2 and cause cord injury. Therefore fiber-optic intubation with cervical collar in situ is preferred for these patients. These patients should not have flexion or extension movement of neck during intubation. [17],[18]
Figure 2: Top row shows mobile AAD in flexion/extension, middle row shows hypermobile AAD in flexion/extension and bottom row shows rotatory dislocation, rotated left C2 facet being posterior to C1

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Rotatory dislocation

This type of dislocation is usually in one plane (axial) and in one direction only i.e. to the right or to the left. This is due to incompetence of the alar ligament. It usually occurs in children and is visible as the classical Cocked Robin position of the head. [19],[20] One can clearly see in CT that on one side the AA joint is normal but there is posterior displacement of the C2 facet in the joint at the other side [Figure 2].

Central dislocation

The opposing facetal surfaces of the normal C1-C2 joints are horizontal and parallel in the sagittal plane as explained earlier [Figure 1]. However, if these are oriented obliquely in the sagittal plane as shown in [Figure 1] then the C2 body has a tendency to slip upwards due to the weight of the patient's head during flexion movements. Therefore the bilateral sagittal plane obliquity of these joints will result in the telescoping of the C2 body into the C1 ring and result in central dislocation. This telescoping will also invariably result in the posterior movement of C2. The CT picture of this type of dislocation is shown in [Figure 3]. The C1 is assimilated with the occiput and there is C2-C3 (C3-third cervical vertebra) fusion. Many neurosurgeons and radiologists report this type of picture (after seeing only sagittal views) as dolichoodontoid and basilar invagination (BI). In fact it is fusion of the C2-C3 bodies, therefore the C2 body with the odontoid as a whole is mistaken for dolichoodontoid. After doing so many transoral operations I have come to the conclusion that the dolichoodontoid entity does not exist. This radiological finding is not BI either; it is central dislocation of C2. One can see the body of C2 behind the anterior arch of atlas in the axial view of the CT scan and there is also posterior dislocation of C2.
Figure 3: Central dislocation, top row-sagittal section and bottom row-axial section

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Basilar invagination or something else?

Quite often one diagnoses a patient as having AAD with BI. However, this diagnosis is not always correct. The term 'BI' is used too casually. In BI, basal structures that form the foramen magnum should go up or invaginate into the posterior cranial fossa. The invagination of the foramen magnum will automatically take the C1 and C2 upwards with it. However, the relationship of C1 and C2 should remain normal in BI [Figure 4]. If C2 goes up into the C1 ring then the diagnosis should either be that of central dislocation (in milder forms) or axial invagination (in extreme forms). Basilar invagination is the abnormal relationship between foramen magnum and posterior fossa while central dislocation or axial invagination is the abnormal relationship between C1 and C2. BI may or may not coexist with central dislocation or axial invagination. [21]
Figure 4: Basilar invagination

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Mixed dislocation

This dislocation is in two planes and directions: (a) central and posterior-as already discussed central dislocation may be associated with posterior dislocation in many cases [Figure 3]; (b) posterior dislocation and rotatory dislocation [Figure 5].
Figure 5: Mixed dislocation

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Surgical management of atlanto-axial dislocation

From the surgical point of view there are two broad categories of AAD and the categorization is based on whether C1 and C2 can be aligned or not. If C1 and C2 can be aligned by flexion/extension movements of the neck or by putting cervical traction, then the AAD is called reducible AAD (RAAD). If it is not possible to align them it is called as fixed or irreducible AAD (IAAD). [15],[16],[17],[22],[23],[24],[25] The aims of the surgical treatment of AAD are: (1) All dislocations should be reduced; (2) If the AAD cannot be reduced by the closed method (traction), then the open method (i.e. by opening the joints) should be used for reducing the AAD; and (3) If the dislocation is irreducible by the above means then sufficient decompression of underlying neural structures should be done. In all cases, after achieving the above aims, arthrodesis has to be done for achieving permanent bone fusion to prevent movement between C1 and C2. [11],[12]

Reducible atlanto-axial dislocation

The surgical procedures which are being discussed here [Table 1] are used for the internal immobilization of C1 and C2 after reduction of AAD. Bone grafts have to be used in all the procedures for ultimate bone fusion (arthrodesis).
Table 1: Procedures for internal immobilization of C1 and C2 in RAAD

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Sub-laminar wiring

This is a technique that was described long back and is still in use by a very large number of surgeons in India. Various techniques of sub-laminar wiring have been described. [26],[27],[28] There are two main problems associated with sub-laminar wiring. One is the possibility of the wire cutting through the arch of atlas and the other is the absorption of the bone graft (interposed bone graft); and both problems give rise to redislocation [Figure 6]. To avoid wire cutting through the arch of atlas, one should study the posterior arch of C1 in the CT scan and see if the posterior arch is bifid (even a hairline defect) or hypoplastic. If such defects are seen then sub-laminar wiring should not be done. The normal posterior arch is very strong and does not break or get cut through. [12] To avoid redislocation due to bone graft absorption we should never put a piece of graft between the C1 posterior arch and laminae and the spinous process of C2 (interposed bone graft) and then tie a wire on it. Instead one should first bring C1 and C2 close together, then tie the wire and place an onlay graft [Figure 7]. One should always do three-point fixations if sub-laminar wiring technique alone is used. One central braided wire and two paramedian wires should be used. Notches of about 1 mm should be made to avoid sideways slipping and loosening of these wires. Bone-graft pieces should be inserted in the crevices between the wire and the bone and on the bare bone. [11],[12] It is a good idea to open both AAJs to remove the cartilage and abrade the facet surfaces with a cutting drill to achieve bony fusion at the joint level itself where the problem actually exists. In cases where the posterior arch of C1 is assimilated with the occiput or is bifid, a similar procedure of sub-laminar wiring can be performed after creating an artificial arch as shown in [Figure 7]. [29],[30] This technique cannot be used when ACM is associated with AAD; when posterior elements of the axis are hypoplastic [31] or when one has to remove the bone for posterior decompression. [32],[33]
Figure 6: Mobile AAD Top row: Preoperative X-ray; Middle row: Immediate postoperative X-ray; Bottom row: 6 months later redislocation

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Figure 7: Top row-Arch of C1 (left) and artificial arch (right) touching spinous process of C2 Middle row-Sub-laminar wiring, diagrammatic and operative pictures; Lower row-Contoured steel rod occipito-cervical fusion in dislocated position

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Occipito-cervical fusion

Many techniques have been described for occipito-cervical fusion. Most often contoured rods are used for this purpose . [34],[35],[36] In my experience it is very difficult to contour steel rods. The currently available occipito-cervical titanium constructs are easier to contour. Usually, contoured rod fusion extends from the occiput to the C4 level. This unnecessarily restricts neck movements at more levels than required. This also puts more strain at a lower level below the fusion which may result in adjacent segment disease. In my view, fusion of more levels than C1 and C2 should not be done in cases where the problem lies only at the AAJ and there is no reason to take the occiput or lower levels in fusion. Another problem with this technique is that the fusion of C1 and C2 often occurs in their dislocated position [Figure 7]. To avoid this problem, one should always hold the C1 arch/artificial arch by the sub-laminar wire and pull it backwards so that it closely touches the rod after the wire is tightened. There are some situations in which this technique is indicated, e.g.there was a patient who had assimilated arch of C1, mobile AAD, C2-C3 prolapsed intervertebral disc and listhesis of C3-C4 in extension, therefore the occipito-cervical fusion of these levels was done with C2 and C3 laminectomy [Figure 8].
Figure 8: Occipito-cervical fusion

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C1-C2 spacer jamming

C1-C2 spacer jamming technique has been described to do posterior fusion in AAD. [37],[38],[39] This was a patient who had central and posterior dislocation of C2. The AAJs were oblique. The AAD was reduced per-operatively and cages with bone-graft were put in both the joints [Figure 9]. He gradually improved completely from quadruparesis within 15 days. However, after one and a half month he slipped on a wet floor and expired instantaneously. We do not know the exact cause of death but there is a possibility that there was gross dislocation of AAJs due to the fall as these joints were widely opened and distracted. The lesson we learnt is that if this technique is used one should also always do posterior fusion by sub-laminar wiring or fusion of C1-C2 with instrumentation.
Figure 9: C1-C2 spacer jamming, preoperative in top row and postoperative bottom row, right joint sagittal section, mid-sagittal section, left joint sagittal section, both joints coronal section are shown in that order from left to right

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Trans-articular screw with sub-laminar wiring

This is the most elegant and most stable method of posterior fusion. [40],[41] As shown in laboratory studies, the range of movements at AAJ is minimal when Magerl technique (Transarticular fixation) is used as compared to other methods. An example is shown in [Figure 10].
Figure 10: Trans-articular screw with sub-laminar wiring Top row: flexion/extension study; 2nd row shows MRI and CT; 3rd and 4th rows show postoperative CT

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Fixed irreducible atlanto-axial dislocation

Conventionally and traditionally, if AAD does not reduce in flexion and extension studies of the cervical spine in lateral view radiograph, it is called irreducible AAD (IAAD). This is only indirect information since it is gathered by seeing the relationship of the odontoid of the C2 body to the anterior arch of the C1 without seeing the actual AA joints where the problem exists. Therefore this definition has the following pitfalls: (1) there may be inadequate extension of the neck for C2 to come in alignment with C1 ; (2) there may be pain and muscle spasm making C2 immobile even on adequate extension ; and (3) the odontoid may have slipped behind the transverse ligament [Figure 11].
Figure 11: Odontoid slipping behind intact transverse ligament

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It is therefore suggested that one should not rely only on flexion/extension studies. First of all, one should know whether flexion and extension were done to a sufficient extent or not. This should be done by looking at the distance between the mandible and the cervical spine. The angle of the mandible almost touches the spine in flexion and it should move significantly away from the spine in extension. Sometimes the flexion and extension is done only at lower cervical spine and the distance between the mandible and the spine does not change significantly resulting in an incorrect study. If one is satisfied with the flexion/extension studies and concludes that the AAD is not reducible, one should focus on the AAJ in coronal and sagittal views of MRI and CT. If the AA joints' facets have near normal orientation [Figure 1], then the AAD should be considered reducible. All these will reduce either on traction or during surgery by opening the joints as in this case, a proper flexion/extension study shows IAAD. However, the C1 and C2 facets are seen nicely on MRI and CT scan and they could be completely and easily reduced by posterior sub-laminar wire technique [Figure 12]. Finally I would like to state that if O-C1-C2 (O- occiput) is normal on plain X-ray, the dislocation should be reducible. If there is any congenital anomaly (such as, assimilation of C1 and/or C2-C3 fusion) associated with AAD, and the AAD is irreducible on proper flexion/extension studies, one should study the AAJ in MRI and/or CT scan. If the AAJ facet surfaces are normal, the IAAD can be reduced on cervical traction or by surgery.
Figure 12: Top row shows preoperative flexion/extension and postoperative X-ray, 2nd and 3rd rows: MRI and CT show right joint, mid, left joint sagittal sections (left to right)

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The true IAAD should have some bone to bone contact in such a way that mobilization of AAJ is not possible. If the joints become almost vertical in sagittal and in coronal planes giving rise to complete telescoping of the C2 body inside the C1 ring or if the dislocation is in more than two planes (multiplanar) and AA joints are not seen [Figure 13], then it is likely to be true IAAD and it may not be possible to align the facet joints for the purpose of reducing the dislocation. [12],[15],[16],[17],[22]

Axial invagination

When the AAJ becomes almost vertical in both planes as described above, the C2 body invaginates into the C1 ring. This should be called axial invagination (AI). Here it is the axis vertebra which is invaginating inside the C1 ring and foramen magnum. However, if one draws various lines which were described during the era of plain X-ray and tomography, one would call these cases as BI which would be wrong [Figure 13].
Figure 13: First and second columns show multiplanar dislocations-posteriorly, centrally, rotationally, and towards right lateral dislocation of C2; third and fourth columns show axial invagination

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Tran-oral surgery for irreducible atlanto-axial dislocation

The generally used term 'transoral odontoidectomy' for surgery of congenital IAAD is not correct. In fact those patients who can be treated by only odontoid removal all have the reducible variety of AAD. It is actually corpectomy (the extent of which depends upon axial invagination of C2 and C3) with odontoidectomy which is required to provide sufficient decompression of the underlying cervicomedullary neural structures. This is clarified in the case shown in [Figure 14], wherein the C2 body is still causing severe canal compromise after insufficient odontoidectomy, which was corrected after redo transoral surgery . Therefore transoral odontoidectomy seems to be a misnomer. It should be called transoral decompression (TOD) and the bony decompression should be sufficient to provide space for the underlying medulla and cervical spinal cord. [42]
Figure 14: Top row: after inadequate decompression; 2nd row: after proper transoral decompression

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What is sufficient transoral decompression?

The normal contour of the craniovertebral junction is funnel-shaped which becomes hourglass-shaped in true IAAD/AI. Therefore sufficient bone has to be removed for generous three-dimensional decompressions i.e. antero-posterior (z axis), transverse (x axis), and vertical (y axis) to regain this funnel shape which will then provide sufficient space for the underlying cervicomedullary structures without any compression. This should be planned before surgery on CT scan to do sufficient TOD [Figure 15]. [42]
Figure 15: CT myelogram- preoperative three-dimensional planning of transoral decompression in upper row and postoperative sufficient decompression in lower row

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Fixed chronic/remote post-traumatic atlanto-axial dislocation

These are usually Type II odontoid fractures which get malunited or nonunited with pseudoarthrosis in dislocated position. There may be abnormal callous formation preventing reduction of dislocation. All these dislocations can be reduced by transoral removal of the offending bone, callous and fibrous tissue [Figure 16]. [43],[44]
Figure 16: Nonunited fracture of dens with fixed AAD-preoperative CT in top row and postoperative CT in lower row

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 » Conclusions Top

  1. If O-C1-C2 are normal on plain X-ray and there is AAD, the AAD should be reducible.
  2. In case there are congenital anomalies at O-C1-C2 and there is IAAD as well (seen on flexion/extension studies of cervical spine), one should look at the C1-C2 joints on a CT. If C1-C2 joint facet surfaces are normal, the AAD should be reducible by cervical traction or surgery by mobilizing the joints.
  3. Dolichoodontoid entity does not exist. It is invariably a C2-C3 fusion which appears like a dolichoodontoid on plain X-ray or on mid-sagittal section of MRI or CT.
  4. Central dislocation and axial invagination should not be confused with BI.
  5. Detailed radiological assessment is important to understand the dynamics at the AAJ before terming AAD as IAAD.
  6. Every AAD should be considered for reduction by all means before terming it IAAD.
  7. Transoral odontoidectomy alone is never sufficient in cases of congenital IAAD. Adequate generous three-dimensional decompressions, i.e. anterior posterior (z axis), transverse (x axis), and vertical (y axis), should be achieved while protecting the underlying cervicomedullary neural structures.
  8. Fixed chronic/remote post-traumatic AAD-these are usually Type II odontoid fractures which get malunited or nonunited with pseudoarthrosis in dislocated position. All these dislocations can be reduced by transoral removal of the offending bone, callous and fibrotic tissue.

The fact that despite these advancements some patients do not improve to normalcy points to the challenges that lie ahead in the management of AAD.

 » Acknowledgment Top

The concepts which have been discussed here are the result of my discussions with my colleagues Prof Sanjay Behari, Department of Neurosurgery and Prof R.V. Phadke, Department of Radiology, both at SGPGI, Lucknow. We often had differing views while discussing the management of cases of atlanto-axial dislocations admitted under our care. Our discussions helped me to crystallize my views which have been presented here.

 » References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16]

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