Goel's classification of atlantoaxial "facetal" dislocation.

Atlantoaxial facet joints are the center of mobility and also center for instability of the atlantoaxial region. It is one of the most mobile joints of the body. It may not be an over-exaggeration to state that all craniovertebral instability can be incriminated to atlantoaxial dislocation. Occipitoaxial joint is the center for stability. Instability related to occipitoaxial joint is remarkably rare and identified infrequently in children with syndromic issues and rarely in cases with trauma. The atlantoaxial joints of two sides along with the occipitoaxial joints form the rostral two limbs of the “Y” shaped configuration of the human spinal support pillar. Any instability of the atlantoaxial region starts from the joint and manifests in the rest of the components of bones of the region. The concept of atlantoaxial instability based on facetal alignment or mal-alignment has not been discussed earlier in the literature. The atlantoaxial instability has been traditionally gauzed by the atlantodental interval that signifies the abnormal movement of the odontoid process away from the circle of atlas and toward the neural structures in the spinal canal. The indentation of the odontoid process into the critical cervicomedullary neural structures result in related symptoms. Such instability is the more common form of atlantoaxial dislocation. The facetal instability can be observed on good quality and modern computer-based imaging. Such visualization was either not possible or was unclear with plain radiography that formed the core investigation not too long ago. To assess the instability of the atlantoaxial joint, it is necessary that the facetal alignment is appropriately evaluated, and the implications of mal-alignment of facets are understood. Three-dimensional computed tomography images provide a perspective of the relationship and alignment of the facets.

The facets of atlas and axis are strong in their material content and larger in size when compared to all the other facets of the spine.[1] Two large superior articular facets of axis flank the odontoid process. Superior facet of C2 vertebra differs from the facets of all other vertebrae in two important characters. First is that the superior facet of C2 is present in proximity to the body when compared to other facets, which are located in proximity to the lamina. The second is that the vertebral artery foramen is present partially or completely in the inferior aspect of the superior facet of C2, while in other cervical vertebrae, vertebral artery foramen is located entirely in relationship with the transverse process. Unlike superior facets of all other vertebrae, they do not form a pillar with the inferior facets, being considerably anterior to these. The course of the vertebral artery in relationship to the inferior aspect of the superior articular facet of the C2 makes its susceptible to injury during Magerl's transarticular and Goel's inter-facetal screw implantation techniques.[2345] The inferior facet of the atlas is almost circular in most of the vertebrae without any significant difference in the mean anteroposterior and transverse (15 mm) dimensions.

All types of dislocations can be divided into mobile and reducible varieties. The dislocation is mobile and reducible when dynamic images with head inflexion show the dislocation and head in extension shows complete reduction of the dislocation. Although such a dislocation can be observed relatively easily by the increase in the atlantodental interval on flexion of head, facetal mal-alignment can also provide additional information about the instability and sometimes can provide a clue as to the pathogenesis of instability. The dislocation can be completely reducible, or it can be partially or incompletely reducible. The classification of any dislocation into reducible and irreducible varieties has considerable therapeutic implications. Whilst fixation in reduced position is necessary in reducible atlantoaxial dislocation anterior or posterior decompressive surgery is possible in cases where the dislocation is fixed. Irreducible dislocations formed a distinct category until not long ago. The dislocation in basilar invagination was also considered to be irreducible. Goel introduced a concept that stated that atlantoaxial dislocation is never or only extremely rarely fixed or irreducible.[6] More importantly, the concept stated that the dislocation could be manually reducible. Even in basilar invagination the dislocation is mobile and manually reducible. This concept has revolutionized the treatment paradigm of a number of pathological entities involving the craniovertebral junction, particularly those where the atlantoaxial dislocation was considered to be irreducible or fixed. Decompressive bone surgery, both from anterior transoral route and also from posterior route are seldom considered in the present day treatment.

The head positioning and natural curvatures of the spine makes it susceptible for anterior dislocation of the facet of atlas over the facet of axis. Such anterior facetal dislocation [Figure 1a] results in posterior movement of the odontoid process away from the anterior arch of atlas and into the spinal canal with eventual possibility of compromise of the critical neural structures at the craniovertebral junction [Figure 1b]. We labeled such an anterior atlantoaxial facetal dislocation as Type 1 dislocation.

Figure 1a

Computed tomography scan showing basilar invagination, assimilation of atlas, os-terminale and severe cord compression

Figure 1b

Sagittal cut of computed tomography scan passing through the facets, showing Type 1 atlantoaxial facetal dislocation

We identified another type of atlantoaxial instability wherein the facets of atlas are dislocated posterior to the facet of axis on lateral profile [Figure 2]. This type of posterior atlantoaxial facetal dislocation has not been described earlier in the literature. The relationship of the facets on dynamic imaging is inconsistent in these cases. We labeled such instability as Type 2 dislocation [Figures 2c–e]. We identified yet another type of dislocation wherein there was no facetal mal-alignment or any alterations in the atlantodental interval [Figures 3a–e]. The instability or dislocation could only be identified by manual manipulations during surgery and can be understood on appropriate clinical evaluation. We labeled such instability as “central” atlantoaxial facetal dislocation. Essentially, it means that in these cases, the instability is present but dynamic images are unable to identify it.

Figure 2a

T2-weighted magnetic resonance imaging with the head in extension position showing basilar invagination, atlantoaxial dislocation, and Chiari malformation

Figure 2c

Computed tomography scan with the head inflexion showing the atlantoaxial facets in alignment

Figure 2e

Computed tomography scan with the head in extensions shows Type 2 atlantoaxial facetal dislocation

Figure 3a

T2-weighted magnetic resonance imaging showing basilar invagination, Chiari malformation and syringomyelia

Figure 3e

Sagittal image of postoperative computed tomography scan with the cut through the facets showing the atlantoaxial implants and an intra-articular spacer

Computed tomography scan with the head in extension position showing a reduction of the atlantoaxial dislocation

Sagittal cut of computed tomography scan through the facets showing no mal-alignment of the facets. This type of instability is Type 3 atlantoaxial facetal instability

Postoperative computed tomography scan showing a reduction of basilar invagination

In Type 2 and Type 3 dislocation, the atlantodental interval is frequently unaffected, and the odontoid process may or may not indent into the neural structures [Figures 2b and 3b]. As the neural compromise is not a prominent or an early feature, Type 2 and Type 3 dislocation are associated with chronic instability and the entire process is longstanding and relentlessly progressive. Although musculoskeletal alterations and neural structural malformations are frequently associated with Type 1 dislocation they are significantly remarkable and hallmarks of Type 2 and Type 3 facetal dislocation. Basilar invagination Group B is frequently associated with Type 2 and Type 3 atlantoaxial facetal dislocation. According to our current concept, all these structural musculoskeletal and neural malformations are natural processes meant to reduce the effect of instability and prevent neural compression and compromise.[7]

Figure 2b

Computed tomography scan with the head inflexed position shows assimilation of atlas and an increase in the atlantodental interval suggesting instability

Figure 3b

Sagittal cut of computed tomography scan showing assimilation of atlas and basilar invagination

Our experience with direct facetal handling and manipulations suggests that the atlantoaxial dislocation is not fixed or irreducible in general and particularly in cases with basilar invagination as was previously considered, but instability of the region is “profound.” We also observed that as soon as stabilization surgery is done, clinical neurological improvement occurs instantly and musculoskeletal and neural alterations reverse. It is as if the cord is stunned in this situation without being compressed. Signals of instability are passed on and reparative natural processes begin and proceed over long time durations. It does seem that instability of the atlantoaxial joint is the primary event and all the other musculoskeletal and neural alterations are secondary and basically protective. It may be that in some cases, the instability is initiated during fetal life, or early infancy and the reparative processes begin at that stage.

Lateral atlantoaxial facetal dislocation [Figures 4a and b] is when the facet of the atlas is dislocated lateral in relationship with the facet of axis.[8] Such a dislocation is frequently identified when the ring of the atlas is bifid or fractured resulting in lateral migration of the facet of atlas in relationship with the facet of axis. Vertical atlantoaxial dislocation is when the odontoid process migrates superiorly on flexion the head and returns back entirely or incompletely to normal position on head extension [Figures 5a–d]. Such a dislocation is related to incompetence of the facet joint. We labeled such a form of dislocation as “vertical mobile and reducible” atlantoaxial dislocation.[9] Rotatory atlantoaxial dislocation [Figures 6a and b] is when the facet of the atlas is dislocated posterior in relationship with the facet of axis on one side and anterior in its relationship on the contralateral side. Such a dislocation results in torticollis.[10] Translatory atlantoaxial dislocation is a clinical situation when the facets of atlas of both sides are dislocated anterior to the facets of axis.[10]

Figure 4a

Coronal image of computed tomography scan showing lateral displacement of facet of atlas in relationship with the occipital condyles and facets of axis

Figure 4b

Axial image of computed tomography scan showing fracture of anterior and posterior arches of atlas

Figure 5a

Computed tomography scan with the head in flexed position showing marked basilar invagination

Figure 5d

Postoperative computed tomography scan showing fixation in reduced position

Figure 6a

Three-dimensional computed tomography scan showing facetal mal-alignment related to rotatory atlantoaxial dislocation. The facet of the atlas is anterior to the facet of axis on one side and posterior to it on the other side

Figure 6b

File photograph of the child showing torticollis

Computed tomography scan with the head in extension showing a reduction of the invagination

Computed tomography scan cut through the facets showing suggestions of incompetence

Surgery that involves direct facetal fixation like transarticular fixation or interfacetal fixation is mechanically and philosophically stronger than those that involve fixation of midline structures.[11] Such fixation procedures provide a “zero movement” situation that is conducive to early bone fusion and arthrodesis.