Indicators of atlantoaxial instability

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Leonardo Da Vinci

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Henry David Thoreau

The atlantoaxial joint is the most mobile joint of the body and is involved in circumferential neck movements, in saying both “yes” and “no” and in assisting the occipitoatlantal joint in firmly stabilizing the head on the shoulders. Whilst this joint is most mobile, it is also most prone to develop instability. The flat and round atlantoaxial joint that allows movements in all directions also makes it susceptible to develop instability. Our experience in the field suggests that atlantoaxial joint instability is an under-recognized and under-treated clinical entity.

Craniovertebral junction comprises of occipitoatlantal and atlantoaxial joints. Whilst occipitoatlantal joint is the center for stability, atlantoaxial joint is the center for mobility. Both mobility and stability are hallmarks of the craniovertebral junction. Whilst the supremely designed architecture of the craniovertebral junction caters simultaneously to movements and to stability, it also protects the critical neural and vascular structures and facilitates their smooth traverse.

The transverse processes of atlas and spinous processes of the axis bone are the largest and strongest when compared to those in the rest of all the vertebrae in the spine. The muscles attached to the transverse processes of the atlas are involved in the rotatory movements at the atlantoaxial joint and have their fulcrum at the articulation between the posterior surface of the anterior arch of the atlas and the anterior surface of the odontoid process. The muscles attached to the posterior elements of the axis in general and the spinous process, in particular, are involved in flexion and extension movements of the neck. The fulcrum of movements of these muscles is at the atlantoaxial facetal articulations. Majority of muscle bulk in the occipitocervical region and in the rest of the spine is located in the posterior aspect of the vertebral column and apart from active neck movements participate in facilitating the life-long standing human position. The flat articular surfaces of facets of atlas and axis are located one above the other in brick over brick fashion that lay the foundation of the spinal pillar. Strong and steel-like ligaments keep the facetal articulations opposed to each other during all movements.

The only validated parameter that establishes the presence of atlantoaxial instability is the pathological alteration of atlantodental interval on dynamic flexion-extension images of the craniovertebral junction. Although neural compromise or compression of the cervicomedullary cord opposite the odontoid process indicates abnormality at the craniovertebral junction definite presence of instability is not confirmed till the abnormal movements at atlantodental articulation are radiologically established. Vertical,[1] lateral[2] and rotatory[3] atlantoaxial dislocations have defined parameters for establishing diagnosis.

Our studies over several years have identified that atlantoaxial instability can be present even in the absence of any abnormal alteration in atlantodental interval on dynamic imaging. Atlantoaxial instability and related symptoms can be present even in the absence of any direct radiological evidence of neural compression. A host of cranial and spinal symptoms that are seemingly unrelated to craniovertebral junction region neural compression can be related to unstable atlantoaxial joint instability.

Weakness of the muscles related to disuse, abuse, or injury and abnormal ligamentous laxity related to syndromic affliction, particularly in young or pediatric age can lead to instability of the atlantoaxial joint. “Degeneration” related to life long stress of continuous movements can frequently affect atlantoaxial articulation and lead to instability.[4] Cervical myelopathy related to ossification of the posterior longitudinal ligament can frequently be linked to atlantoaxial instability.[5] Tuberculosis and rheumatoid arthritis can lead to the destruction of bones of the articulation and are more common causes of instability and subsequent deformity of the craniovertebral junction.[2,6] Hirayama disease, an entity associated with multilevel cervical spinal instability, can be associated with atlantoaxial instability.[7] Even in the absence of any known primary or initiating factor, there can be atlantoaxial instability. In general, atlantoaxial instability is relatively common and can lead to crippling neurological deficits. Identification and treatment of atlantoaxial instability in such cases can provide an opportunity to treat disabling and devastating neurological symptoms.

There are two discrete forms of atlantoaxial instability. Acute atlantoaxial instability is manifested by rather sudden onset symptoms like severe pain and muscle spasm in the nape of the neck, and neurological symptoms and deficits. Chronic atlantoaxial instability is associated with relatively subtle but longstanding and relentlessly progressive symptoms and neurological deficits that are associated with a range of musculoskeletal and neural alterations. Cranial symptoms such as giddiness, visual obscurations, and sleep disorders can be prominent.[8] Validated radiological evidence of instability may or may not be present in such cases.

In the year 2004, we proposed that alignment of facets on lateral profile imaging with the head in neutral position can be an additional and important guide to suggest the presence of atlantoaxial instability.[9] We divided atlantoaxial instability on the basis of facetal alignment into 3 types. In Type 1 atlantoaxial facetal dislocation there is facetal listhesis or the facet of atlas is dislocated anterior to the facet of axis. This type of instability is usually associated with acute symptoms. Atlantodental interval is frequently abnormally altered and there are evidence of neural compression by the odontoid process. In Type 2 atlantoaxial facetal dislocation, the facet of the atlas is dislocated posterior to the facet of the axis, or there is retrolisthesis of facets. Type 2 atlantoaxial facetal dislocation can also be labeled as partial rotatory atlantoaxial dislocation. In Type 2 atlantoaxial facetal dislocation, there can be complete rotatory atlantoaxial instability wherein the facet of atlas is dislocated behind the facet of axis on one side and anterior to the facet of the axis on the contralateral side.[3] In Type 3 atlantoaxial facetal instability, the facets are in alignment, and instability is assessed by telltale clinical and associated radiological evidence and is confirmed by direct manipulation of bones and identification of instability during surgery. Types 2 and 3 are usually associated with chronic atlantoaxial instability. Types 2 and 3 atlantoaxial facetal instability have been labeled as “central” or “axial” atlantoaxial instability as the atlantodental interval may not be altered and there may be no radiological evidence of neural compression and related altered cord signal.[10] Essentially, there can be atlantoaxial instability and its related musculoskeletal and neural alterations and symptoms without any validated radiological guide that confirms the presence of instability. Identification of instability and its treatment can relieve the patient of his symptoms and provide an opportunity to lead a normal life.[10,11]

Our earlier articles identify a host of musculoskeletal and neural alterations in the face of chronic atlantoaxial instability. These alterations that are generally considered to be “pathological” or abnormal but are protective designs of Nature initiated and established in an attempt to limit the consequences of potential or manifest neural compression. These secondary or protective processes can lead to the identification of the presence of atlantoaxial instability. It is erroneous to label these secondary processes as craniovertebral “anomalies.” We prefer to label them as craniovertebral junction “alterations.”

Basilar invagination is an “umbrella” term used to define the musculoskeletal alterations in the region of the craniovertebral junction. Basilar invagination Type A that is associated with manifest evidence of atlantoaxial instability is usually associated with relatively acute atlantoaxial instability. There is abnormal alteration of atlantodental or clivodental interval and “invagination” or herniation of odontoid process into the foramen magnum leading to neural compression. More often there is Type 1 atlantoaxial facetal instability in these cases.[12]

Basilar invagination Type B is associated with a more chronic form of atlantoaxial instability and the musculoskeletal and neural alterations are prominent and starkly evident in these cases.[13,14] Basilar invagination is associated with several musculoskeletal alterations. Bone fusion alterations include platybasia, assimilation of atlas, C2–3 fusion and Klippel-Feil abnormalities, and pan cervical vertebral fusions.[15,16,17] Other bone abnormalities that include bifid arches of atlas, absent posterior arch of axis and os-odontoideum are also secondary alterations that are dynamic and protective.[18,19,20] Retroodontoid and retro-C2 body pseudotumor and cyst formation are secondary alterations and have a neural protective function.[21] Retroodontoid pseudotumor mimics osteophyte formation in subaxial spine. Like retroodontoid pseudotumors, subaxial spinal osteophytes are also secondary formations and related to segmental instability and have a protective role.[22]

Neural alterations include Chiari formation, syringomyelia, syringobulbia, external syringomyelia, and external syrinogobulbia.[12,13,14,23,24,25] The spinal and cranial neural girth reduces, a process labeled by us earlier as self-neural destruction, is an attempt of Nature to minimize trauma to the more critical nerve fibers. Identification of the fact that both Chiari formation and syringomyelia are secondary alterations related to atlantoaxial instability and have a protective role can radically alter the thinking process behind its surgical treatment.

The short neck can be associated with torticollis and with “degenerative” or kyphotic changes in the subaxial cervical spine.[26,27,28] Short spine can be associated with dorsal spinal kyphoscoliosis. In general “short head” is associated with basilar invagination. Essentially, the process of basilar invagination results in vertical reduction and anteroposterior increase in spinal canal and head dimensions. The vertical reduction of spinal canal length relaxes the neural structures during its course over the odontoid process. Anteroposterior increase in the dimensions of the spinal canal facilitates introduction of extra volume of cerebrospinal fluid in the spinal canal (syringomyelia, external syringomyelia) and cranial cavity (syringobulbia, external syringobulbia) that allows the cord to float away from the potentially indenting odontoid process.[12,13,14]

Diagnosis of atlantoaxial instability is important to initiate correct treatment. The presence of each one of these multiple secondary musculoskeletal and neural alterations either in a cohort or singly indicates the presence of atlantoaxial instability and can direct the treatment towards atlantoaxial stabilization. The concept of central or axial atlantoaxial instability has the potential to radically alter the understanding of instability of the craniovertebral junction and in formulating the treatment strategy.