Traumatic Atlantoaxial Dislocation
23 October 2019 (online)
Atlantoaxial dislocation (AAD) refers to the relative motion between C1 and C2 and results from abnormal articulation between the C1 (atlas) and C2 (axis). It occurs across all age groups. C1–C2 instability can be congenital or it can result from traumatic or inflammatory etiologies. Osteoarthritis or degenerative arthritis is a less known cause of C1–C2 instability. We will focus on traumatic AAD in this article.
It is prudent to discuss the anatomy of craniovertebral junction (CVJ), which is quite complex. The C1–C2 joint stabilizes the head in addition to being one of the most mobile joints of the body, allowing a great amount of movement. About half of the total cervical flexion occurs at the occipito-C1 joint, while similar amount of total cervical rotation occurs at the C1–C2 joint.  Other unique anatomical variation in this region is the dens and the transverse ligament, and the absence of intervertebral disc between C1 and C2. The vertebral bodies of C1 and C2 do not directly bear the weight of head, rather it is transferred from occipital condyles to C1 lateral masses and then to the C2 lateral mass. The lateral masses of the C1 and C2 form a pillar of stability and mobility for the head and neck.
Ligaments play a greater role in the stabilization of these joints as compared with other joints in the body. Transverse ligament runs across the posterior aspect of dens and is attached to the lateral masses of C1 on either side and prevents anterior translation of C1 over C2. Alar ligaments extend from the odontoid in a lateral and cephalad direction to the basilar portion of the occiput and provide additional stability. C1–C2 facets are arranged in axial plane and no bony structure is there to prevent dislocation, in contrast to subaxial spine. Transverse ligament presents dislocation in the sagittal plane.
Traumatic AAD can occur in one of the following settings:
We will further discuss the following entities in detail:
Traumatic transverse atlantal ligament (TAL) insufficiency.
Atlantoaxial rotatory subluxation/dislocation.
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