The atlas is the top most vertebra and chief peculiarity of atlas is that is has no body, it is ring like and consist of anterior and posterior arch and two lateral masses
What is axis?
The 2nd cervical vertebra has a concave under side and convex from side to side. The most distinctive characteristic of this bone is strong odontoid process, the dens.
Atlanto-occipital joint
This is the joint between the atlas and the occipital bone. It consists of a pair of condylar joints which are synovial socket type joints.
Atlantoaxial joint
The atlantoaxial joint a complex of 3 synovial joints which join the atlas (C1) to the axis (C2).
It is made up of
Paired lateral atlanto- axial joints: classified as planar type joint between the lateral masses of C1 and C2 though somewhat more complex in shape with concavity of the superior axial facets.
Medial atlanto-axial (atlanto-dental) joint: Pivot joint with anterior and posterior articulations of odontoid process/ dens of C2 and anterior arch and transverse ligament of C1.
Fibrous capsules of the lateral and medial joints are thin and provide only minimal support, therefore ligamentous attachments provide the majority of support
Attachments
- Anterior longitudinal ligament: thick fibrous band attaches from inferior border of anterior arch of C1 to the anterior axial body.
- Posterior atlanto occipital membrane (continuation of ligamenta flava): attaches from lower border of atlantal arch (and more superiorly to the occipital bone) and upper borders of lamina of C2.
- Cruciform/ cruciate ligaments: superior, transverse and inferior bands posterior to dens; transverse is strongest portion which stabilises odontoid to the lateral mass of C1 limiting lateral motion of C1 over C2 and limiting subluxation of the joint.
- accessory atlanto-axial ligament: from posterior body of C2 to lateral mass of C1
- there are a number of ligaments which attach the axis to the occipital bone including the tectorial membrane, the paired alar ligaments, medial apical ligaments and the longitudinal parts of the cruciform ligaments.
Function
The atlanto-axial joint allows 10-15 degress of flexion/ extension and 30 degrees of axial rotation
Atlantoaxial Instability
AAI may occur as a result of abnormalities or trauma associated with the C1-2 articulation, causing excessive movement around this joint. This includes the articulation between the anterior arch of C1 and the odontoid process of C2, as well as the facet joints of the posterior elements. The following three patterns are noted:
- Flexion-extension
- Distraction
- Rotation
The most common abnormalities involve the transverse ligament or odontoid process. The strong transverse ligament and the facet capsules maintain the integrity of the atlantoaxial articulation. The transverse ligament is the primary restraint against anterior translation of the C1 on C2, whereas the odontoid is the primary restraint against posterior translation. Cadaveric studies have demonstrated that the transverse ligament is stronger in resisting anteroposterior (AP) forces before failure as compared with lateral forces.
AAI is defined as an atlantodental (or atlantodens or atlas-dens) interval (ADI) of greater than 3 mm in adults and of greater than 5 mm in children as measured on plain radiography. The ADI is the distance between the odontoid process and the posterior border of the anterior arch of the atlas.
Symptomatic AAI occurs when subluxation or dislocation causes the odontoid process, or posterior arch of the atlas, to impinge on the spinal cord and cause neurologic manifestations. In addition, motion of the C1-2 segment can cause compression of adjacent or exiting nerve roots. No evidence exists that individuals with asymptomatic AAI are at higher risk for the development of symptomatic AAI.
Vertical displacement of the atlas requires widening of the C1-2 facet joint. The underlying problem is the disruption of the alar ligament, the superior longitudinal band of the cruciate ligament, the tectorial membrane, or a combination of these structures.
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