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STRUCTURE AND FUNCTION

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Atlanto-Occipital and Atlantoaxial Joints

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Anatomy
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The atlanto-occipital joint is composed of the right and left deep concave superior facets of the atlas (C1) that articulate with the right and left convex occipital condyles of the skull (Fig. 11.1).

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FIGURE 11.1

A lateral view of a portion of the atlanto-occipital joint shows the superior atlantal articular process of the atlas (C1) and the corresponding occipital condyle. The joint space has been widened to show the articular processes.

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The atlantoaxial joint is composed of three separate articulations: the median atlantoaxial and two lateral joints. The median atlantoaxial joint consists of an anterior facet on the dens (the odontoid process of C2) that articulates with a facet on the internal surface of the atlas (C1). The two lateral joints are composed of the right and left superior facets of the axis (C2) that articulate with the right and left slightly convex inferior facets on the atlas (C1) (Fig. 11.2).

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FIGURE 11.2

A posterior view of the atlantoaxial joint and the superior, inferior, and transverse bands of the cruciate ligament.

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The atlanto-occipital and atlantoaxial joints are reinforced anteriorly by the anterior-occipital and atlantoaxial membranes (Fig. 11.3A) and posteriorly by the posterior atlanto-occipital, atlantoaxial, and tectorial membranes (Fig. 11.3B).

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FIGURE 11.3

A: The anterior atlanto-occipital and atlantoaxial membranes help to support the anterior aspect of the atlanto-occipital and atlantoaxial joints. B: The posterior atlanto-occipital, atlantoaxial, and tectorial membranes help to support the posterior aspect of the atlanto-occipital and atlantoaxial joints. The tectorial membrane is an extension of the posterior longitudinal ligament.

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Osteokinematics
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The atlanto-occipital joint is a condylar synovial joint that permits active flexion-extension as a nodding motion.1 However, a very limited amount of axial rotation and lateral flexion may be produced passively1 Flexion-extension takes place in the sagittal plane around a medial-lateral axis. Extremes of flexion are limited by osseous contact of the anterior ring of the foramen magnum with the dens. Normally flexion is limited by tension in the posterior neck muscles and tectorial membrane and by impaction of the submandibular tissues against the throat. Extension is limited by the occiput compressing the suboccipital muscles.1 Combined flexion-extension is reported to be between 20 degrees2 and 30 degrees3 and is usually described as the amount of motion that occurs during nodding of the head. However, according to Cailliet,4 the range of motion (ROM) in flexion is 10 degrees and the range in extension is 30 degrees. Maximum rotation at the atlanto-occipital joint is between ...

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