Dalley, 10th Edition

103

Vertebral Column

band extends between the tubercles on the medial aspects of the lateral masses of C1 vertebrae (see Fig. 2.7A). Vertically oriented but much weaker superior and inferior longitudinal bands pass from the transverse ligament of the atlas to the occipital bone superiorly and to the body of C2 in feriorly. The cruciate ligament of the atlas , so named be cause of its resemblance to a cross, consists of the transverse ligament of the atlas plus the longitudinal bands (Fig. 2.22A). The alar ligaments extend from the sides of the dens of the axis to the lateral margins of the foramen magnum. These short, rounded cords, approximately 0.5 cm in diam eter, attach the cranium to the C2 vertebra and act as check ligaments in preventing excessive rotation at the joints. The tectorial membrane (Figs. 2.22A and 2.23A) is the strong superior continuation of the posterior longitudinal ligament that broadens and passes posteriorly over the me dian atlantoaxial joint and its ligaments. It runs superiorly from the body of C2 through the foramen magnum to attach to the central part of the oor of the cranial cavity, formed by the internal surface of the occipital bone. Movements of Vertebral Column The range of movement of the vertebral column varies ac cording to the region and the individual. Contortionists, who begin their training during early childhood, become capable of extraordinary movements. The normal range of move ment possible in healthy young adults is typically reduced by 50% or more as they age. The mobility of the vertebral column results primarily from the compressibility and elasticity of the IV discs. The vertebral column is capable of exion, extension, lateral ex ion and extension, and rotation (torsion) (Fig. 2.25). Bending of the vertebral column to the right or left from the neutral (erect) position is lateral exion ; returning to the erect pos ture from a position of lateral exion is lateral extension . The range of movement of the vertebral column is lim ited by the • thickness, elasticity, and compressibility of the IV discs • shape and orientation of the zygapophysial joints • tension of the joint capsules of the zygapophysial joints • resistance of the back muscles and ligaments (e.g., the ligamenta ava and posterior longitudinal ligament) • attachment to the thoracic (rib) cage • bulk of surrounding tissue Movements are not produced exclusively by the back muscles. They are assisted by gravity and the action of the anterolateral abdominal muscles. Movements between ad jacent vertebrae occur at the resilient nuclei pulposi of the IV discs (serving as the axis of movement) and at the zyg apophysial joints (see Figs. 2.16 and 2.17). The orientation of the latter joints permits some move ments and restricts others. With the exception perhaps of C1–C2, movement never occurs at a single segment of the column. Although movements between adjacent verte brae are relatively small, especially in the thoracic region,

Anterior atlanto occipital membrane

Anterior atlantoaxial membrane Anterior longitudinal ligament

Joint capsule of atlantooccipital joint

2

Axis

(A) Anterior view

Foramen for vertebral artery Posterior atlantooccipital membrane

Occipital bone

Joint capsule of lateral atlantoaxial joint Joint capsule of atlanto occipital joint

Posterior atlantoaxial membrane

(B) Posterior view

The cranium and C1 are also connected by anterior and posterior atlantooccipital membranes , which extend from the anterior and posterior arches of C1 to the anterior and pos terior margins of the foramen magnum (Figs. 2.23A and 2.24). The anterior membranes are composed of broad, densely wo ven bers (especially centrally where they are continuous with the anterior longitudinal ligament). The posterior membranes are broad but relatively weak. The atlantooccipital membranes help prevent excessive movement of the atlantooccipital joints. Atlantoaxial Joints. There are three atlantoaxial articu lations (Figs. 2.22 and 2.23B, C): two (right and left) lateral atlantoaxial joints (between the inferior facets of the lateral masses of C1 and the superior facets of C2) and one median at lantoaxial joint (between the dens of C2 and the anterior arch of the atlas). The lateral atlantoaxial joints are gliding-type syno vial joints, whereas the median atlantoaxial joint is a pivot joint. Movement at all three atlantoaxial joints permits the head to be turned from side to side (Fig. 2.23D), as occurs when ro tating the head to indicate disapproval (the “no” movement). During this movement, the cranium and C1 rotate on C2 as a unit. During rotation of the head, the dens of C2 is the axis or pivot that is held in a socket or collar formed anteriorly by the anterior arch of the atlas and posteriorly by the transverse lig ament of the atlas (Figs. 2.22A and 2.23A, B, D). This strong FIGURE 2.24. Membranes of craniovertebral joints. A. Anterior membranes and ligaments. Only the thicker, most anterior part of the anterior longitudinal ligament is included here to demonstrate its superior continuation as the anterior atlantoaxial membrane and anterior atlantooccipital membrane. Laterally, the membranes blend with the joint capsules of the lateral atlantoaxial and atlantooccipital joints. B. Posterior membranes. The posterior atlantooccipital and atlantoaxial membranes span the gaps between the posterior arch of the atlas (C1) and the occipital bone (posterior margin of the foramen magnum) superiorly, and the laminae of the axis (C2) inferiorly. The vertebral arteries penetrate the atlantooccipital membrane before traversing the foramen magnum.

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