Applied Anatomy of the Normal and Aging Spine

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2 Applied Anatomy of the Normal and Aging Spine

Rajesh G. Arakal, Malary Mani, Ravi Ramachandran

KEY POINTS

• Cervical disc herniations most often affect the exiting root.
• Lumbar posterolateral herniations most often affect the root of the respective lower foramen.
• Acquired lateral recess stenosis is most often a result of hypertrophy of the superior articulating facet.
• Degenerative spondylolisthesis is most common at L4-5 and can entrap the L4 nerve root.
• Aging affects every aspect of the spine, from mineral density of the bones, to the physiology of the intervertebral discs, to the muscular scaffold around the spine.

“Chance favors the prepared mind.” The spinal column consists of 33 vertebrae and is divided into seven cervical, twelve thoracic, and five lumbar vertebrae. The lumbar vertebrae articulate with the sacrum, which in turn articulates with the pelvis. Below the sacrum are the four or five irregular ossicles of the coccyx.

The Vertebrae

The articulations of the spine are based on synovial and fibrocartilaginous joints. The overall morphology of the vertebral column has a basic similarity, with the exception of the first two cervical vertebrae and the sacrum. A vertebra consists of a cylindrical ventral body of trabecularized cancellous bone and a dorsal vertebral arch that is much more cortical. From the cervical to the lumbar spine, there is a significant increase in the size of the vertebral bodies. An exception is the sixth cervical vertebra, which is usually shorter in height than the fifth and seventh vertebrae. In the thoracic spine, the vertebral body has facets for rib articulations. The posterior aspect of the vertebra starts with a posterior apex or spinous process. This process then flows into flat lamina that arch over the spinal canal and attach to the main body through a cylindrical pillar or pedicle. The transverse processes are found at the junction of the confluence of the laminae and pedicles and extend laterally. In the upper six cervical vertebrae, this component is part of the bony covering of the vertebral arteries. In the thoracic spine, the transverse process articulates with ribs. A mature and robust transverse process is found in the lumbar spine, with the remnant neural arch structure forming a mammillary process (Figure 2-1).

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FIGURE 2-1 The Vertebrae

There are points of articulation between the individual vertebral segments between an inferior and ventral facing facet and a superior and dorsal facing facet. It is a diarthrodial, synovial joint. The shape of the facets is coronally oriented in the cervical spine, thus allowing for flexion-extension, lateral bending, and rotation. The facets are sagitally oriented in the lumbar spine and thus resist rotation, while allowing for some flexion and some translational motion.1 Lateral to these joints are mamillary bony prominences upon which muscles can originate and insert.

The pedicles are the columns that connect the posterior elements to the anterior vertebral body. The transverse pedicle widths vary in size, but generally tend to larger dimension from the midthoracic to the lumbar spine, with a decrease of pedicle width from the lower cervical to the upper thoracic spine. Sagittal pedicle height increases from C3 to the thoracolumbar junction and then decreases from the upper lumbar region to the sacrum. The angles at which the pedicles articulate to the body also vary depending on the level. The windows formed between the pedicles transmit the nerves and vessels that correspond to that body segment.

The portion of the posterior arch most subject to stress by translational motion is the pars interarticularis, which lies between the superior and inferior articular facets of each mobile vertebra. Clinically, fracture of this elongated bony segment in the C2 vertebra results in the hangman’s fracture; in the lower lumbar spine, it results in isthmic spondylolisthesis. The shear forces often result in ventral displacement of the superior articular facet, pedicle, and vertebral body and in maintenance of the attachments of the inferior articular facets and relationships to the lower vertebrae.2 In cadaveric studies, the L5 pars region was particularly susceptible to fracture, given its smaller cross-sectional area of 15 mm2 compared to the L1 and L3 vertebrae, which had over a fourfold increase.3

Cervical Vertebrae

Forward flexion and rotation are largely attributed to the first two cervical vertebrae. The atlas is the first cervical vertebra. It is a bony ring with an anterior and posterior arch connected with relatively two large lateral masses. The superior articular facet of the lateral mass is sloped internally to accommodate the occipital condyles. The inferior portion is sloped externally to articulate with the axis. This inferior articulation allows for rotational freedom while limiting lateral shifts. The posterior arch of C1 is grooved laterally to fit the vertebral arteries as they ascend from the foramen transversarium of C1 to penetrate the posterior atlanto-occipital membrane within 20 to 15 mm lateral to the midline. It is recommended that one remain within 12 mm lateral to midline during dissection of the posterior aspect of the ring.4 The anterior arch connects the two lateral masses, and the anterior tubercle in the most ventral portion is the site of attachment for the longus colli. The ventral side of the anterior arch has a synovial articulation with the odontoid process. The odontoid is restrained at this site with thick transverse atlantal ligaments that attach to the lateral masses (Figure 2-2).

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FIGURE 2-2 Cervical Vertebrae

The axis is the second cervical vertebra. The odontoid process, a remnant of the centrum of C1, projects from the body of C2 superiorly. This anatomy, unique to the cervical spine, allows for a strong rotational pivot with limitations on horizontal shear. Apical ligaments attach superiorly and alar ligaments attach laterally on the odontoid to the base of the skull at the basion. The basion is the anterior aspect of the foramen magnum. The superior aspects of the lateral masses are directed laterally and are convex to accommodate the atlas. The inferior articulations of the axis are similar to the remainder of the subaxial spine with a 45 degree sagittal orientation of the facets.

The cervical vertebrae are smaller in dimension than the lumbar vertebrae because they bear less weight than their lumbar counterparts. They are wider in the coronal plane in relation to the sagittal plane. The superior lateral edges of the vertebrae form the uncinate processes. The lateral processes have openings for the superior transit of the vertebral artery; these are called the foramen transversarium. During instrumentation of the lateral masses, it should be noted that as one descends from the upper cervical levels to C6, the foramen is more laterally positioned respective to the midpoint of the lateral mass. Anterior and posterior cervical musculature attach to their respective tubercles in the lateral portions of the transverse process. The seventh cervical vertebra is a transitional segment and has a long spinous process or vertebra prominens. The vertebral arteries usually enter the transverse foramen at C6 and omit the passage through the C7 foramen.

Thoracic Vertebrae

The thoracic vertebrae are heart-shaped and have dual articulations for both ribs as well as for the superior and inferior vertebrae. The transverse diameter of the pedicles is smallest from T3 to T6. At T1, the transverse diameter is larger, with an average of 7.3 mm in men and 6.4 mm in women.5 The first thoracic vertebra has a complete facet on the side of the body for the first rib head and an inferior demifacet for the second rib head. The ninth to twelfth vertebrae have costal articulations with their respective ribs. The last two ribs are smaller and do not attach to the sternum. The thoracic facets are rotated 20 degrees forward on the coronal plane and 60 degrees superiorly on the sagittal plane (Figure 2-3).

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FIGURE 2-3 Thoracic Vertebrae

Lumbosacral Spine

The lumbar vertebrae are much larger in overall relative proportion. The articular facets are concave and directed approximately 45 degrees medially on the coronal plane. The fourth transverse process tends to be smallest in comparison to the proximal lumbar segments. The fifth transverse process is the most robust (Figure 2-4).

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