1 Introduction

The back forms the axis (central line) of the human body and consists of the vertebral column, spinal cord, supporting muscles, and associated tissues (skin, connective tissues, vasculature, and nerves). A hallmark of human anatomy is the concept of “segmentation,” and the back is a prime example. Segmentation and bilateral symmetry of the back will become obvious as you study the vertebral column, the distribution of the spinal nerves, the muscles of the back, and its vascular supply. Functionally, the back is involved in three primary tasks, as follows:

2 Surface Anatomy

Figure 2-1 shows key surface landmarks of the back, including the following bony landmarks:

3 Vertebral Column

The vertebral column (spine) forms the central axis of the human body, highlighting the segmental nature of all vertebrates, and usually is composed of 33 vertebrae distributed as follows (Fig. 2-2):

The actual number of vertebrae can vary, especially the number of coccygeal vertebrae.

Viewed from the lateral aspect (Fig. 2-2), one can identify the following:

Typical Vertebra

A “typical” vertebra has the following features (Fig. 2-3):

• Arch: a projection formed by paired pedicles and laminae.

• Articular processes (facets): two superior and two inferior facets for articulation with adjacent vertebrae.

• Body: the weight-bearing portion of a vertebra that tends to increase in size as one descends the spine.

• Intervertebral foramen (foramina): the opening formed by the vertebral notches that is traversed by spinal nerve roots and associated vessels.

• Lamina (laminae): paired portions of the vertebral arch that connect the transverse processes to the spinous process.

• Pedicle: paired portions of the vertebral arch that attach the transverse processes to the body.

• Transverse foramina: apertures that exist in transverse processes of cervical vertebrae only and transmit the vertebral vessels.

• Transverse processes: the lateral extensions from the union of the pedicle and lamina.

• Spinous process: a projection that extends posteriorly from the union of two laminae.

• Vertebral foramen (canal): a foramen formed from the vertebral arch and body that contains the spinal cord and its meningeal coverings.

• Vertebral notches: superior and inferior semicircular features that in articulated vertebrae form an intervertebral foramen (two semicircular notches form a circle).

Regional Vertebrae

Cervical Vertebrae

The cervical spine is composed of seven cervical vertebrae. The first two cervical vertebrae are unique and called the atlas and axis (Fig. 2-4). The atlas (C1) holds the head on the neck (the titan Atlas of Greek mythology held the heavens on his shoulders as punishment by Zeus). The axis (C2) is the point of articulation where the head turns on the neck, providing an “axis of rotation.”

Table 2-1 summarizes key features of the cervical vertebrae. The cervical region is a fairly mobile portion of the spine, allowing for flexion and extension as well as rotation and lateral bending.

Thoracic and Lumbar Vertebrae

The thoracic spine is composed of 12 thoracic vertebrae (Fig. 2-5 and Table 2-2). The 12 pairs of ribs articulate with the thoracic vertebrae. This region of the spine is more rigid and inflexible than the cervical region.

The lumbar spine is composed of five lumbar vertebrae (see Figs. 2-3 and 2-5 and Table 2-2). The lumbar vertebrae are comparatively large for bearing the weight of the trunk and are fairly mobile, but not nearly as mobile as the cervical vertebrae.

Joints and Ligaments of Craniovertebral Spine

The craniovertebral joints include the atlanto-occipital (atlas and occipital bone of the skull) and atlanto-axial (atlas and axis) joints. Both are synovial joints that provide a relatively wide range of motion compared with other joints of the vertebral column. The atlanto-occipital joint permits one to nod the head up and down (flexion and extension), whereas the atlanto-axial joint is a pivot joint that permits one to rotate the head from side to side, as if to indicate “no” (Fig. 2-6 and Table 2-3).

Joints and Ligaments of Vertebral Arches and Bodies

The joints of the vertebral arches (zygapophysial joints) occur between the superior and inferior articular processes (facets) of adjacent vertebrae and allow for some gliding or sliding movement (Fig. 2-7 and Table 2-4). These joints slope inferiorly in the cervical spine (facilitate flexion and extension), are more vertically oriented in the thoracic region (limit flexion and extension but allow for rotation), and are interlocking in the lumbar spine (but do allow flexion and extension, but not to the degree present in the cervical spine). Corresponding ligaments connect the spinous processes, laminae, and bodies of adjacent vertebrae (see Tables 2-2 and 2-3). Strong anterior and posterior longitudinal ligaments run along most of the length of the vertebral column. Of these two ligaments, the anterior longitudinal ligament is stronger and prevents hyperextension (see Table 2-4).

The joints of the vertebral bodies (intervertebral joints) occur between the adjacent vertebral bodies (see Fig. 2-7 and Table 2-4). The intervertebral joints are lined by a thin layer of hyaline cartilage with an intervening intervertebral disc (except between first two cervical vertebrae). These stable, weight-bearing joints also absorb pressure because the intervertebral disc is between the bodies. Intervertebral discs are composed of a central nuclear zone of collagen and hydrated proteoglycans called the nucleus pulposus, which is surrounded by concentric lamellae of collagen fibers that compose the anulus fibrosus. The inner gelatinous nucleus pulposus (remnant of embryonic notochord) is hydrated and acts as a “shock absorber,” compressing when load bearing and relaxing when the load is removed. The outer fibrocartilaginous anulus fibrosus, arranged in concentric lamellae, is encircled by a thin ring of collagen and resists compression and shearing forces.

The lumbar are the thickest and the upper thoracic the thinnest intervertebral discs. The anterior and posterior longitudinal ligaments help to stabilize these joints (see Table 2-4).

Clinical Focus 2-6   Intervertebral Disc Herniation

The intervertebral discs are composed of a central nuclear zone of collagen and hydrated proteoglycans called the nucleus pulposus, which is surrounded by concentric lamellae of collagen fibers that compose the anulus fibrosus. The nucleus pulposus is hydrated and acts as a “shock absorber,” compressing when load bearing and relaxing when the load is removed. Over time, the repeated compression-relaxation cycle of the intervertebral discs can lead to peripheral tears of the anulus fibrosus that allow for the extrusion and herniation of the more gelatinous nucleus pulposus. This often occurs with age, and the nucleus pulposus becomes more dehydrated, thus transferring more of the compression forces to the anulus fibrosus. This added stress may cause thickening of the anulus and tears. Most disc herniations occur in a posterolateral direction because the anulus fibrosus tears often occur at the posterolateral margins of the disc (rim lesions). Moreover, the posterior longitudinal ligament reinforces the anulus such that posterior herniations are much less common; otherwise, the disc would herniate into the vertebral canal and compress the spinal cord or its nerve roots.

The most common sites for disc herniation in the cervical region are the C5-C6 and C6-C7 levels, resulting in shoulder and upper limb pain. In the lumbar region the primary sites are the L4-L5 and L5-S1 levels. Lumbar disc herniation is much more common than cervical herniation and results in pain over the sacro-iliac joint, hip, posterior thigh, and leg.


Movements of the Spine

The essential movements of the spine are flexion, extension, lateral flexion (lateral bending), and rotation (Fig. 2-8). The greatest freedom of movement occurs in the cervical and lumbar spine, with the neck having the greatest range of motion. Flexion is greatest in the cervical region, and extension is greatest in the lumbar region. The thoracic region is relatively stable, as is the sacrum.

Again, the atlanto-occipital joint permits flexion and extension (e.g., nodding in acknowledgment), and the atlanto-axial joint allows side-to-side movements (rotation; e.g., indicating “no”). This is accomplished by a uniaxial synovial joint between the dens of the axis and its articulation with the anterior arch of the atlas. The dens functions as a pivot that permits the atlas and attached occipital bone of the skull to rotate on the axis. Alar ligaments limit this side-to-side movement so that rotation of the atlanto-axial joint occurs with the skull and atlas rotating as a single unit on the axis (see Fig. 2-6).

Movements of the spine are a function of the following features:

Jun 16, 2016 | Posted by in ANATOMY | Comments Off on Back
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