Chapter 4: Back

Intro

Overview of Back and Vertebral Column

The back comprises the posterior aspect of the trunk, inferior to the neck and superior to the buttocks. It is the region of the body to which the head, neck, and limbs are attached. The back includes the:

  • Skin and subcutaneous tissue.
  • Muscles: a superficial layer, primarily concerned with positioning and moving the upper limbs, and deeper layers (“true back muscles”), specifically concerned with moving or maintaining the position of the axial skeleton (posture).
  • Vertebral column: the vertebrae, intervertebral (IV) discs, and associated ligaments (Fig. 4.1).
  • Ribs (in the thoracic region): particularly their posterior portions, medial to the angles of the ribs.
  • Spinal cord and meninges (membranes that cover the spinal cord).
  • Various segmental nerves and vessels.

Figure 4.1. Vertebral column and its five regions.

Vertebral column and its five regions

The isolated vertebrae between (A) and (B) are typical of each of the three mobile regions of the vertebral column. The continuous, weight-bearing column of vertebral bodies and IV discs increases in size as the column descends. Zygapophysial (facet) joints representative of each region are circled. The posterior view (C) includes the vertebral ends of ribs, representing the skeleton of the back. The bisected vertebral column in (D) demonstrates the vertebral canal. The intervertebral (IV) foramina (also seen in B) are openings in the lateral wall of the vertebral canal through which spinal nerves exit.

Because of their close association with the trunk, the back of the neck and the posterior and deep cervical muscles and vertebrae are also described in this chapter. The scapulae, although located in the back, are part of the appendicular skeleton and are considered with the upper limb (Chapter 6).

Study of the soft tissues of the back is best preceded by examination of the vertebrae and the fibrocartilaginous intervertebral discs that are interposed between the bodies of adjacent vertebrae. The vertebrae and IV discs collectively make up the vertebral column (spine), the skeleton of the neck and back that is the main part of the axial skeleton (i.e., articulated bones of the cranium, vertebral column, ribs, and sternum) (Fig. 4.1D). The vertebral column extends from the cranium (skull) to the apex of the coccyx. In adults, it is 72–75 cm long, of which approximately one quarter is formed by the IV discs that separate and bind the vertebrae together. The vertebral column:

  • Protects the spinal cord and spinal nerves.
  • Supports the weight of the body superior to the level of the pelvis.
  • Provides a partly rigid and flexible axis for the body and an extended base on which the head is placed and pivots.
  • Plays an important role in posture and locomotion (the movement from one place to another)

Vertebrae

The vertebral column in an adult typically consists of 33 vertebrae arranged in five regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal (Fig. 4.1A,B,C–D). Significant motion occurs only between the 25 superior vertebrae. Of the 9 inferior vertebrae, the 5 sacral vertebrae are fused in adults to form the sacrum, and after approximately age 30, the 4 coccygeal vertebrae fuse to form the coccyx. The lumbosacral angle occurs at the junction of the long axes of the lumbar region of the vertebral column and the sacrum (Fig. 4.1D). The vertebrae gradually become larger as the vertebral column descends to the sacrum and then become progressively smaller toward the apex of the coccyx (Fig. 4.1A,B,C–D). The change in size is related to the fact that successive vertebrae bear increasing amounts of the body’s weight as the column descends. The vertebrae reach maximum size immediately superior to the sacrum, which transfers the weight to the pelvic girdle at the sacro-iliac joints.

The vertebral column is flexible because it consists of many relatively small bones, called vertebrae (singular = vertebra), that are separated by resilient IV discs (Fig. 4.1D). The 25 cervical, thoracic, lumbar, and first sacral vertebrae also articulate at synovial zygapophysial (facet) joints (Fig. 4.2D), which facilitate and control the vertebral column’s flexibility. Although the movement between two adjacent vertebrae is small, in aggregate the vertebrae and IV discs uniting them form a remarkably flexible yet rigid column that protects the spinal cord it surrounds.

Figure 4.2. Typical vertebra, represented by the 2nd lumbar vertebra.

Typical vertebra, represented by the 2nd lumbar vertebra

A. Functional components include the vertebral body (bone color), a vertebral arch (red), and seven processes: three for muscle attachment and leverage (blue) and four that participate in synovial joints with adjacent vertebrae (yellow). B and C. Bony formations of vertebrae are demonstrated. The vertebral foramen is bounded by the vertebral arch and body. A small superior vertebral notch and a larger inferior vertebral notch flank the pedicle. D. The superior and inferior notches of adjacent vertebrae plus the IV disc that unites them form the IV foramen for passage of a spinal nerve and its accompanying vessels. Each articular process has an articular facet where contact occurs with the articular facets of adjacent vertebrae (B–D)

Structure and Function of Vertebrae

Vertebrae vary in size and other characteristics from one region of the vertebral column to another, and to a lesser degree within each region; however, their basic structure is the same. A typical vertebra (Fig. 4.2) consists of a vertebral body, a vertebral arch, and seven processes.

The vertebral body is the more massive, roughly cylindrical, anterior part of the bone that gives strength to the vertebral column and supports body weight. The size of the vertebral bodies increases as the column descends, most markedly from T4 inferiorly, as each bears progressively greater body weight.

The vertebral body consists of vascular, trabecular (spongy, cancellous) bone enclosed by a thin external layer of compact bone (Fig. 4.3). The trabecular bone is a meshwork of mostly tall vertical trabeculae intersecting with short, horizontal trabeculae. The spaces between the trabeculae are occupied by red bone marrow that is among the most actively hematopoietic (blood-forming) tissues of the mature individual. One or more large foramina in the posterior surface of the vertebral body accommodate basivertebral veins that drain the marrow (see Fig. 4.26).

Figure 4.3. Internal aspects of vertebral bodies and vertebral canal.

Internal aspects of vertebral bodies and vertebral canal

The bodies consist largely of trabecular (spongy) bone—with tall, vertical supporting trabeculae linked by short horizontal trabeculae—covered by a relatively thin layer of compact bone. Hyaline cartilage “end plates” cover the superior and inferior surfaces of the bodies, surrounded by smooth bony epiphysial rims. The posterior longitudinal ligament, covering the posterior aspect of the bodies and linking the IV discs, forms the anterior wall of the vertebral canal. Lateral and posterior walls of the vertebral canal are formed by vertebral arches (pedicles and laminae) alternating with IV foramina and ligamenta flava.

In life, most of the superior and inferior surfaces of the vertebral body are covered with discs of hyaline cartilage (vertebral “end plates”), which are remnants of the cartilaginous model from which the bone develops. In dried laboratory and museum skeletal specimens, this cartilage is absent, and the exposed bone appears spongy, except at the periphery where an epiphysial rim or ring of smooth bone, derived from an anular epiphysis, is fused to the body (Fig. 4.2B).

In addition to serving as growth zones, the anular epiphyses and their cartilaginous remnants provide some protection to the vertebral bodies and permit some diffusion of fluid between the IV disc and the blood vessels (capillaries) in the vertebral body (see Fig. 4.26). The superior and inferior epiphyses usually unite with the centrum, the primary ossification center for the central mass of the vertebral body (Fig. 4.2B), early in adult life (at approximately age 25) (see “Ossification of Vertebrae,”).

The vertebral arch is posterior to the vertebral body and consists of two (right and left) pedicles and laminae (Fig. 4.2A & C). The pedicles are short, stout cylindrical processes that project posteriorly from the vertebral body to meet two broad, flat plates of bone, called laminae, which unite in the midline. The vertebral arch and the posterior surface of the vertebral body form the walls of the vertebral foramen (Fig. 4.2A & B). The succession of vertebral foramina in the articulated vertebral column forms the vertebral canal (spinal canal). The canal contains the spinal cord and the roots of the spinal nerves, along with the membranes (meninges), fat, and vessels that surround and serve them (Figs. 4.1D and 4.3). (See the blue box “Laminectomy,”.)

The vertebral notches are indentations observed in lateral views of the vertebrae superior and inferior to each pedicle between the superior and inferior articular processes posteriorly and the corresponding projections of the body anteriorly (Fig. 4.2C). The superior and inferior vertebral notches of adjacent vertebrae and the IV discs connecting them form intervertebral foramina (Fig. 4.2D) through which the spinal nerves emerge from the vertebral column (see Fig. 4.27). Also, the spinal (posterior root) ganglia are located in these foramina.

Seven processes arise from the vertebral arch of a typical vertebra (Fig. 4.2A,B–C):

  • One median spinous process projects posteriorly (and usually inferiorly, typically overlapping the vertebra below) from the vertebral arch at the junction of the laminae.
  • Two transverse processes project posterolaterally from the junctions of the pedicles and laminae.
  • Four articular processes (G. zygapophyses)—two superior and two inferior—also arise from the junctions of the pedicles and laminae, each bearing an articular surface (facet)

The spinous and transverse processes provide attachment for deep back muscles and serve as levers, facilitating the muscles that fix or change the position of the vertebrae.

The articular processes are in apposition with corresponding processes of vertebrae adjacent (superior and inferior) to them, forming zygapophysial (facet) joints (Figs. 4.1B and 4.2D). Through their participation in these joints, these processes determine the types of movement permitted and restricted between the adjacent vertebrae of each region.

The articular processes also assist in keeping adjacent vertebrae aligned, particularly preventing one vertebra from slipping anteriorly on the vertebra below. Generally, the articular processes bear weight only temporarily, as when one rises from the flexed position, and unilaterally when the cervical vertebrae are laterally flexed to their limit. However, the inferior articular processes of the L5 vertebra bear weight even in the erect posture.

Regional Characteristics of Vertebrae

Each of the 33 vertebrae is unique; however, most of the vertebrae demonstrate characteristic features identifying them as belonging to one of the five regions of the vertebral column (e.g., vertebrae having foramina in their transverse processes are cervical vertebrae) (Fig. 4.4). In addition, certain individual vertebrae have distinguishing features; the C7 vertebra, for example, has the longest spinous process. It forms a prominence under the skin at the back of the neck, especially when the neck is flexed (see Fig. 4.8A).

Figure 4.4. Comparison of presacral vertebrae.

Comparison of presacral vertebrae

As the vertebral column descends, bodies increase in size in relationship to increased weight-bearing. The size of the vertebral canal changes in relationship to the diameter of the spinal cord.

In each region, the articular facets are oriented on the articular processes of the vertebrae in a characteristic direction that determines the type of movement permitted between the adjacent vertebrae and, in aggregate, for the region. For example, the articular facets of thoracic vertebrae are nearly vertical, and together define an arc centered in the IV disc; this arrangement permits rotation and lateral flexion of the vertebral column in this region (Fig. 4.7). Regional variations in the size and shape of the vertebral canal accommodate the varying thickness of the spinal cord (Fig. 4.1D).