Back

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:

• Support. The vertebral column forms the axis of the body and is critical for upright posture (standing or sitting), as a support for the head, as an attachment point and brace for movements of the upper limbs, and as a support for transferring the weight of the trunk to the lower limbs.

• Protection. The vertebral column protects the spinal cord and proximal portions of the spinal nerves before they distribute throughout the body.

• Movements. Muscles of the back function in movements of the head and upper limbs and in support and movements of the vertebral column.

2

Surface Anatomy

Fig. 2.1 shows key surface landmarks of the back, including the following bony landmarks:

• Vertebrae prominens: the spinous process of the C7 vertebra, usually the most prominent process in the midline at the posterior base of the neck.

• Scapula: a part of the pectoral girdle that supports the upper limb; note its spine, inferior angle, and medial border.

• Iliac crests: felt best when you place your hands “on your hips.” An imaginary horizontal line connecting the iliac crests passes through the spinous process of vertebra L4 and the intervertebral disc of L4-L5, providing a useful landmark for a lumbar puncture or an epidural block (see Clinical Focus 2.11 ).

• Posterior superior iliac spines: an imaginary horizontal line connecting these two points passes through the spinous process of S2 (second sacral segment).

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 ):

• Cervical: seven vertebrae; the first two called the atlas (C1) and axis (C2).

• Thoracic: 12 vertebrae; each articulates with a pair of ribs.

• Lumbar: five vertebrae; large vertebrae for support of the body’s weight.

• Sacral: five fused vertebrae for stability in the transfer of weight from the trunk to the lower limbs.

• Coccyx: four vertebrae, but variable; Co1 often is not fused, but Co2-Co4 are fused (a remnant of the embryonic tail).

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

Clinical Focus 2.1

Scoliosis

Scoliosis is an abnormal lateral curvature of the spine, which also includes an abnormal rotation of one vertebra upon another. In addition to scoliosis, accentuated curvatures of the spine include kyphosis (hunchback) and lordosis (swayback).

Viewed from the lateral aspect ( Fig. 2.2 ), one can identify the following:

• Cervical curvature (cervical lordosis): a secondary curvature acquired when the infant can support the weight of the head.

• Thoracic curvature (thoracic kyphosis): a primary curvature present in the fetus (imagine the spine in the “fetal position”).

• Lumbar curvature (lumbar lordosis): a secondary curvature acquired when the infant assumes an upright posture and supports its own weight.

• Sacral curvature : (sacral kyphosis) a primary curvature present in the fetus.

Typical Vertebra

A “typical” vertebra has the following features ( Fig. 2.3 ):

• Arch: a projection formed by paired pedicles and laminae and the spinous processes; the arch serves as the site for articulation with adjacent vertebrae and also as the attachment point for ligaments and muscles.

• 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; C7 transmits only the vertebral vein and is usually smaller.

• 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 triangular 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.

Clinical Focus 2.2

Cervical Fractures

Fractures of the axis (C2) often involve the dens and are classified as types I, II, and III. Type I fractures are usually stable, type II fractures are unstable, and type III fractures, which extend into the body, usually reunite well when immobilized. The hangman fracture, a pedicle fracture of the axis, can be stabilized, if survived, with or without spinal cord damage. A Jefferson fracture is a burst fracture of the atlas (C1), often caused by a blow to the top of the head.

TABLE 2.1

Key Features of the Cervical Vertebrae (C1-C7)

VERTEBRAE	DISTINGUISHING CHARACTERISTICS
Atlas (C1)	Ringlike bone; superior facet articulates with occipital bone.
	Two lateral masses with facets
	No body or spinous process
	C1 rotates on articular facets of C2.
	Vertebral artery runs in groove on posterior arch.
Axis (C2)	Dens projects superiorly.
	Strongest cervical vertebra
C3 to C7	Large, triangular vertebral foramen
	Transverse foramen through which vertebral artery passes (except C7)
	Narrow intervertebral foramina
	Nerve roots at risk of compression
C3 to C5	Short, bifid spinous process
C6 to C7	Long spinous process
C7	Vertebra prominens; nonbifid

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.

TABLE 2.2

Key Features of Thoracic, Lumbar, Sacral, and Coccygeal Vertebrae

VERTEBRAE	DISTINGUISHING CHARACTERISTICS
Thoracic (T1-T12)	Heart-shaped body, with facets for rib articulation
	Small circular vertebral foramen
	Long transverse processes, with facets for rib articulation in T1-T10
	Long spinous processes, which slope posteriorly and overlap next vertebra
Lumbar (L1-L5)	Kidney-shaped body, massive for support
	Midsized triangular vertebral foramen
	Facets face medial or lateral direction, which permits good flexion and extension
	Spinous process is short and strong.
	L5: largest vertebra with massive transverse processes
Sacrum (S1-S5)	Large, wedge-shaped bone that transmits body weight to pelvis
	Five fused vertebrae, with fusion complete by puberty
	Four pairs of sacral foramina on dorsal and ventral (pelvic) side
	Sacral hiatus, the opening of sacral vertebral foramen
Coccyx (Co1-Co4)	Co1 often is not fused.
	Co2 to Co4 are fused.
	No pedicles, laminae, or spines
	Remnant of our embryonic tail

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.

Sacrum and Coccyx

The sacrum is composed of five fused vertebrae that form a single, wedge-shaped bone ( Fig. 2.5 and Table 2.2 ). The sacrum provides support for the pelvis. The coccyx is a remnant of the embryonic tail and usually consists of four vertebrae, with the last three often fused into a single bone. The coccyx lacks vertebral arches and has no vertebral canal.

The features and number of vertebrae can vary, and clinicians must always be aware of subtle differences, especially on radiographic imaging, that may be variants within a normal range.

Clinical Focus 2.3

Osteoarthritis

Osteoarthritis is the most common form of arthritis and often involves erosion of the articular cartilage of weight-bearing joints, such as those of the vertebral column.

Joints and Ligaments of Craniovertebral Spine

The craniovertebral joints include the atlanto-occipital (atlas and occipital bone of the skull) and atlantoaxial (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 atlantooccipital joint permits one to nod the head up and down (flexion and extension), as if to indicate “yes,” whereas the atlantoaxial 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 ).

TABLE 2.3

Key Features of Atlantooccipital and Atlantoaxial Joints

LIGAMENT	ATTACHMENT	COMMENT
Atlantooccipital (Biaxial Condyloid Synovial) Joint
Articular capsule	Surrounds facets and occipital condyles	Allows flexion and extension
Anterior and posterior membranes	Anterior and posterior arches of C1 to foramen magnum	Limit movement of joint
Atlantoaxial (Uniaxial Synovial) Joint
Tectorial membrane	Axis body to margin of foramen magnum	Is continuation of posterior longitudinal ligament
Apical	Dens to occipital bone	Is very small
Alar	Dens to occipital condyles	Limits rotation
Cruciate	Dens to lateral masses	Resembles a cross; allows rotation