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Anatomy of the Spine

Bones

Each individual vertebra has unique features depending on the region in which it is found. Every vertebra, regardless of location, has three basic functional parts: (1) the drum-shaped vertebral body, designed to bear weight and withstand compression or loading; (2) the posterior (backside) arch, made of the lamina, pedicles and facet joints; and (3) the transverse processes, to which muscles attach.

The vertebral body is composed of hard cortical bone on the outside and less dense cancellous bone on the inside. The top and bottom of the vertebral body are called the end plates. The intervertebral disc, sandwiched between two vertebral bodies, is attached to the end plates. Changes in the disc may be accompanied by changes in the end plates.

The pedicle is a paired, strong, tubular bony structure made of hard cortical bone on the outside and cancellous bone on the inside. Each pedicle comes out of the side of the vertebral body and projects to the back. Pedicles act as the lateral (side) walls of the bony spinal canal that protects the spinal cord and cauda equina, or nerve roots, in the lumbar region. There is also a space created between the facet joints and pedicles of one vertebral body and the next, called the intervertebral foramen, through which the spinal nerves branch out to the rest of your body.

The lamina are shingle-like plates of bone coming from the pedicles to arch over the nerves and join at the midline. The lamina are shorter than the vertebral bodies so that there is a gap between any two laminae, bridged by soft tissue called the ligamentum flavum. This provides additional protection for the nerves that lie underneath it. Together, the lamina and pedicles form the vertebral arch.

As the lamina come together at the back of the spinal column, they join to form the spinous process, the bony part of the spine that you can feel at the midline when you rub your back. There is an interspinous ligament that runs between the spinous processes of the vertebrae and a supraspinous ligament that runs on top of them from the cervical region to the sacrum.

Each vertebral body has two articular processes at the top and bottom where the lamina and pedicle meet. These articular processes create a joint, called the facet joint, between the stacked vertebral bodies. There is a facet joint on each side of the vertebral body. The facet joint typically lies behind the spinal nerves as they emerge from the central spinal canal. The surfaces of the facet joint are capped with cartilage and the joint is contained in a capsule lined by synovium, much like the knee joint. The two facet joints and the intervertebral disc at each level allow for motion between the vertebral bodies.

The typical vertebral body has two transverse processes, or lateral projections, one on each side. These projections serve as points of attachment for muscles and ligaments in the spine. In the cervical spine, the transverse processes each have a foramen or canal through which the vertebral artery and vein travel. The intertransverse ligaments connect the transverse processes of the vertebrae on each side of the spinal column.

In the lumbar spine, identification of the pars interartcularis is important because it is the site of pathology related to spondylolisthesis, or 'slipped vertebra.' This is another paired structure on the back side of the spine and it links the pedicle, transverse process, lamina and articular facets on each side of the vertebrae.

Sacrum

The sacrum is somewhat triangular in appearance. It is made up of naturally fused vertebrae, with sacral foramina (canals) into intervertebral foramen through which the spinal nerves leave the bottom portion of the spinal canal. The lamina of the lowest segment of the sacrum is not completely formed, and there may be a gap called the sacral hiatus. At the end of the sacrum is the coccyx (sometimes called the tailbone) which typically is composed of four vertebra. The lowest segments are usually fused together.

The sacrum is wedged between the ilium, which contribute to the pelvic girdle. The sacroiliac joints (SI joints) are where the sacrum meets the iliac bones on each side. The sacrum connects the spine to the pelvis and the lower half of the skeleton. The SI joint has both a synovial joint portion (like the knee) and a fibrous joint. The joint surfaces are irregular so that the bones can fit together in an almost locking construct, allowing for only a small amount of motion during weight bearing and forward flexion.

Spinal Canal

When the vertebral bodies are stacked one on top of another, they create a vertical tunnel behind the vertebral bodies called the spinal canal or neural canal. The front wall of the spinal canal is created by the back of the vertebral bodies; the sides are formed by the pedicles at each level; and the lamina form the posterior wall. The spinal canal runs from the cervical region to the sacrum, and contains the spinal cord, which ends at the top of the lumbar region, where it becomes the conus medullaris and then the cauda equina. These terms are described below.

On both sides of the spinal canal, there are neural foramen at each level of the cervical, thoracic and lumbar spine small canals, through which the paired spinal nerves travel. The foramen on each side are created by the space between two pedicles, above and below, in addition to the side of the vertebral body and the facet joint. The spinal nerves are often referred to as nerve roots.

The Spinal Column

The spine (also called the vertebral column or spinal column) is composed of a series of bones called vertebrae stacked one upon another. There are four regions of the spine: cervical (neck), thoracic (chest/trunk), lumbar (low back), and sacral (pelvic)

The cervical spine is made up of seven cervical vertebrae. The main function of the cervical spine is to support the weight of the head which is approximately 10-12 pounds. The cervical spine has the greatest range of motion, in part because of two specialized vertebra that move with the skull. Cervical vertebrae are the smallest of the vertebrae. The first cervical vertebra is called the atlas and is significantly different from the other vertebrae. It is ring-like in shape with two large protrusions on the sides to support the weight of the head. The second cervical vertebra is called the axis.The axis is also unique in that it has a bony peg-like protrusion, called the dens or odontoid on its upper surface that fits within the ring of the atlas. The curve of the neck is described as a lordosis or lordotic curve, and looks like a “C” in reverse.

The main function of the thoracic spine is to protect the organs of the chest, especially the heart and lungs. There are 12 thoracic vertebrae with one rib attached on each side, to create a thoracic cage, which protects the internal organs of the chest. The thoracic spine has a normal kyphosis, or “C” curve. The thoracic spine is less mobile than the cervical and lumbar spine because of the thoracic cage.

The lumbar spine has five lumbar vertebrae, which are the largest vertebrae. These vertebrae are also aligned in a reverse “C” like the cervical spine, creating a normal lumbar lordosis. The five lumbar vertebral bodies are the weight-bearing portion of the spine and are the largest in diameter compared to the thoracic and cervical vertebral bodies. They sit atop the sacrum, which is formed by five vertebrae fused together into a solid unit. There are usually no identifiable disc spaces between the sacral segments. At the end of the spinal column is the coccyx or tailbone. Most people have 33 vertebrae in total, although there may be 32 or 34. Variations are usually found in the lumbar or sacral regions.

Muscles

Muscles are often categorized according to their function using the terms 'flexor' or 'extensor'. The muscles and ligaments in the spine work to hold the spine upright, and to allow forward bending (flexion) and backward bending (extension), as well as rotation from side to side and combinations of those activities.

There are many muscles that contribute to motion and stability in the cervical spine. The trapezius muscle is a flat, triangular muscle covering the upper and back part of the neck and shoulders. Other muscles that lie deeper in the neck are called the scalene muscles. They flex and rotate the cervical spine.

The muscles of the lumbar region of the back act in various combinations to allow flexion, extension, lateral flexion and rotation. Flexion and rotation can also occur together, in a coupled motion.

Soft Tissue

The intervertebral disc is found between the vertebral bodies. It is a complex structure that supports the weight of the body and, with the facet joints, permits a significant range of motion. The disc is made up of fibrocartilage and has two parts: the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is a gel-like material located in the center of the disc. It has a high water content, which allows it to act as a cushion and distribute loads onto the vertebral body end plates and to the annulus. The water content of the nucleus decreases with age, which can contribute to some of the conditions discussed in later chapters. The disc has no blood supply of its own. The annulus fibrosus is the outer portion of the disc. The annulus consists of 15 to 25 layers of collagen, much like the layers of a truck tire. This structural design allows the annulus to contain the nucleus under pressure, and to help hold the vertebral bodies in place. The annulus is weakest at the back, on either side of the midline, which is often the location of disc herniations.

The bony end plate covers the top and bottom surfaces of the disc, and is the interface between the nucleus and the vertebral body. The posterior longitudinal ligament connects all the vertebral bodies along the back and lines the spinal canal. It is a fibrous band that extends from the base of the skull to the sacrum. The anterior longitudinal ligament is a broad band on the front surfaces of the vertebral bodies. The supraspinous ligament runs across the top of the spinous processes. The paired ligament that lies between adjacent lamina, and protects the neural elements beneath it, is the ligamentum flavum, sometimes referred to as the “yellow ligament.” It is made up of elastic fibers, and there is an opening in the middle between the two paired halves. The intertransverse ligaments connect adjacent transverse processes. The interspinous ligament connects adjacent spinous processes. The ligamentum nuchæ is a fibrous membrane in the neck. It extends from the external occipital protuberance and median nuchal line to the spinous process of the bottom cervical vertebra.

Nerves

The spinal cord is composed of millions of nerve fibers, which form a tube-like structure that extends from the brain to the area between the L1 and L2 vertebrae in the upper lumbar region. The nerve fibers branch off from the spinal cord and form the nerve roots, which are paired at each spinal level for a total of 32 pairs. These nerve fibers are directed to various parts of the body. The cervical nerve roots connect to the upper body and arms and hands, and the nerve roots in the thoracic spine go to the chest and abdomen. The nerve roots in the lumbar spine innervate the legs, bowel and bladder. The dorsal root ganglion is a nodule on the dorsal (posterior) root portion of the spinal nerve that contains cell bodies of outgoing spinal nerve neurons.

The end of the spinal cord is tapered and is called the conus medullaris. There is a thread that continues from the conus called the filum terminale. These structures are surrounded by the roots of the caudal spinal nerves (nerves at the lower end) and, all together, are termed the cauda equina. The nerve roots each leave the vertebral canal through an intervertebral foramen, so the roots in the cauda equina get fewer as it gets farther down the spinal canal. The nerve roots in the cauda equina go to the lower extremities and the bowel and bladder.

The thecal sac is a protective membrane that covers the spinal cord and cauda equina and contains cerebrospinal fluid that provides nutrition to the spinal cord. The membrane is composed of several layers: the outermost is the dura mater, the middle layer is the arachnoid mater and the inner layer is called the pia mater. The thecal sac is separated from the wall of the vertebral canal by the epidural space, which contains epidural fat.

The Motion Segment

The motion segment is the functional unit of the spinal column. The combination of both bony and soft tissue structures forms a motion segment, which is composed of two adjacent vertebral bodies, the facet joint created by their articular processes, the intervertebral disc between them and the associated soft tissue structures already mentioned. The intervertebral disc and the facet joints (one on each side of the midline) allow for motion in flexion, extension, side bending and rotation at the level of the motion segment.

Motion and Biomechanics of the Spine
The lower portion of the spine both bears the most weight and allows the greatest amount of motion. Lumbar spine motion is very complex because of the need for both stability and mobility. The motion segment allows motion within a restrained range and provides stability. Lateral bending occurs mostly in the upper lumbar motion segment. In the lower lumbar spine and lumbo-sacral region, flexion and extension are the primary motions.

The anterior column of the spine, consisting of the stacked vertebral bodies and discs, supports a majority of the body weight in the upright position. Biomechanical studies have shown that the posterior elements (lamina and facet joints) support about one-seventh of the axial load (the vertical load, perpendicular to the ground). When the intervertebral disc is loaded in the sitting or upright position, its two components, the annulus and the disc, behave differently: with lateral bending, the annulus bulges toward the direction of motion while the nucleus slides away from the pressure.

After age 30, the nucleus gradually begins to lose water content and the volume of the nucleus shrinks. The load borne by the annulus subsequently increases and the annulus is subject to weakening and tearing. Resistance to loading is diminished, and motion of the functional spinal unit changes. Disc degeneration is the term used to describe these changes, which as they progress, may result in decreased disc height, irregular bony end plates of the vertebrae and osteophyte (bone spur) formation. Disc degeneration and the accompanying changes in soft tissues and bone can result in increased instability (eg, spondylolisthesis) in some people, and decreased motion in others.

In the lumbar region, the facet joints are positioned to resist rotation and allow flexion and translation. The normal lordotic curvature allows the facet joints in the lumbar spine to bear weight. Disc degeneration and disc space narrowing force the lumbar facet joints to bear increased loads and likely lead to earlier degenerative changes.