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)
- 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.
Vertebrae
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.
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 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.