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Human BackThe Human Back

In order to understand the diagnosis and treatment of back pain it is necessary to understand the complex anatomy of the human back. The back is made up of an assortment of bones, muscles, tendons, and ligaments extending from the base of the skull to the pelvis. The bones of the back are the shoulder blades (scapulae), rib cage (thoracic cage) and the spine (vertebral column).

Shoulder Blades (scapulae)

The shoulder blades are flat, mobile, triangular bones located in the upper back. The blade connects the upper arm bone with the collar bone. The blades are kept in position by a complex assortment of muscles, tendons and ligaments. Some of the shoulder blade muscles are attached to the spine, while others move the shoulder. These include the levator scapulae which raises the shoulder blade; the pectoralis minor which lowers shoulder blade; the trapezius which lifts and rotates the collar bone; the rhomboideus major which rotates the blade inwards; and, the serratus anterior which stabilizes the shoulder blade.

Rib Cage (thoracic cage)

The rib cage (also known as the thoracic basket) is a semi-rigid structure made of bone and cartilage which surrounds the chest (thoracic) cavity), The structure encloses the heart and lungs, and supports the shoulder. The rib cage typically consists of 24 ribs, the breast bone (sternum), costal cartilages (positioned between the rib and sternum to provide flexibility and connection to the vertebrae), and the 12 thoracic vertebrae. Ribs that are attached to the breast bone are known as true ribs. Ribs not connected to the sternum are known as false ribs. They are attached to the true ribs. The bottom two ribs, known as floating ribs, are attached directly to the spine.

Spine (vertebral column)

The spine, or backbone, extends from the base of the skull to the pelvis. It is the most important component of the human back, responsible for the structure and shape of the back. Although it may be intuitive to consider the spine to be a straight column, it has three gradual curves. The curvature increases spine strength and protects the spinal cord from injury.

The spine consists of 33 vertebrae stacked in a column. The vertebrae are made of dense cortical bone which differ in size and shape in the different regions of the spinal column. The four regions of the spine are:

  • The cervical or neck region - has seven vertebrae (labeled C1–C7). Each vertebrae has a nerve numbered the same as the vertebrae. However, there is a nerve root numbered C8 with no corresponding vertebrae.
  • The thoracic or upper back region - has twelve vertebrae (labeled T1–T12). These vertebrae increase in size as they descend from T1 to T12.
  • The lumbar or lower back region - has five vertebrae (labeled L1-L5) which are involved in the bending forward, backward and side movements. Stress or damage to the spine at the L1-L5 level is commonly associated with pain in the back. Spinal nerve L5 articulates with the sacrum which is attached to the pelvis.
  • The sacrum and coccyx region - located at the base of the spine has 9 fused, or immobile, vertebrae (5 in the sacrum and 4 in the coccyx).

Each of the 24 moveable vertebrae in your spine have two bony processes (pedicles) made of dense cortical bone that attach to other spinal structures. During some spinal surgeries titanium screws are inserted into the pedicles to hold the spine together and immobile. This is done to allow bone placed along the side of the spine, or in between the vertebral bodies, to fuse. Two facet joints on the vertebrae allow the spine to bend forward, bend backward, and twist.

The vertebrae are separated and cushioned by an intervertebral disc. These fibrocartilaginous discs, which are approximately three quarters of an inch thick have a soft, jelly-like center and a fibrous outer ring. The discs serve as shock absorbers and allow movement of the spinal column. The discs account for one third the length of the spine and one quarter of the length of the spinal cord (average length 17.5 in.). Changes in the intervertebral discs as a result of trauma, disease or the aging process affects the way the disc can respond to mechanical stress. This can cause changes in spinal muscles and ligaments, resulting in mild to severe back pain.

The twenty-four moveable vertebrae are hollow in the center, forming the spinal canal. Inside the spinal canal is the spinal cord. The spinal cord, along with the brain, makes up the central nervous system of the human body. The cord has a diameter about the size of the little finger and averages approximately 17 ½ inches in length. It carries nerve impulses between the brain and spinal nerves. The spinal cord terminates at the L1 level in the lumbar region (the cauda equina) where it splits into nerve roots that extend into the lower body and legs.

As mentioned in a previous section, the spine is not straight. It has three gradual curves. A pronounced forward curve of the lumbar spine is a condition known as lordosis, colloquially referred to as “sway back.” A pronounced curve of the thoracic spine is known as kyphosis, also called “hunchback.” In those instances where the spine abnormally curves from side-to-side, the condition is known as scoliosis. When the curvature is greater than 20 degrees it can become a health concern by limiting physical activity. When the scoliosis curve exceeds 45 degrees it is deemed a serious health concern, as this curvature not only limits physical activity, but affects heart and lung function. Scoliosis can be the result of a congenital condition, poor posture, spinal imbalance caused by one leg being shorter than the other, or paralysis of the muscles on one side of the body. The treatment of the scoliosis usually consists of the individual being fitted with braces. When this fails to correct the condition surgery is necessary.

A severe spinal cord injury can prevent the flow of information from the point of impact down the spine. This means that the arms, legs, and other parts of the body may be partially or completely cut off from the brain. In the worst case scenario, the individual may have a loss sensation, be unable to move the arms and/or legs, and experience a failure of the respiratory system.

In total, there are 31 pairs of nerves that branch off from the spinal cord. The spinal nerves are numbered according to the vertebrae above which it exits the spinal canal: the eight cervical spinal nerves are C1 through C8; the twelve thoracic spinal nerves are numbered T1 through T12; the five lumbar spinal nerves are L1 through L5; and the five sacral spinal nerves are S1 through S5. There is 1 coccygeal nerve. Two spinal nerves exit the spinal canal between the vertebrae. One nerve goes to the left side of the body, the other to the right. The exits (foramina) are positioned behind the intervertebral disc and in front of the facet joint. Because of their proximity to the nerve, a damaged or ruptured disc impacting the nerve results in pain in the back.

The spinal nerves go to a specific area of the body to control sensation and movement. The nerves from the thoracic region go to the chest wall and abdomen where they receive sensations from the following:

  • Arms
  • Bronchial tubes
  • Coronary arteries
  • Esophagus
  • Gallbladder
  • Hands
  • Heart
  • Kidneys
  • Liver
  • Pancreas
  • Small intestines
  • Spleen
  • Stomach
  • Trachea, lungs
  • Ureters
  • Adrenal glands

The nerves from the lumbar spine go to the lower abdomen, pelvis and down the legs. They experience sensations from the lower back, large intestine, appendix, bladder, reproductive organs, legs and feet. There sensations are then returned to the brain for interpretation.

Pinched Nerve

The most common injury to the spinal cord is that of a pinched nerve caused by a bulging or herniated disc. When a nerve is pinched the pain can usually be felt in a specific site. The pain may also be felt in multiple sites, such as pain in the buttocks and down the leg to the ankle. This condition is known as radiculopathy. Another example of radiculopathy, related to the cervical spine, is that of a pain being felt in the hand and fingers due to a pinched nerve in the wrist (carpal tunnel syndrome).

Pinched nerve pain is caused by too much pressure being applied to a nerve by a bone, cartilage, tendon or muscle as a result of trauma, disease or inflammation. The pain may be accompanied by numbness, muscle weakness or tingling. Bone spurs, caused by arthritic degeneration of the vertebral body, can also impinge upon the spinal nerves causing pain. Fortunately, in many cases, pinched nerves can be treated by conservative means. In the majority of cases, pain in the back is felt in the low back and/or legs.

In addition to the discs, the spine is surrounded by many muscles and ligaments which give it strength and flexibility. The muscles are defined according to their function, i.e., flexion, extension or rotation. The muscles and ligaments support the spine, hold it upright, and control movement during rest and activity.

Ligaments

The fibrous bands of tissue that link two or more bones, cartilages, or structures together are known as ligaments. One or more ligaments provide stability to a joint during rest and movement. Some ligaments prevent movement in a given direction, while others prevent excessive movements such as hyper–extension or hyper–flexion.

Laminae

Laminae are thin, plate-like, bony structures connected to the vertebrae by the ligamentum flavum. Displacement or slippage of the vertebrae (spondylolisthesis) can cause pain in the back due to the compression of nerve roots. This is especially true in the low-back or lumbosacral region. There are a number of types of spondylolisthesis including:

  • Congenital spondylolisthesis — is a birth defect caused by the abnormal formation of the vertebrae, putting the vertebrae at greater risk of slipping
  • Degenerative spondylolisthesis — this is the most common form of the disorder caused by the natural aging process
  • Isthmic spondylolisthesis — a condition that leads to small vertebral stress fractures that can cause the vertebrae to slip out of place
  • Pathological spondylolisthesis - where the spine is weakened by osteoporosis or other disease, tumor or infection
  • Post-surgical spondylolisthesis - vertebral slippage that occurs or becomes worse after spinal surgery
  • Traumatic spondylolisthesis – caused by an injury to the spine that results in slippage
Gary Kraus, MD,
Neurosurgeon, is Board Certified
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Masaki Oishi, MD,
Spine Fellowship at the University
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