Anatomy of The Lower Back
The first step to understanding the various causes of low back pain is learning about the normal design (anatomy) of the tissues of this area. Important structures of the low back that can be related to symptoms there include the bony lumbar spine (vertebrae, singular = vertebra), discs between the vertebrae, ligaments around the spine and discs, spinal cord and nerves, muscles of the low back, internal organs of the pelvis and abdomen, and the skin covering the lumbar area.
The bony lumbar spine is designed so that vertebrae "stacked" together can provide a movable support structure while also protecting the spinal cord from injury. The spinal cord is composed of nervous tissue that extends down the spinal column from the brain. Each vertebra has a spinous process, a bony prominence behind the spinal cord, which shields the cord's nervous tissue from impact trauma. Vertebrae also have a strong bony "body" (vertebral body) in front of the spinal cord to provide a platform suitable for weight bearing of all tissues above the buttocks. The lumbar vertebrae stack immediately atop the sacrum bone that is situated in between the buttocks. On each side, the sacrum meets the iliac bone of the pelvis to form the sacroiliac joint of the buttocks.
The discs are pads that serve as "cushions" between the individual vertebral bodies. They help to minimize the impact of stress forces on the spinal column. Each disc is designed like a jelly donut with a central softer component (nucleus pulposus) and a surrounding outer ring (annulus fibrosus). The central portion of the disc is capable of rupturing (herniating) through the outer ring, causing irritation of adjacent nervous tissue and sciatica as described below.
Ligaments are strong fibrous soft tissues that firmly attach bones to bones. Ligaments attach each of the vertebrae to each other and surround each of the discs.
The nerves that provide sensation and stimulate the muscles of the low back as well as the lower extremities (the thighs, legs, feet, and toes) exit the lumbar spinal column through bony portals, each of which is called a "foramen."
Many muscle groups that are responsible for flexing, extending, and rotating the waist, as well as moving the lower extremities, attach to the lumbar spine through tendon insertions.
The aorta and blood vessels that transport blood to and from the lower extremities pass in front of the lumbar spine in the abdomen and pelvis. Surrounding these blood vessels are lymph nodes (lymph glands) and tissues of the involuntary nervous system that are important in maintaining bladder and bowel control.
The uterus and ovaries are important pelvic structures in front of the pelvic area of women. The prostate gland is a significant pelvic structure in men. The kidneys are on either side of the back of the lower abdomen, in front of the lumbar spine.
The skin over the lumbar area is supplied by nerves that come from nerve roots that exit from the lumbar spine.
The lower back has a lot more motion than the thoracic spine and also carries all the weight of the torso, making it the most frequently injured area of the spine.
The motion in the lumbar spine is divided between five motion segments, although a disproportionate amount of the motion is in the lower segments (L3-L4 and L4-L5). Consequently, these two segments are the most likely to breakdown from wear and tear (e.g. osteoarthritis). The two lowest discs (L4-L5 and L5-S1) take the most strain and are the most likely to herniate. This can cause lower back pain and possibly numbness that radiates through the leg and down to the foot (sciatica).
The vast majority of episodes of lower back pain are caused by muscle strain. Even though a muscle strain doesn't sound like a serious injury, trauma to the muscles and other soft tissues (ligaments, tendons) in the lower back can cause severe back pain. The good news is that soft tissues have a good blood supply, which brings nutrients to the injured area, facilitates the healing process and often provides effective relief of the back pain.
Vertebral Bodies in the Lumbar Spine (Lower Back)
The lumbar spine has five vertebral bodies that extend from the lower thoracic spine (upper back) to the sacrum (bottom of the spine). The vertebral bodies of the lower back are the largest of the spine because they bear the majority of the body’s weight.
The paired facet joints on the back of the vertebral segments are aligned so that they allow flexion/extension but not a lot of rotation. Most causes of back pain originate in the lumbar spine.
Vertebral Components of the Lumbar Spine
The thick oval segment of bone forming the front of the vertebral segment is the vertebral body. Each segment of the lumbar spine is comprised of the following structures:
The vertebral bodies are attached to a bony arch through which all the nerve roots run. The vertebral arch is comprised of two pedicles, the short stout processes that extend from the sides of the vertebral body and two laminae, the broad flat plates that project from the pedicles and join in a triangle to form a hollow archway (the foramen).
The vertebral arches are interconnected by paired facet joints, which in combination with the disc, create a three joint complex at each vertebral motion segment. This three-joint complex at each vertebral segment (the facet joint) allows for motion in flexion, extension, rotation, and lateral bending.
The facet joints have cartilage on each surface and a capsule around them. The cartilage can degenerate as one ages, and lead to degenerative arthritis.
The spinous process protrudes from the junction of the two laminae and these are the ridges that can be felt through the skin along the back of the spine.
Transverse processes project from the junction of the pedicles and lamina. The structures of the vertebral arch protect the spinal nerves that run through the spinal canal. A back surgery known as a lumbar laminectomy involves the removal of the laminae to gain access to the vertebral canal.
Spinal Discs in The Lower Back
The vertebral disc in the spine is an interesting and unique structure. Its primary purpose is to act as a shock absorber between adjacent vertebrae. Spinal discs also act as ligaments that hold the vertebrae of the spine together and as cartilaginous joints that allow for slight mobility in the spine.
There are a total of twenty-three vertebral discs in the spinal column. Specific problems with any of these discs may prompt different symptoms, including back pain, neck pain and sciatica.
Discs are actually composed of two parts: a tough outer portion and a soft inner core, and the configuration has been likened to that of a jelly doughnut (see a healthy disc with Figure 1).
The outer portion of the disc (annulus fibrosus) is the tough circular exterior composed of concentric sheets of collagen fibers (lamellae) that surround the inner core. The inner core (nucleus pulposus) contains a loose network of fibers suspended in a mucoprotein gel.
The annular fibers hydraulically seal the gelatinous nucleus and evenly distribute pressure and force imposed on the structure. The outer portion and inner core of the spinal disc fit together like two concentric cylinders and are interconnected by cartilagenous end-plates.
At birth, 80 percent of the disc is composed of water. In order for the disc to function properly, it must be well hydrated. The nucleus pulposus is the major carrier of the body’s axial load and relies on its water-based contents to maintain strength and pliability.
Spinal Cord & Spinal Nerve Roots
The spinal cord comes off the base of the brain, runs throughout the cervical and thoracic spine, and ends at the lower part of the thoracic spine. Therefore, spinal cord injury or damage may accompany trauma or diseases of the cervical spine or thoracic spine.
The spinal cord does not run through the lumbar spine (lower back). After the spinal cord stops in the lower thoracic spine, the nerve roots from the lumbar and sacral levels come off the bottom of the cord like a "horse’s tail" (cauda equina) and exit the spine.
Therefore, because the lumbar spine has no spinal cord and comprises a large amount of space for the nerve roots, even serious conditions (such as a large disc herniation) are unlikely to cause paraplegia (loss of motor function in the legs).
Division of the Spinal Cord
The spinal cord can be divided into segments according to the nerve roots that branch off of it. Nerves along the cord consists of 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. The nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine.
Cervical spine nerve roots. In the cervical spine (neck), the nerve root is named for the lower segment that it runs between (e.g. C6 nerve root at C5-C6 segment).
Lumbar spine nerve roots. In the lumbar spine (lower back), the nerve is named for the upper segment that it runs between (e.g. L4 nerve root at L4-L5 segment).
Back Muscles & Low Back Pain
Soft tissues around the spine also play a key role in lower back pain. There is a large and complex group of muscles that work together to support the spine, help hold the body upright and allow the trunk of the body to move, twist and bend in many directions.
Extensor, Flexor and Oblique Muscles and Back Pain
Three types of back muscles that help the spine function are extensors, flexors and obliques.
The extensor muscles are attached to the posterior (back) of the spine and enable standing and lifting objects. These muscles include the large paired muscles in the lower back (erector spinae), which help hold up the spine, and gluteal muscles.
The flexor muscles are attached to the anterior (front) of the spine (which includes the abdominal muscles) and enable flexing, bending forward, lifting and arching the lower back.
The oblique muscles are attached to the sides of the spine and help rotate the spine and maintain proper posture.