What is the anatomy of the low 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 (nervous tissue that extends down the spinal column from the brain) from injury. Each vertebrae has a spinous process, a bony prominence behind the spinal cord, which shields the cord's nervous tissue. They also have a strong bony "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 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 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.

What is the function of the low back?

The low back, or lumbar area, serves a number of important functions for the human body. These functions include structural support, movement, and protection of certain body tissues.

When we stand, the lower back is functioning to support the weight of the upper body. When we bend, extend, or rotate at the waist, the lower back is involved in the movement. Therefore, injury to the structures important for weight bearing, such as the bony spine, muscles, tendons, and ligaments, often can be detected when the body is standing erect or used in various movements.

Protecting the soft tissues of the nervous system and spinal cord as well as nearby organs of the pelvis and abdomen is a critical function the lumbar spine and its adjacent muscles.

What are common causes of low back pain?

Common causes of low back pain include lumbar strain, nerve irritation, lumbar radiculopathy, bony encroachment, and conditions of the bone and joints. Each of these is reviewed below.

1. Lumbar strain (acute, chronic)
   
   A lumbar strain is a stretch injury to the ligaments, tendons, and/or muscles of the low back. The stretching incident results in microscopic tears of varying degrees in these tissues. Lumbar strain is considered one of the most common causes of low back pain. The injury can occur because of overuse, improper use, or trauma. Soft-tissue injury is commonly classified as "acute" if it has been present for days to weeks. If the strain lasts longer than three months, it is referred to as "chronic."
   
   Lumbar strain most often occurs in people in their forties, but it can happen at any age. The condition is characterized by localized discomfort in the low back area with onset after an event that mechanically stressed the lumbar tissues. The severity of the injury ranges from mild to severe, depending on the degree of strain and resulting spasm of the muscles of the low back.
   
   The diagnosis of lumbar strain is based on the history of injury, the location of the pain, and exclusion of nervous system injury. Usually, X-ray testing is only helpful to exclude bone abnormalities.
   
   The treatment of lumbar strain consists of resting the back (to avoid reinjury), medications to relieve pain and muscle spasm, local heat applications, massage, and eventual (after the acute episode resolves) reconditioning exercises to strengthen the low back and abdominal muscles. Long periods of inactivity in bed are no longer promoted, as this treatment may actually slow recovery. Spinal manipulation for periods of up to one month has been found to be helpful in some patients who do not have signs of nerve irritation. Future injury is avoided by using back-protection techniques during activities and support devices as needed at home or work.



2. Nerve irritation
   
   The nerves of the lumbar spine can be irritated by mechanical impingement or disease anywhere along their paths -- from their roots at the spinal cord to the skin surface. These conditions include lumbar disc disease (radiculopathy), bony encroachment, and inflammation of the nerves caused by a viral infection (shingles). See discussions of these conditions below.



3. Lumbar radiculopathy
   
   Lumbar radiculopathy is nerve irritation that is caused by damage to the discs between the vertebrae. Damage to the disc occurs because of degeneration ("wear and tear") of the outer ring of the disc, traumatic injury, or both. As a result, the central softer portion of the disc can rupture (herniate) through the outer ring of the disc and abut the spinal cord or its nerves as they exit the bony spinal column. This rupture is what causes the commonly recognized "sciatica" pain that shoots down the leg. Sciatica can be preceded by a history of localized low-back aching or it can follow a "popping" sensation and be accompanied by numbness and tingling. The pain commonly increases with movements at the waist and can increase with coughing or sneezing. In more severe instances, sciatica can be accompanied by incontinence of the bladder and/or bowels.
   
   Lumbar radiculopathy is suspected based on the above symptoms. Increased radiating pain when the lower extremity is lifted supports the diagnosis. Nerve testing (EMG/electromyogram and NCV/nerve conduction velocity) of the lower extremities can be used to detect nerve irritation. The actual disc herniation can be detected with imaging tests, such as CAT or MRI scanning.
   
   Treatment of lumbar radiculopathy ranges from medical management to surgery. Medical management includes patient education, medications to relieve pain and muscle spasms, cortisone injection around the spinal cord (epidural injection), physical therapy (heat, massage, ultrasound, electrical stimulation), and rest (not strict bed rest, but avoiding reinjury). With unrelenting pain, severe impairment of function, or incontinence (which can indicate spinal cord irritation), surgery may be necessary. The operation performed depends on the overall status of the spine and the age and health of the patient. Procedures include removal of the herniated disc with laminotomy (a small hole in the bone of the lumbar spine surrounding the spinal cord), laminectomy (removal of the bony wall), by needle technique (percutaneous discectomy), disc-dissolving procedures (chemonucleolysis), and others.

Picture of herniated disc between L4 and L5


Cross-section picture of herniated disc between L4 and L5


4. Bony encroachment
   
   Any condition that results in movement or growth of the vertebrae of the lumbar spine can limit the space (encroachment) for the adjacent spinal cord and nerves. Causes of bony encroachment of the spinal nerves include foraminal narrowing (narrowing of the portal through which the spinal nerve passes from the spinal column, out of the spinal canal to the body), spondylolisthesis (slippage of one vertebra relative to another), and spinal stenosis (compression of the nerve roots or spinal cord by bony spurs or other soft tissues in the spinal canal). Spinal-nerve compression in these conditions can lead to sciatica pain that radiates down the lower extremities. Spinal stenosis can cause lower-extremity pains that worsen with walking and are relieved by resting (mimicking the pains of poor circulation). Treatment of these afflictions varies, depending on their severity, and ranges from rest to surgical decompression by removing the bone that is compressing the nervous tissue.



5. Bone and joint conditions
   
   Bone and joint conditions that lead to low back pain include those existing from birth (congenital), those that result from wear and tear (degenerative) or injury, and those that are due to inflammation of the joints (arthritis).



Congenital bone conditions -- Congenital causes (existing from birth) of low back pain include scoliosis and spina bifida. Scoliosis is a sideways (lateral) curvature of the spine that can be caused when one lower extremity is shorter than the other (functional scoliosis) or because of an abnormal architecture of the spine (structural scoliosis). Children who are significantly affected by structural scoliosis may require treatment with bracing and/or surgery to the spine. Adults infrequently are treated surgically but often benefit by support bracing.

Spina bifida is a birth defect in the bony vertebral arch over the spinal canal, often with absence of the spinous process. This birth defect most commonly affects the lowest lumbar vertebra and the top of the sacrum. Occasionally, there are abnormal tufts of hair on the skin of the involved area. Spina bifida can be a minor bony abnormality without symptoms. However, the condition can also be accompanied by serious nervous abnormalities of the lower extremities.

Degenerative bone and joint conditions -- As we age, the water and protein content of the body's cartilage changes. This change results in weaker, thinner, and more fragile cartilage. Because both the discs and the joints that stack the vertebrae (facet joints) are partly composed of cartilage, these areas are subject to wear and tear over time (degenerative changes). Degeneration of the disc is called spondylosis. Spondylosis can be noted on X-rays of the spine as a narrowing of the normal "disc space" between the vertebrae. It is the deterioration of the disc tissue that predisposes the disc to herniation and localized lumbar pain ("lumbago") in older patients. Degenerative arthritis (osteoarthritis) of the facet joints is also a cause of localized lumbar pain that can be detected with plain X-ray testing. These causes of degenerative back pain are usually treated conservatively with intermittent heat, rest, rehabilitative exercises, and medications to relieve pain, muscle spasm, and inflammation.

Injury to the bones and joints -- Fractures (breakage of bone) of the lumbar spine and sacrum bone most commonly affect elderly people with osteoporosis, especially those who have taken long-term cortisone medication. For these individuals, occasionally even minimal stresses on the spine (such as bending to tie shoes) can lead to bone fracture. In this setting, the vertebra can collapse (vertebral compression fracture). The fracture causes an immediate onset of severe localized pain that can radiate around the waist in a band-like fashion and is made intensely worse with body motions. This pain generally does not radiate down the lower extremities. Vertebral fractures in younger patients occur only after severe trauma, such as from motor-vehicle accidents or a convulsive seizure.

In both younger and older patients, vertebral fractures take weeks to heal with rest and pain relievers. Compression fractures of vertebrae associated with osteoporosis can also be treated with a procedure called vertebroplasty, which can help to reduce pain. In this procedure, a balloon is inflated in the compressed vertebra, often returning some of its lost height. Subsequently, a "cement" (methymethacrylate) is injected into the balloon and remains to retain the structure and height of the body of the vertebra.

Arthritis -- The spondyloarthropathies are inflammatory types of arthritis that can affect the lower back and sacroiliac joints. Examples of spondyloarthropathies include reactive arthritis (Reiter's disease), ankylosing spondylitis, psoriatic arthritis, and the arthritis of inflammatory bowel disease. Each of these diseases can lead to low back pain and stiffness, which is typically worse in the morning. These conditions usually begin in the second and third decades of life. They are treated with medications directed toward decreasing the inflammation. Newer biologic medications have been greatly successful in both quieting the disease and stopping its progression.

What are other causes of low back pain?

Other causes of low back pain include kidney problems, pregnancy, ovary problems, and tumors.

Kidney problems

Kidney infections, stones, and traumatic bleeding of the kidney (hematoma) are frequently associated with low back pain. Diagnosis can involve urine analysis, sound-wave tests, or imaging studies of the abdomen.

Pregnancy

Pregnancy commonly leads to low back pain by mechanically stressing the lumbar spine (changing the normal lumbar curvature) and by the positioning of the baby inside of the abdomen. Additionally, the effects of the female hormone estrogen and the ligament-loosening hormone relaxin may contribute to loosening of the ligaments and structures of the back. Pelvic-tilt exercises are often recommended for this pain. Women are also recommended to maintain physical conditioning during pregnancy according to their doctors' advice.

Ovary problems

Ovarian cysts, uterine fibroids, and endometriosis not infrequently cause low back pain. Precise diagnosis can require gynecologic examination and testing.

Tumors

Low back pain can be caused by tumors, either benign or malignant, that originate in the bone of the spine or pelvis and spinal cord (primary tumors) and those which originate elsewhere and spread to these areas (metastatic tumors). Symptoms range from localized pain to radiating severe pain and loss of nerve and muscle function (even incontinence of urine and stool) depending on whether or not the tumors affect the nervous tissue. Tumors of these areas are detected using imaging tests, such as plain X-rays, nuclear bone scanning, and CAT and MRI scanning.

What are uncommon causes of low back pain?

Uncommon causes of low back pain include Paget's disease of bone, bleeding or infection in the pelvis, infection of the cartilage and/or bone of the spine, aneurysm of the aorta, and shingles.

Paget's disease of bone

Paget's disease of the bone is a condition of unknown cause in which the bone formation is out of synchrony with normal bone remodeling. This condition results in abnormally weakened bone and deformity and can cause localized bone pain, though it often causes no symptoms. Paget's disease is more common in people over the age of 50. Heredity (genetic background) and certain unusual virus infections have been suggested as causes. Thickening of involved bony areas of the lumbar spine can cause the radiating lower extremity pain of sciatica.

Paget's disease can be diagnosed on plain X-rays. However, a bone biopsy is occasionally necessary to ensure the accuracy of the diagnosis. Bone scanning is helpful to determine the extent of the disease, which can involve more than one bone area. A blood test, alkaline phosphatase, is useful for diagnosis and monitoring response to therapy. Treatment options include aspirin, other anti-inflammatory medicines, pain medications, and medications that slow the rate of bone turnover, such as calcitonin (Calcimar, Miacalcin), etidronate (Didronel), alendronate (Fosamax), risedronate (Actonel), and pamidronate (Aredia).

Bleeding or infection in the pelvis

Bleeding in the pelvis is rare without significant trauma and is usually seen in patients who are taking blood-thinning medications, such as warfarin (Coumadin). In these patients, a rapid-onset sciatica pain can be a sign of bleeding in the back of the pelvis and abdomen that is compressing the spinal nerves as they exit to the lower extremities. Infection of the pelvis is infrequent but can be a complication of conditions such as diverticulosis, Crohn's disease, ulcerative colitis, infection of the Fallopian tubes or uterus, and even appendicitis. Pelvic infection is a serious complication of these conditions and is often associated with fever, lowering of blood pressure, and a life-threatening state.

Infection of the cartilage and/or bone of the spine

Infection of the discs (septic discitis) and bone (osteomyelitis) is extremely rare. These conditions lead to localized pain associated with fever. The bacteria found when these tissues are tested with laboratory cultures include Staphylococcus aureus and Mycobacterium tuberculosis (TB bacteria). TB infection in the spine is called Pott's disease. These are each very serious conditions requiring long courses of antibiotics. The sacroiliac joints rarely become infected with bacteria. Brucellosis is a bacterial infection that can involve the sacroiliac joints and is usually transmitted in goat's milk.

Aneurysm of the aorta

In the elderly, atherosclerosis can cause weakening of the wall of the large arterial blood vessel (aorta) in the abdomen. This weakening can lead to a bulging (aneurysm) of the aorta wall. While most aneurysms cause no symptoms, some cause a pulsating low back pain. Aneurysms of certain size, especially when enlarging over time, can require surgical repair with a grafting procedure to repair the abnormal portion of the artery.

Shingles

Shingles (Herpes zoster) is an acute infection of the nerves that supply sensation to the skin, generally at one or several spinal levels and on one side of the body (right or left). Patients with shingles usually have had chickenpox earlier in life. The Herpes virus that causes chickenpox is believed to exist in a dormant state within the spinal nerve roots long after the chickenpox resolves. In people with shingles, this virus reactivates to cause infection along the sensory nerve, leading to nerve pain and usually an outbreak of shingles (tiny blisters on the same side of the body and at the same nerve level). The back pain in patients with shingles of the lumbar area can precede the skin rash by days. Successive crops of tiny blisters can appear for several days and clear with crusty inflammation in one to two weeks. Patients occasionally are left with a more chronic nerve pain (postherpetic neuralgia). Treatment can involve symptomatic relief with lotions, such as calamine, or medications, such as acyclovir (Zovirax) for the infection and pregabalin (Lyrica) or lidocaine (Lidoderm) patches for the pain.

How is low back pain treated?

So, how is low back pain treated? Well, as described above, the treatment very much depends on the precise cause of the low back pain. Moreover, each patient must be individually evaluated and managed in the context of the underlying background health status and activity level.

As has been highlighted by research presented at the national meeting of the American College of Rheumatology, a very important aspect of the individual evaluation is the patient's own perception of their particular situation. British researchers found that those who believed that their symptoms had serious consequences on their lives and that they had, or treatments had, little control over their symptoms were more likely to have a poor outcome. This research points out to physicians the importance of addressing the concerns and perceptions that patients have about their condition during the initial evaluations.

Finally, it should be noted that the conditions listed above are intended for general review. There are many other causes of back pain that have not been discussed.

Low Back Pain At A Glance

• Functions of the low back, or lumbar area, include structural support, movement, and protection of certain body tissues.
• Symptoms in the low back can relate to the bony lumbar spine, 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.
• Treatment of low back pain is optimally directed toward a diagnosed or suspected specific cause.

REFERENCES:

Koopman, William, et al., eds. Clinical Primer of Rheumatology. Philadelphia: Lippincott Williams & Wilkins, 2003.

Kelley's Textbook of Rheumatology, W B Saunders Co, edited by Shaun Ruddy, et al., 2000.

American College of Rheumatology, Annual Scientific Meeting, 2007.

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