Back pain can be caused by a wide variety of factors. These include structural problems of the back, inflammation, muscle and soft tissue injury, secondary response to other diseases or conditions, imbalances in body mechanics, and psychological/social factors, among others. A substantial proportion of patients who develop chronic low back pain have no identifiable structural pathology capable of explaining the pain. Pain specialists sometimes attribute the pain in these patients to unknown musculoskeletal factors; sometimes the term "idiopathic" is used and means that the cause is entirely unknown.
It is important to identify the type of problem that is causing chronic low back pain, if possible. A diagnosis can help guide the therapeutic strategy.
The causes of low back pain are commonly grouped into categories that reflect the underlying source of the problem.
The low back architecture consists of:
- Vertebral bodies: the bones of the spine
- Vertebral discs: cushions between the bones
- Cartilage: lines the bones that connect with other bones (i.e. spine to ribs)
- Supportive structures surrounding the spine: muscles, tendons (connecting muscle to bone), ligaments (connecting bone to bone)
Muscle Strain: The major cause of acute low back pain (70%) can be found in the myofascial system (the muscles, ligaments and tendons that bind the spine together and support it).
Osteoarthritis: Osteoarthritis is a condition in which the cartilage in the disks or the moving joints of the spine are damaged. Often the body's reaction is to grow calcium deposits over the joint, which can lead to pinching of the nerve as it exits the spine through the foramen (opening). Just as most people, as they age, have evidence of osteoarthritis of other joints in the body, most people have evidence of osteoarthritis of the spine when a MRI is performed. Usually, these abnormalities seen on the MRI are painless. Why some people with arthritic spine disease develop pain is not known and is presumed to be complex.
Spinal Stenosis: Spinal stenosis refers to narrowing of the spinal canal. There are a variety of causes. The most common cause is a combination of degenerative spine disease (osteoarthritis of the spine) and bulging or herniated discs. From one perspective, spinal stenosis refers to the architecture of the spine and is something that is seen on a MRI. From the clinical perspective, spinal stenosis is associated with a specific type of pain syndrome-pain in the legs with walking-that is called "neurogenic claudication." Sometimes, patients with other kinds of pain have a MRI, which reveals stenosis, and the pain is attributed to the stenosis. This can confuse the picture. Given the definite link between stenosis and neurogenic claudication, it is best to attribute pain to the stenosis only when the pain fits the description of the claudication syndrome. Different kinds of pain may be caused by the spinal pathology associated with the stenosis (such as the arthritis), but saying that any kind of pain can be caused by stenosis would not be accurate. Some studies suggest that spinal stenosis accounts for approximately 3% of low back pain.
Discogenic Disk Disease: Disk disease probably accounts for about 10% of low back pain. Chronic back pain may be caused by degenerative disk disease, which refers to a thinning and drying out of the disks over time that can lead to altered spine mechanics and perhaps to nerve irritation. Although pain also may be caused by chronically herniated disks or bulging disks, the relationship between disc herniations and low back pain is poorly defined. Most patients older than the age of 40 will have some lumbar disk abnormalities found if an MRI is performed (Boos, 2000).
Spondylolisthesis: Spondylolisthesis is a condition in which one vertebral body slips forward over another. This can cause worsening of osteoarthritis, altered spine mechanics, nerve root compression, or spinal stenosis. A fracture of disconnection in the bones that surround the spinal canal, a condition known as spondylolysis, predisposes to spondylolisthesis. Spondylolysis is often congenital.
Vertebral Fracture: Compression fractures of the vertebrae occur most often in patients with osteoporosis. Osteoporosis is most common in postmenopausal women, the elderly, patients with a family history of the disease, and patients with a history of malnutrition (especially with calcium or Vitamin D), chronic corticosteroid usage or immobility.
It is important to screen for the condition and treat it if it is present.
Congenital diseases: In addition to spondylolysis, congenital abnormal curvatures of the spine may predispose to chronic pain. For example, scoliosis accounts for less than 1% of low back pain---usually in people with more severe curvatures.
While this group of conditions represents only a very small fraction of the causes of low back pain, it is important to be aware of them. Treatment for the underlying condition may be part of the therapeutic approach to the pain.
Ankylosing Spondylitis: Ankylosing spondylitis is a chronic inflammation of the spine that can lead to fusion of the spinal vertebrae over time. It is believed to be a hereditary disease in most cases.
Neoplasms: Cancer commonly invades the spine. In addition to metastatic solid tumors, such as breast and prostate cancer, lymphomas and myeloma often involve vertebral bodies and produce back pain. Tumors arising from the spinal cord, nerve roots, or surrounding tissues are much less common. Some benign tumors, such as spinal meningiomas and neurofibromas, also cause pain when they occur.
Infections: Back pain can be caused by numerous types of infections. Bacteria can invade the vertebral bodies (osteomyelitis) or causes abscesses inside the spine. Chronic infections, such as Lyme disease, can involve the structures inside the spine. Viruses, such as the zoster virus causing shingles, can attach specific levels of the spine and cause back pain.
Atherosclerosis: Atherosclerosis (hardening of the arteries) involving the large blood vessels that run just in front of the spine (such as the aorta) may be a cause of back pain in some patients (Kauppila 1997,Kurunlahti 1999). For example, an abdominal aortic anuerysm, a complication of advanced atherosclerosis, may cause fluctuating back pain, at least for a period of time. How often atherosclerosis is involved in the development of chronic pain is not known.
Diseases of the Viscera: Diseases of other organs, such as uterine or pelvic infections in women, kidney infections, and gastrointestinal disease such as pancreatitis and cholecystitis, can also cause pain that can be referred to the low back.
Nerve roots, which originate at the spinal cord and travel within the spine until they exit to become nerves, can be injured by spinal disorders. Most commonly, the nerve roots are injured where they exit the spine by diseased disks, bony abnormalities (such as those accompanying osteoarthritis of the spine), or some combination of these disorders. Some patients with chronic pain that develops after surgery or trauma, the injury to the nerve roots is presumably related to scarring. Other problems, such as tumors, are very uncommon. Injury to nerves can refer pain down the leg. When severe, it can cause weakness or numbness. Very severe injury, which occurs rarely, can cause a caudia equina syndrome, in which there is injury to all the nerve roots, which can cause weakness and numbness of both legs, and bowel and bladder problems.
Poor posture and movement patterns can contribute to chronic low back pain. Although there is little scientific understanding of the processes involved, it is believed that these problems can cause wear and tear on the spine and large joints, and also lead to muscle spasm.
Psychological factors often play a significant role in the development of and adaptation to chronic low back pain (Goldberg 1999, Kendall 1999). Pre-existing depression, anxiety and stress, together with lack of effective coping skills, may predispose individuals to back pain. Research has shown that having a variety of coping strategies, a person's belief in their ability to control their pain, and the level of catastrophizing about their condition is strongly associated with healthy adjustment in chronic pain patients and their families (Simmonds, 1996). Some believe that psychological factors such as these are primary causes of back pain (Sarno, 1991). Distress, whether secondary to physical restriction or to pain, may aggravate the pain and thus the disability (Simmonds 1996). A vicious cycle can be established in which pain causes stress or stress causes pain, and both produce more stress, anxiety and/or depression which causes more low back strain and pain.
Once low back pain has occurred, it will impact on the quality of life of the individual and the family. Often, while this can be a disruptive influence, it can also produce what is termed "secondary gain". This describes a situation in which an illness can produce advantages for a patient, such as increased attention and family support or disability from work. Addressing these kinds of reactions to chronic pain are also an important part of successful treatment (Stenger, 1992).