Most of the time, the exact cause of back pain is not easily determined. Fortunately, about 90 percent of cases improve on their own.
People under age 60 generally have acute backaches—that is, backaches that are sudden and short lived. These most often occur as pain limited to the back and result from a sprain or strain (it can be hard to distinguish between the two), a spasm, or, less frequently, a degenerated or herniated disk. In older people, chronic conditions such as degenerative changes of the spinal bones and disks, vertebral compression fractures, spinal stenosis (narrowing of the spinal column), and spinal deformities are the most common sources of back pain.
Other back-related disorders may cause symptoms that are not limited to back pain. For example, sciatica—pain that radiates into the buttocks, down the thighs to below the knees, and into the calves and often even the feet—is caused by irritation of a nerve leaving the spinal canal (the hollow channel through which the spinal cord runs) in the lumbar region of the back. Stiffness and loss of muscle strength may accompany back pain.
Although it may seem that back pain occurs suddenly (such as when bending down to pick up something), underlying problems—including normal aging, a weak back and weak abdominal muscles, obesity, and postural imbalances —set the stage for such "back attacks." The frequency of sprains and strains tends to decline after age 60, in part because older adults are less likely to participate in the kind of vigorous activities that lead to these problems. In addition, the disks between the vertebrae become less pliable in older adults and, as a result, are more likely to herniate. Such rigidity also may lead to degeneration of the various spinal bones.
Sprains, Strains, and Spasms
Sprains, strains, and spasms usually result from activity or injury. The term sprain is used when a ligament is partly torn, while strain applies to cases when a muscle is overstretched. A muscle spasm refers to a contraction of muscles in response to injury, which may occur in a ligament, muscle, disk, or joint.
Although the pain may be intense, muscle spasms ultimately protect you. Pain is your body's way of making sure that the injured area remains immobile, thus preventing further damage. Unlike back pain due to the compression of a nerve, the presence of a muscle spasm cannot be confirmed by imaging technology such as an x-ray or magnetic resonance imaging (MRI).
Degenerative changes to the disks and facet joints in the lumbar spine, often referred to as degenerative osteoarthritis or spondylosis, are an inevitable consequence of aging. They usually begin around age 20, but imaging studies have detected changes within the disks of teenagers. Degenerative changes, along with vertebral compression fractures and kyphosis, are responsible for the loss of height experienced by many people over age 50.
As we age, lumbar disks wear out because they are subjected to such large loads. Depending on what you do for a living and what types of activity you engage in during your leisure time, pressure on the lumbar spine can range from one to 11 times your body weight. Standing erect places a load on a lumbar disk that is equivalent to your body weight; bending, twisting, and lifting increase the load in proportion to the activity and the amount of weight lifted. Age-related loss of strength in the back muscles—which normally bear about one third of the load on the spine—also increases the stress on the disks and facet joints.
Over the years, the center of the disk slowly loses its water content and shrinks. This flattening of the disks leads to a narrowing of the space between the vertebrae. Further, as a result of supporting heavy loads, the vertebrae may develop bone spurs (osteophytes) that can press on a spinal nerve and cause pain.
Another possible cause of pain is deterioration of the facet joints due to disk wear and vertebral changes. In addition, arthritic changes (caused by a gradual erosion of the cartilage that lines the facet joints of the spine) can lead to pain and impair the smooth and coordinated movement of the spine. Pain also can occur if the nerves in the outer portion of the disks themselves are irritated.
Originally published in The Johns Hopkins White Papers: Back Pain and Osteoporosis (2011)
Lee H. Riley III, M.D.
Chief, Orthopaedic Spine Division, Johns Hopkins Medicine
Suzanne M. Jan de Beur, M.D.
Chief, Division of Endocrinology, Johns Hopkins Bayview Medical Center