December 16, 2018
Lower back pain.

Back to Basics: The Anatomy of Low Back Pain

by Wellness Letter  

Four out of five adults experience significant low back pain at some point in their lives, and many contend with ongoing pain; pain triggered or exacerbated by certain positions, movements, or activities; or intermittent episodes of debilitating pain. These problems may persist for years. Low back pain is the second most common medical complaint after headache and a leading cause of doctor visits and missed days from work.

The prevalence of low back pain is not surprising given the amount of daily stress placed on the back over the course of a lifetime. The human back supports the weight of the upper body not only when standing, walking, and lifting, but also when turning, twisting, and bending. These movements are affected by the strength, flexibility, and alignment of many body parts, including the spine, its ligaments, and the “core” muscles of the torso that help provide both movement and stability to the vertebral column. As a result, seemingly simple movements—how we habitually stand or sit, for instance—can adversely affect our backs, sometimes to the point of injury.

Your aching back

To understand why low back pain is so common and so complex, it helps to know a little about how your back is built. The human spine consists of 33 bones called vertebrae, including five that fuse to form the sacrum (part of the pelvis) and four that fuse to form the coccyx, or “tailbone.” When stacked on each other, separated by cushioning intervertebral discs, the front (anterior) portions of the vertebrae—the “vertebral bodies”—form a strong yet flexible pillar that supports the trunk and head.

The rear (posterior) portion of each vertebra is a bony arch. Outgrowths on each arch form joints with the vertebrae above and below. These “facet joints” are constructed to allow varying degrees of movement between vertebrae, but only in certain directions. Together, the arches form a long, cylindrical channel that surrounds and protects the spinal cord.

The interlocking vertebrae rest upon one another at a slight angle, which results in an S-­shaped curve (when viewed from the side), optimal for shock absorption. Throughout childhood and adolescence, the vertebrae grow thicker and stronger, and the sacral and coccygeal vertebrae gradually fuse so that, by adulthood, most people have 24 distinct vertebrae plus the sacrum and coccyx.

Regions of the back

The vertebrae are grouped into regions. The top seven make up the cervical (neck) region of the spine, which supports the head. The next 12 make up the thoracic (chest) region, which supports the weight of the chest wall and arms as well as the neck and head. The thickest, sturdiest vertebrae make up the lumbar region of the lower back, which supports the body above the hips. Because the lumbar and sacral regions may both be involved in low back pain, they are sometimes referred to together as “lumbosacral.”

The sacrum itself is a triangular bone oriented “point down” at the base of the spine and forming the middle of the pelvis’ rear wall. Its upper surface is separated from the lowest (fifth) lumbar vertebra by an intervertebral disc. Its other two sides are each connected to the adjacent pelvic bone, called the ilium or “hipbone,” by the sacroiliac joints. Below the sacrum is the coccyx.

Intervertebral discs—resilient pads of cartilage-­like tissue that prevent the bone surfaces from grinding together and serve as a cushion between vertebrae—are high in water content (especially during the early decades of life) and are thus very elastic. In conjunction with the facet joints, these tough yet pliable discs give the spine its remarkable flexibility.

Soft tissue and nerves

The spine’s wide range of motion would not be possible without the ligaments, muscles, and tendons that connect to the spinal bones. These non-bony elements both allow and limit spinal movements and provide stability. Over 140 muscles attach to the bony spine, and these can directly power spinal movements, though most are actually used more for stabilizing. In addition, the abdominal muscles help to stabilize the lumbar spine.

Down the length of the spinal cord, nerves branch off in pairs to all parts of the body. The roots of these nerves pass through narrow channels called foramina. The foramina lie close to the junction of the vertebral arches with the vertebral bodies, and thus the nerves passing through are close to the intervertebral discs.

The spinal cord itself ends around the top of the lumbar region, but nerves to the lower body travel farther down the spinal canal to exit at their individual foramina. The bundle these nerves make below the end of the spinal cord is known as the cauda equina (Latin for “horse’s tail,” owing to its appearance). With nerve tissue exiting the spinal column in close proximity to hard, unyielding bone and intervertebral discs, a number of problems can arise—resulting in inflammation or compression of the nerves and thus pain.

Acute vs. chronic back pain

When we say “back pain” in this article, we’re talking about low back pain, unless otherwise specified—not pain in the neck or upper back. Back pain is usually classified as acute or chronic.

  • Acute back pain occurs as an isolated episode, usually lasting less than a few weeks. It is often (but not always) relatively intense and can make normal activities unpleasant or difficult, if not impossible. Some people will have only one episode of acute back pain in their life, whereas others may have repeated episodes, with long or short pain-free periods in between.
  • Chronic back pain is persistent and lasts for longer than three months—sometimes for years. Symptoms may ease during periods of rest, but the problem never seems to go away completely and is often quick to flare up. Pain intensity can wax and wane over time and can range from mild, nagging discomfort to debilitating agony. (A third category, sub-acute back pain, lasts longer than a few weeks but less than three months.)

Whether your pain is acute or chronic, it’s important to note that back pain is not, unto itself, a diagnosis. It is a symptom of some underlying problem. Discovering the nature of the underlying problem can be an important first step to getting effective treatment and, ultimately, relief.

Deciding to see a doctor

If you have low back pain, have adequate control of the pain, and can either function somewhat normally or are rapidly getting to that point, you don’t need to see a doctor or other health-care provider. But if the measures you take (like avoiding positions or movements that bring on the pain or make it worse, not over-exerting yourself, applying heat or cold to the affected area, or taking over-the-counter pain relievers) don’t help enough, you should see a doctor. That’s also the case if there are symptoms such as numbness, tingling, or weakness in the lower extremities. If you suffer fever or loss of bladder or bowel control, seek immediate medical attention.

Your primary-care doctor will examine you (typically without the need for X-rays or other imaging tests), make recommendations on how best to manage your pain, and advise you on what to expect. If standard measures don’t help, your doctor can refer you to a specialist. Those who deal with backs all or much of the time include spinal surgeons (orthopedic surgeons and neuro­surgeons who have undergone special training in the surgical treatment of back problems) and physiatrists (specialists in physical medicine and rehabilitation).

Getting the right diagnosis

With back pain, obtaining the proper diagnosis can be notoriously difficult and elusive. Why? Because we do not understand the reason—or reasons—for back pain in most people. Furthermore, disruptions in normal function elsewhere in the body, including the bones and ligaments of the pelvis and muscles in the legs, buttocks, trunk, and shoulders, can affect how the spine moves and functions. Since so many different and interrelated variables are in play, pinpointing a single clear cause for back pain can all too often be an exasperating challenge—resulting in the frustrating diagnosis of “nonspecific back pain.” Some 85 percent of all back pain cases fall into this category.

High-tech limitations

Another problem with getting a clear diagnosis is the tendency to rely too much on imaging technology. Although an MRI, CT scan, or X-ray can be useful for confirming certain diagnoses, joint guidelines on low back pain from the American College of Physicians and the American Pain Society emphasize the importance of talking to and examining the person with back pain rather than relying on imaging tests. Imaging the spine produces too many false alarms to be a good routine test for people with nonspecific back pain. That is, the scans may reveal an abnormality that’s assumed to be the cause of the pain but isn’t, potentially leading to unnecessary and possibly risky treatments like epidural injections or surgery.

MRIs find spinal abnormalities in most people, including those who have no back pain. The overuse of MRIs helps explain why the rate of back surgery in the U.S. is more than twice as high as in other countries—and why, all too often, the surgery doesn’t help. In many cases, surgery corrects a “problem” that is unrelated to the cause of the pain.

Given the limitations and high false-positive rate of MRIs, if your doctor recommends such scans, ask about the pros and cons beforehand.

This article first appeared in the UC Berkeley Wellness Letter.

Also see Back Pain: An Ounce of Prevention.