If you’ve ever complained to a friend about your aching back, chances are he or she responded with some version of: Been there, had that. Back pain is the second most common reason Americans go to the doctor (after colds and the flu). Studies show that four out of five adults have back pain at some time in their lives.

With an aging U.S. population, back pain may be growing into an even more common problem, since osteoarthritis (OA) typically emerges in people over age 40, and the spine is a prime target for this common joint condition. But just how much of our nation’s back pain is caused by OA? “That’s the $64,000 question,” says University of Pittsburgh pain specialist Debra K. Weiner, MD. Back pain is a complex condition, say Dr. Weiner and other doctors who treat it. OA is often just one piece of the puzzle.

OA and the Spine
OA is a disease of the joints, and your back has plenty of those. The spine is a column of 33 bones called vertebrae. Five of the vertebrae are fused into a single unit at the base of the spine, known as the sacrum; another four vertebrae combine to form the coccyx, or tailbone. The rest of the vertebrae are connected by so-called facet joints. Tucked between each vertebra are cushion-like pads called disks, which protect the spine and give it flexibility. Resting within the column of vertebrae lies the spinal cord, which connects the brain to every part of the body through an unfathomably complex networks of nerves.

Viewed from the side, the spine forms a graceful S shape. Yet this elegant-looking structure is vulnerable to OA—especially in the lower, or lumbar, region—thanks in large part to one of the very qualities that sets humans apart from the rest of the animal kingdom: We walk on two legs. “That produces a constant loading on the spine,” says Michael Marks, MD, a spokesperson for the American Academy of Orthopedic Surgeons. As we age, persistent pressure on the spine wears away the tough cartilage lining facet joints. Meanwhile, disks in the spine begin to narrow due to water loss, adding to pressure on the facet joints (and subtracting from your overall height, which is why people shrink as they age). As a result of these changes, says Dr. Marks, the facet joints can develop inflammation.

What’s more, OA can make the spine unstable, explains Dr. Marks. In response, growths called osteophytes, or bone spurs, can form in the joints. While bone spurs appear to be the body’s attempt to restore stability, they can cause the spine to stiffen. As OA worsens, bone spurs can narrow the “frames” where nerves exit the spinal cord. This condition, called spinal stenosis, can pinch nerves and cause numbness and weakness in the legs.

Not Just Joints
Trauma to the spine, from an accident or engaging in certain activities, increases your chances of having spinal OA (also known as spondylosis). “If you were a football lineman with constant impact all the time, you’re more likely to have it [spinal OA] than someone who was a swimmer,” says Dr. Marks. However, genetics plays a role too; if one or both of your parents had OA in the spine, your odds rise. Dr. Marks estimates that OA is the likely culprit in about 20 percent of 40-year-old men and women who develop back pain, a figure he says rises to as much as 75 percent among people over age 60.

However, some doctors who treat back pain feel that OA gets too much of the blame. They are quick to note this phenomenon: X-rays reveal that the vast majority of middle-aged and older men and women have OA and disk degeneration in their spinal joints, yet many have no symptoms. “They’re walking around on the street, pain free,” says Norman Marcus, MD, author of End Back Pain Forever (Atria, 2012) and director of the division of muscle pain research in the department of anesthesiology at the New York University School of Medicine.