"Oh, my aching back!" It's one of the most common complaints in medicine. But why is the back so prone to pain?
Henry Dietrich Fernandez, an architect who lives in Cambridge, Mass., sees the human spine as a magnificent piece of architecture. It can bear great weights but also flex.
The best analogy, says Fernandez, "is the steel skeleton of a high-rise, having to bend and shift to accommodate itself to dynamic forces, like the wind."
But it's the spine's complexity that makes it vulnerable.
"Because of its flexibility and because of its multijointed structure, there are so many opportunities for so many things to go wrong," says Fernandez.
He knows what he's talking about. Four years ago Fernandez was crossing the street when he was hit by a car. It wasn't a big accident. But ever since, he hasn't had a day without back pain.
"It's with me all day long, no surgery nor drugs can fix it. It's just with me now for life," says Fernandez.
He has lots of company. Eight of out 10 people will suffer back pain at some point. Most can't point to a specific injury; the pain seemed to come out of nowhere.
Dr. Augustus White of Harvard Medical School has spent most of his life treating back pain.
Nearly 70 percent of all back pain is caused by strains and sprains. That is, injury to the muscles and ligaments that support the spine. These soft-tissue injuries usually heal within six weeks or so. Chronic back pain is more likely to be caused by degenerative changes in the spine's structure.
White says most chronic back pain can be traced to spinal deterioration that starts surprisingly early in life.
"The changes occur between the ages of 30 and 50," White explains. "And that's when most of the back pain occurs, although there's plenty of it before 30 and there's certainly plenty of it afterwards."
Much of the pain comes from the discs, especially in the lower back. Like a stale jelly doughnut, these shock-absorbing discs dry out and change shape.
"Narrowing of the disc, fragmentation of the disk, drying of the nucleus -- the jelly part of the jelly doughnut -- are all aging processes," he adds. "And it happens in varying degrees to all of us."
When the disk bulges or ruptures, the cushioning jelly can push into the spinal canal and press on the nerves.
Another source of back pain is the facet joints. These are the meeting points of winglike projections on the back of each vertebra.
"These little facet joints can act like arthritis just about anywhere in the body," says White. "The same way the knee joint can be painful or the hip joint can be painful, these little facet joints can be painful."
And then there's spinal stenosis. That's the narrowing of the spinal canal where nerves run through or the passages in each vertebra where nerves exit. Many things can cause it: injury, arthritis, and sometimes the ligaments that hold the spine together get floppy and fold over.
White says imaging devices like CT and MRI offer detailed pictures of these problems. But that often doesn't reveal which changes explain the pain.
"Some of them become painful, and some of them don't. And that's where the big question is. We tend to assume that the pain is coming somehow from the degeneration. But we don't have good proof of that," he says.
And because diagnosis is so often difficult, White counsels caution and patience in searching for treatments.