I am Dr. Grant Cooper from the Princeton Spine and Joint Center. I have been asked to talk to you today about what causes lower back pain. When we talk about lower back pain we have to make an important distinction when we talk about the causes between acute lower back pain and chronic lower back pain.
Chronic is anything that lasts for longer than three months. Acute lower back pain, there are a lot of different causes, starting with muscle strains, ligament sprains, and so forth. Once pain in the lower back has lasted for at least three months, we give it the classification of chronic lower back pain and that’s important because there are basically about three things that account for approximately eighty percent of chronic lower back pain, and that’s what we’re going to talk about today.
If we take the lower back, the kind that looks like this. This is the front, this is the back. You get five vertebrae (one, two, three, four, five). Sacrum’s over here, and the pelvis is over here. Spinal cord is in the back. The discs are in between the bones over here.
The three most common causes, the most common is the disc. You’ve heard of disc, herniated disc, this and that. Pain that comes from a disc is not the same thing as a herniated disc. Herniated discs, herniate out and they start to irritate nerves and that sends pain, numbness, sometimes weakness down the leg.
Pain that comes from a disc is when there’s actually a tear inside of the disc. A tear inside the disc basically, the disc is like a jelly donut. There’s this inner jelly that has all these inflammatory proteins. They’re called inflammatory proteins because when they get next to nerves, they cause pain, sometimes numbness, weakness, they cause inflammation. Luckily there’s no nerve fibers in the middle of the disc, in the jelly of the disc. But in the outer third of the crust of the disc, sometimes in the outer two-thirds, you have these little nerve fibers. And so what happens when people have pain that comes from a disc is there’s a tear where some of the inflammatory proteins in the jelly has oozed out and now it’s irritating the outer third or outer two-thirds of the crust of the disc and that’s when discs are actually causing lower back pain.
Now, it’s true that often when somebody has a herniated disc, they may also have a tear inside of the disc, and then, of course, you can have the back pain as well with the leg pain or numbness or weakness. It’s also important to remember that just because you have a tear in the disc doesn’t mean that you are necessarily going to have pain from that. People certainly do have some asymptomatic tears, but that’s the most common cause. In younger people, people under the age of forty, it accounts for about 40 percent of chronic lower back pain. It’s also a very common cause in older people as well. In general, it tends to be more painful when you sit; it tends to be more painful when you bend forward because those are the positions that put more mechanical pressure on the discs, which puts more pressure on the jelly and on the nerves inside the disc.
The second most common cause of chronic lower back pain are these joints in the back of the spine called facet joints. Facets are like knees, shoulders, hips, fingers—they’re synovial joints, they have all the same basic parts as a knee, shoulder, or hip. They’re just a little more exotic because they’re deep in the spine. You can’t see them without an x-ray. And you can’t push on them unless you poke it with a needle. But basically, just like a knee or shoulder or hip that can become arthritic, and painful, so can these facet joints. Facet joints tend to be more painful when you stand and extend and they tend to feel a little bit better when you sit, in general. You can imagine these are the joints back here. The facets, standing puts a lot of load on them, and that’s why they tend to be more painful when you’re standing or when you twist and turn. In older people, after the age of 65, they tend to account for about 30 to 40 percent of chronic lower back pain. In younger people, it’s less common from facet joint; more like 15 percent or thereabouts.
The third most common cause of chronic lower back pain is the sacroiliac joint. Remember this is the lower back, right? Front, back. Here’s the sacrum. This bone over here is the ilium. There is a joint line here called the sacroiliac joint. That tends to be pain more in the lower portion, more in the buttocks. It accounts for 10 to 15 percent, some people even say as many as 20 percent of chronic lower back pain.
And those three things account for the most common causes of chronic lower back pain. I hope this has been helpful.