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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 of acute lower back pain and chronic lower back pain. Chronic is anything that lasts for more than three months. With 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 classifications of chronic lower back pain and that’s important because there are basically three things that account for approximately eighty percent of chronic lower back pain and what we’re going to talk about today.
If we take the lower back, it looks like this. This is the front, this is the back. You have five lumbar vertebrae. The sacrum is over here. Pelvis is over here. The spinal cord is in the back. The discs are between the bones here. There are three most common causes of lower back pain.
The most common cause is the disc. You’ve heard of the 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 irritate the nerves and that sends pain, numbness, sometimes weakness in the leg. Pain that actually comes from a disc is when there is 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 are no nerve fibers in the middle of the disc – in the jelly of the disc – but the outer third crust of the disc; sometimes the outer two-thirds have these little nerve fibers. 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 have oozed out and now it’s irritating the outer third or outer two-thirds of the crust of the disc and that’s where discs are actually causing lower back pain. It’s true that often when somebody has a herniated disc, they may also have a tear inside of the disc and then you can have the back pain along 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 for younger people – people under the age of forty – that accounts for about forty percent of chronic lower back pain. It is also a fairly 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 lean forward because those are the positions that put more mechanical pressure on the discs, which puts more pressure on the jelly and 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, fingers – they’re synovial joints with the same basic parts as the 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. You can’t push on them to see particular movement. Basically, just like a knee or shoulder or hip that can become arthritic, 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 if you sit in general. You can imagine these are the joints right here. The facets standing puts a lot of load on it, 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 maybe thirty to forty percent of chronic lower back pain. In younger people, it’s less common for facet pain; more like fifteen percent or thereabouts.
The third most common cause of chronic lower back pain is the sacroiliac joint. Remember this is the lower back – here’s the front, back. Here’s the sacrum. This bone over here is the ilium. There is a joint line here that’s called the sacroiliac joint. That tends to be pain more in the lower portion, more in the buttocks. It accounts for ten to fifteen percent, some people say as many as twenty percent of chronic lower back pain.
Those three things account for the most common causes of chronic lower back pain. I hope this has been helpful.