Oh My Aching Body - 6 - Spinal Discs

This month's article focuses on spinal discs: the fantastic function they provide every day without our acknowledgement, and the awful things that happen when something goes wrong with them. 

Article #4 of Oh My Aching Body is called "Back Pain." It talks about how difficult it is to discern the exact cause of back pain, because there are so many bones, ligaments, and muscles so close together. 

What that article didn't discuss at all was the vertebrae, the intervertebral discs, and the nerves. This article is a brief effort of explaining that. 

We separate the spine into five distinct parts. 

• There are 7 bones in the neck, which flex and rotate to allow impressive range of motion. We call this the cervical spine or c-spine. (Fun fact. Giraffes also have 7 vertebrae in their necks. But each giraffe neck vertebra is almost a foot tall, while the entire human neck averages is between 4.5" and 5" long.)
• There are 12 bones in the mid-back, which connect to the ribs. They don't do a lot of flexing or twisting, but they support the cage that protects our vital organs. We call this the thoracic spine.
• There are 5 bones in the low back. These bones are also capable of twisting and bending. We call this the lumbar spine.
• There are 5 bones fused into a single mass that connects the two sides of the pelvis to the spine. We call this the sacrum.
• Finally, there are the tiny bones referred to as "the tailbone." This "bone" is actually another 3-5 small bones that are fused together at the base of the spine. We call this the coccyx. (It's pronounced cock-six. Really.) The tailbone doesn't wag, so as tails go it's unimpressive. But the lack of wagging makes it possible to play poker without revealing your hand. 

I'm pausing here to draw your attention to the unique shapes of the vertebrae that allow for the incredible mobility of the neck and back, while simultaneously protecting the critical and vulnerable spinal cord. Every vertebra protects the spinal cord that runs through a channel behind the vertebral bodies. 

So despite the fact that it's incredibly flexible, allowing twisting and bending in multiple directions, our spine also protects the spinal cord when we do really stupid things.

The vertebrae are stacked on top of one another from the base of the skull to the tailbone. Between the vertebrae are discs that keep the bones from eroding each other. If the bones are compared to bricks in a wall, the discs are the mortar. When it comes to shape and structure, our discs can be compared to a jelly donut. The outer (donut) part is tough and rubbery, surrounding a soft, gelatin-like filling.

In the picture above you see the "jelly donut" disc, and right above it is the spinal cord, with spinal nerves emerging from it. Simply put, spinal nerves conduct sensations from the body up to the brain, and motor commands from the brain down to the body. 

But sometimes our discs aren't perfect, and those nerves become compromised.

Disc Herniation

If a disc is compressed repeatedly or forcefully, the jelly might squish out of the donut. Below is an image of a herniated disc. This is sometimes referred to as a "slipped disc" or "ruptured disc" or "protruding disc". It's all the same thing. The top view in the image below is looking down on the disc from above. The lower image is looking at the right side of the spine.

With a disc herniation, the jelly squishes out of the donut and pushes against a nerve root, so everything that nerve root innervates is getting mixed signals. With a compressed spinal nerve in your neck, you might feel zinging, tingling, or heat in your entire arm. The muscles might get weaker, because you don't have the ability to maximally contract the muscles that are powered by that nerve. When there's pain or numbness in a muscle, we tend to favor it and avoid using it. We might bend to one side or the other in an attempt to relieve the pressure, thus altering our postural alignment. Every time you bend your spine, it sends jelly out onto the nerve root, which sends a zing to whichever parts that nerve innervates. This results in stiffness, muscle guarding, and limited range of motion.

To relieve pressure on the nerve, we need to get the jelly off of it. 

In the donuts above, the jelly is being contained nicely within the donuts. Imagine slowly compressing one of those donuts between your thumb and fingers. The jelly starts to ooze slowly out of the jelly hole. There's a point at which you can release your compression, pressing on the sides instead of top and bottom. With this counter-pressure, the jelly gets sucked back into the donut. You can do this at home with a toothpaste tube. Slight pressure sends the toothpaste slightly out of the tube, but pressure on the other sides of the tube sucks it back in.

This is why we use traction in the physical therapy clinic. Creating space between the vertebrae might allow the nucleus of the disc to return to where it belongs. For the neck we place a special strap around the chin and base of the skull, then attach the other end to a traction unit that pulls on the skull for about 20 minutes. As the muscles of the neck relax, the traction unit separates the vertebrae, and the patient experiences relief from the disc's pressure on the nerve root. The amount of weight on the traction unit is in the 20-50 pound range.

We do the same thing for the low back, but the muscles are so much thicker and stronger in the low back that we use 20% to 50% of the patient's body weight in pulling strength. For a 200 pound patient we'd use up to 100 pounds of traction, so to keep the patient from being pulled off the table we have to anchor the rib cage to the top of the treatment table. 

Traction works for a lot of patients. Many will feel gradual relief over a period of several visits. A handful will feel immediate relief. 

But for those who have squished out too much jelly, they won't experience any relief at all. 

When your disc looks like the donut above, no amount of counter-pressure will suck the jelly back in. At this point you'll need a surgeon to remove the jelly that's pressing on the nerve. This is called a discectomy. Sometimes this procedure is done with an incision and tissue spreaders, and other times with a much smaller incision and a scope.

When the spongy center of the jelly donut is removed, that gel-like nucleus doesn't grow back. Instead, it is replaced with scar tissue that is less voluminous and less yielding, which results in less mobility of the spine at that spinal level, and less height between the vertebrae. This leads to a higher risk of compression of the nerves emerging between these vertebrae, and quicker degeneration of the disc. Over time, more problems emerge. You already know this: as we age we get more problems with our aching bodies.

Degenerative Disc Disease

Almost everyone over the age of 40 has some degeneration in their discs. This is the main cause of people becoming shorter as they age. There are 23 discs in the spine, so if each one only compresses 1/8" over a lifetime, it will translate to a total of 3" reduction in height. In fact, if you measure yourself in the morning, you'll likely be taller than you are at night. This is due to the compression of the discs throughout the day.

When a disc becomes too damaged to simply clean up the squished out jelly part, it has to be removed entirely. But the bones can't be left to just rub against each other. Options include fusing the spinal vertebrae together with bone or with metal (or both), or replacing the damaged disc.

The most common procedure is the most limiting. To fuse the vertebrae together, the surgeon first places a spacer between the vertebrae. This helps to maintain the space needed to keep from compressing the nerves emerging from between the vertebrae. With that spacer in place, the surgeon removes some bone from the hip (the part where moms rest their hands while scolding) and fuses it to the space between the vertebrae. 

An alternative to spinal fusion with a bone graft is spinal fusion with metal hardware. 

The vertebrae now have no motion at those levels. They are essentially a single, tall vertebra. If you've ever seen a giraffe try to drink water, you know why it's not ideal to have tall vertebrae. Giraffes don't have very much range of motion in those long necks.

 

Additionally, if you don't have any motion at one level of the spine, that need for motion gets transferred to the other levels of the spine, putting more of a demand on the other vertebrae, which speeds the deterioration at those levels. 

An option that doesn't involve fusing the vertebrae is replacement of the deteriorated disc. The disc might come from a cadaver, or it might be a synthetic disc. Replacement with a synthetic is more common than the use of a donor disc, and it carries fewer problems.

With this procedure they carefully measure the angle, circumference, and height of the space between the two vertebrae, then implant a metal plate into the bones above and below that space, then insert a rubbery prosthetic disc into the space between the metal plates.

Following any type of surgery on the spine, there are significant recovery steps involved, with some period of rest followed by physical therapy. But once recovered, people who have experienced disc replacement or spinal fusion are usually much better off in function and quality of life than before the surgery. Having your nerve roots pressed is a terribly uncomfortable thing, so measures as drastic as fusing the spine or inserting synthetic discs are really complicated, and yet preferable to the alternative.

To summarize, spines are super complicated. If you're having chronic back pain, it might be addressed in one of the two posts I've dedicated to back pain so far, or it might be caused by one of the myriad other conditions I haven't even mentioned. (Spondylolisthesis, spondylolisis, arthritis, ankylosing spondylitis, foraminal stenosis, and other difficult to pronounce conditions are plentiful.) If you're having pain in your back, and especially if it's causing pain or function loss in your extremities, please go see an orthopedic specialist and find out what's wrong. There's a good chance they can figure things out and get you on the road to recovery. Don't expect a quick fix, but also don't resign yourself to a life of limitation or pain.

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Is there another topic you'd like to see addressed here? Let me know!