Intervertebral discs. The all-important mighty disc. The center of spine dysfunction. The bringer of pain. The elusive mystical creature that lives in your back and makes pain travel down your leg. Discs have gained quite a reputation in society today. Everyone seems to “know” a little about discs and have a lot more to say about them. Most tend to view discs as quite fragile, easily injured, and even easily mobile structures in your spine. But how accurate are the common beliefs? As I continue to have conversations with patients AND health care providers it becomes more evident that an accurate perception of these anatomic structures in relation to anatomy, function, and pain isn’t all that clear.
Anatomy So what role does this disc play? What is the actual anatomy of the disc? How often is it really injured? Can it heal? Does it “slip”? These are all valid questions we should take some time to answer. As you can see from the following diagram the disc lies between your vertebrae.
Every segment in your spine has a disc except for the C1-C2 level and the level between the bottom of your skull and the top vertebrae (C0-C1). Each disc forms a fibrocartilaginous joint between two vertebrae to allow slight movement of the vertebrae, and acts as a ligament to hold the vertebrae together. This type of fibrocartilage joint can also be referred to as a symphysis. If you know anything about symphysis joints, you know that they are EXTREMELY STRONG. Other examples are the pubic symphysis which connects to two pubic bones in your anterior pelvis, as well as the joint between the sternum and manubrium in your chest. All of these are examples of very robust joints that can withstand tremendous amounts of force without fail. Discs also play an important role as shock absorbers in the spine. Their structure is uniquely designed to allow this to happen effortlessly.
Discs are composed of an outer fibrous ring referred to as the annulus fibrosus. The annulus fibrosus consists of several layers or rings of fibrocartilage made up of both type I and type II collagen. These layers surround an inner gel-like center material referred to as the nucleus pulposus. The nucleus pulposus contains loose fibers suspended in a muco-protein gel which helps to distribute pressure evenly across the disc and prevent excessive forces on the vertebral end plate. In the following pictures you can appreciate the approximate anatomy of both the disc and the vertebral end plate.
The interesting thing about the vertebral end plate is that there is both a bony and cartilaginous endplate that create an exceptionally strong connection to the annulus of the disc making it IMPOSSIBLE for the disc to SLIP. Yes, you heard that right. DISCS CANNOT JUST SLIP OUT OF PLACE willy-nilly like a bar of soap in the shower. The endplates function to hold discs in place, evenly spread applied loads, and to provide anchorage for the collagen fibers of the disc.
Discs don’t slip I can’t count the number is times I’ve heard the following quotes. “I’ve had back pain ever since I slipped a disc back when I was 29”. “Don’t bend over like that you’ll slip a disc”! “Don’t lift that, it’ll slip your disc”! “I can’t do that I’ve got a slipped disc”. The problems with these statements are that discs DO NOT and CAN NOT slip.
Can discs get injured? Sure. Can discs heal? Absolutely. But how strong are discs really? This study here examined the compressive and tensile strength of thoracic discs in both young and older populations. (28 +/- 8 and 70 +/- 7 years of age respectively). They found that it takes about 740lbs of force to compress the disc height 1mm in young subjects and 460lbs of force to compress the disc height 1mm in older subjects. Keep in mind this is in cadaveric discs with bony support cut away and no active co-contraction in any surrounding muscle tissue. Obviously, in a normal well-functioning human there is quite a bit of extra strength and stability added by active contractile structures. End story is that discs are VERY strong. However, we know that shearing forces are much more likely to injure discs, just as they are more likely to injure ligaments. This is why it is likely not a good idea to lift heavy items off of the ground with end range spinal flexion in a jerking twisting motion.
Additionally, a disc “injury” quite often happens in the absence of any pain and you don’t even know about it. Plenty of high quality research shows that the odds are you have a pain free and benign disc “injury” as you sit here and read this article.