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The Lumbar Spine Masterclass, Part 2

In the last blog post we talked about the boney structure of the lumbar spine and how it is different from the rest of the spine.

Let’s move on to the next important piece of that area of the spine. The lumbar disc…

Lumbar discs sit between the vertebrae and are named for the vertebrae above. For example, the disc sitting between the L1-L2 vertebrae is considered the L1 disc. The discs are the largest in the body as they absorb and transfer a lot of forces from one vertebrae to the next. Lumbar discs actually grow throughout your life! Yes that is correct,it is normal for them to grow. The discs are usually 1-2 mm taller at age 60, than they are at age 20. Most people are very surprised to learn that, it has been a common thought pattern that lumbar disc shrink and degenerate but that is not the reality of the situation. The disc seems to be the perfect object to sit between two vertebrae. Not only does it permit 360° of movement but it also helps to absorb and transmit loads from one vertebrae to another. The lumbar disc is made up of two components: the annulus fibrosus and the nucleus pulposus.

The annulus fibrosus, or the annulus, is the outer component of the disc. The annulus is composed of 60-70% water. The annulus slowly dehydrates throughout our lifetime. Type 1 collagen primarily makes up the annulus. That type of collagen is strongest under tension, being pulled. The annulus is made out of 10-20 concentric rings of collagen. The collagen fibres, in the rings, are orientated about 65° from the vertical. Each ring is orientated in the opposite direction. For example, the first ring is orientated to the right, the next is to the left, the next is to the right… The orientation of the rings, that 65° number, is nearly the perfect orientation to give maximum support to protect against rotation. There is only one problem, only half the disc resists rotation at one time. If I rotate to the left, half the disc, the rings orientated to the left, take up the tension, become taut and limit my rotation. However, the other half of the ring, orientated to the right, become slack. The rings or laminae of the disc, are thickest in the front and sides of the disc. The laminae are the thinnest in the posterior and posterolateral corners, this is where disc herniations occur most commonly. In addition, upto 50% of the disc rings in the posterior lateral corners are incomplete. That area is the weak link in the chain for tearing the annulus fibres. The outer rings of the annulus are considered a ligament that helps to limit movement in the lumbar spine. The outer laminae have nerve supply and a blood supply. This means that if you tear the outer annulus, it will hurt. However, the tear can heal because of the blood supply! People that think that a disc tear/bulge last for ever, are incorrect. Thank goodness!

The nucleus pulposus, aka nucleus, is the center area of the disc. The nucleus is made up of Type 2 collagen. That type of collagen differs from Type 1 collagen. Type 2 collagen has a higher compressive strength. Just like the annulus, the nucleus is made up of water, 90% when we are younger and dehydrates to about 70% later in life. The nucleus does not have any nerve innervation and it has no blood supply. So how does it survive? Everything needs blood and oxygen to survive in the body. The secret to allowing the nucleus to survive, we think, is the vertebral end plate. The vertebral end plate is the top and bottom of the vertebral body. It is made out of cartilage, hyaline and fibrocartilage, in young people. When you get to my age, it is mostly fibrocartilage. The functions of the vertebral end plate are to attach the disc to the vertebrae, it is a zone of growth and diffusion of nutrition to the disc/ nucleus. Being a growth zone, the end plate would have a lot of blood supply. The oxygen would diffuse across from the end plate to the disc/nucleus. Compression and transmitting loads would create a pump like system to push fluid across the cartilage and the unloading of the end plate would create a sponge like effect to pull the waste away from the center of the disc. It is so neat how it all works.

Let’s have a brief example of how the discs and the end-plates work together to transmit loads. When there is weight put on the spine, the pressure increases in the nucleus which pushes it outwards, in all directions into the annulus. The annulus then tightens up and this force is exerted back onto the nucleus to stop it from expanding. The pressure is then exerted onto the vertebral end plates. The end plates then transmit the load through the vertebrae and onto the next vertebrae. You read it and it seems to take 10-15 seconds. In reality, it occurs in a fraction of a second. This is not without risk though. If the load comes on too fast or too much load is bore on the vertebrae, the vertebral end plate may fracture. The vertebral end plate is the weakest part in the transmission of forces. If there is a vertebral end plate fracture, the disc might migrate into the body of the lumbar vertebrae. This creates what is called a Schmorl’s node. These nodes can be seen on x-rays or other diagnostic imaging. There is a thought that if caught soon enough, with diagnostic imaging, that degenerative discs in the lumbar spine all start with end plate fractures and Schmorl’s nodes can be seen. Very interesting, indeed!

So we know that there are two vertebrae and a disc hold the vertebrae together. What else helps to support the spinal column? Ligaments, of course.

If you have any questions or comments about the Lumbar spine, please click here to contact New Leaf Physiotherapy, a mobile Kelowna physio clinic.

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