Anatomy Series: Back to Bascis – The Lower Back

I deal with ALOT of back pain patients, and I think that a huge part of treating back pain is giving the patient the ability to understand WHY they are in pain. I won’t lie the anatomy of the spine is complex and can be difficult to explain in a way that people with no anatomical knowledge will understand.

So I have decided to do a couple of anatomy blogs to try and give readers a basic understanding of the body, how it works like it does, and why it often breaks down and you end up in a consultation with me!

The spine itself is made up of 33 vertebrae that is classed into 5 regions.spine 1

  • The cervical region (or your neck) has 7 vertebrae
  • The thoracic region (mid back) has 12 vertebrae
  • The lumbar region (low back) has 5 vertebrae
  • The Sacrum (joints the spine to the pelvis) has 5 vertebrae which are all fused as one
  • The coccyx has 4 small vertebrae all fused together

Often you will hear people say humans have 24 vertebrae which is correct if you count the fused sacrum and coccyx as one vertebrae each.

This anatomy series will focus on the lumbar spine or lower back. One of the most commonly injured and treated problems in our physio practise.

What is your low back?

As mentioned above the lumbar spine is formed by 5 vertebrae. The lowest (known as L5) articulates with the sacrum which then feeds down into the tail like coccyx portion. The lumbar spine itself is the chunkiest part of the spine as it is required to support the rest of the spine which towers above it.

When you look at yourself from side on you should notice that the lower back has a subtle inwards curve. This is known as the lumbar lordosis, maintenance of this curvature is essential to optimal spinal health. Too little or too much curve results in excessive weight being borne through different parts of the spine inevitably resulting in back pain.

To understand back pain one must understand the basic anatomical makeup of the spine. Most disorders of the lower back originate from the ‘disc’ or the facet joint. Apologies if this gets a little technical but I have attempted to break it down to the bare basics.

The vertebrae are the ‘building blocks’ of the spine. The front compartment is made up of the vertebral body which is circular in shape and designed to bear weight. It isn’t however a solid mass of bone, formed instead by a honeycomb bone structure called trabeculae. Literally picture the lattice formation of honeycomb. This sponge like arrangement contributes to its ability to absorb shock and transmit compressive forces that are placed through the spine. It also creates a reservoir to store blood and nutrients which we will soon learn is vital to the health of the intervertebral disc.

On the top and bottom of each vertebral body are the ‘end plates’ on to which the intervertebral discs attach.

spine 3The intervertebral discs sit between each adjacent vertebra and are the shock absorbers of the spine, like small water sacs to cushion weight bearing forces.  The anatomy of the disc is important. The outer most layer is known as the ‘annulus’ and forms a hard outer casing for the disc which is like fibrous mesh running on all different angles. This makeup provides maximum strength, yet also stretches to allow us the ability to twist and bend during movement, we would otherwise be like a rigid pole. The annulus also functions to hold the jelly like ‘nucleus’ (centre of the disc) within its boundaries.The outer annulus is the only portion of the disc with a nerve supply, thus is the only portion that can produce pain.

As mentioned earlier discs are water filled cushions, and unsurprisingly the health of a disc is heavily reliant on its hydration. The nucleus gets its nutrients and hydration via an osmotic exchange process between the fluid stored in vertebral body. Each day with general day to day activities our lumbar discs lose 20 per cent of their fluid, interestingly the first 10 percent is lost within the first 2 hours of sitting. At night (when there is no weight borne through your spine they undergo a re-hydration process). This often explains early morning stiffness as when you wake your discs are fully hydrated and bursting with fluid which gradually deflates as your daily activities take place.

In order to maintain disc health and hydration we need to keep our spine mobile. Having full range of spinal activity (ability to bend in all directions) maintains the elasticity of the disc wall and also acts as a suction mechanism to suck small amounts of fluid into the disc across the end plate of the vertebral body.

The back compartment of the vertebra is a formed by a ring of bone with three small projections, one out either side which are known as the transverse processes and a central projection called the spinous process (if you run your finger down the centre of your spine these are the bony points you can feel). These all serve as attachment sites for muscles and ligaments responsible to the stability and mobility of the spinal column. This ring of bone also creates the vertebral foramen which is a canal encasing the spinal cord which originates at the base of the brain. At each spinal level nerve roots exit the spine through the intervetebral foramen.

spine 4

In the back (posterior) compartment are the facet joints. These are the junctions where each vertebrae stack ontop of one another. These joints do not bear much weight unless the intervertebral disc is thin or the lumbar lordosis is excessive. However over time they do experience general wear and tear as they continually control the movement of the spine. These joints permit bending forward of the spine however limit other movements particularly twisting as this can be harmful to our discs.

The facet joints are pretty much wired to cause pain. They have an exceptional blood and nerve supply (very unlike the intervertebral disc) and for this reason an inflamed facet joint is quite often the cause of back pain.

Putting it all together

So I have touched on the basic anatomical makeup of the spine. (This was exceptionally hard, the design of the spine really is amazingly complicated). In the next series I will touch on the spinal and trunk muscles which are obviously also very important.

If you finish reading this blog understanding one thing though I would like it to be this. The health of our intervetebral disc is probably the single most important factor that drives back pain. In many cases thinning and loss of water from the disc through factors such a:

  • Repetitive compression forces
  • Sustained poor posture (sitting at a desk is a terrible position for the spine and places great stress on a disc)
  • Age (yes they do undergo natural degeneration)
  • Acute injury or trauma
  • Weak ‘core’ or tummy muscles

This places more stress on the facet joints, which we all know now are not designed to bear weight. This increased stress can irritate the joint, causing it to become inflamed and ‘stiff’. A stiff facet is like a rusty link in a bike chain, restricting spinal movement and causing surrounding muscles to go into a protective like spasm. The result of all this = Back pain.

Please remember this is the mere basics of back pain! There is still much to learn in the next post where we will begin to discuss the role of the spinal muscles and the theory behind a ‘slipped disc’.

If you suffer from back pain or are just keen to do some extra reading I would highly recommend

Sarah Key’s – Back Sufferers’ Bible

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