Structure of the lumbar discs
The lumbar discs are interesting and complex structures which can shed light on the function of the spine and what happens when we get lower back pain.
Their main function is to transfer the compression forces from one vertebra to another as loads are put on the back by the activities we perform.
To do this they have three main components with different jobs to perform:
The annulus is the tough outer covering of the disc, built to resist the forces put on the spine. It acts with the nucleus to construct a structure which is stiff enough to transmit forces and pliable enough to allow movement. That’s a neat trick.
If you look at the annulus in cross-section, it looks like the rings of an onion. There are 10 to 20 sheets of collagen making up the annulus, and these sheets are called lamellae, packed together to form a fibrous capsule. This capsule can deform in shape to allow movement and accommodate to pressure, but also with stiffness to resist stresses. However, it does need to be kept stretched to work properly, rather like a balloon needs air in it to keep its shape. To do this it needs the nucleus.
Each layer of the annulus has its collagen fibres arranged in a certain direction, running obliquely from one vertebral body to another. Each succeeding layer has its fibres oriented in the opposite direction, and this criss-crossing structure gives the disc strength to avoid splitting. Whichever movement is put upon the disc, there will be some fibres which are in the right direction to resist the movement and maintain the safe alignment of the spine.
The nucleus is a water-rich gel in the centre of the disc. When the pressure of the bodyweight is put on it, it expands sideways like a bag of water would if you pressed down on it. The expansion is held back by the walls of the annulus, and the nucleus braces the annulus from the inside so it cannot buckle. In this manner the disc is stiff enough to cope with the pressures put on it, yet be pliable enough to allow movements between the vertebrae.
The material within the nucleus is made up of complex proteins and sugars, chemicals which can attract and hold quantities of water. This allows the nucleus to behave as it does, distributing the pressures evenly throughout the disc.
If the internal disc chemistry alters and loses its ability to hold water well, the nucleus is no longer able to brace and support the annulus. The disc then loses its stiffness and ability to resist the loads put upon it. As time goes on, such an altered disc will work less well and narrow with the forces put upon it.
The endplates are areas of cartilage which cover the upper and lower surfaces of the vertebrae at the top and bottom of the disc. It covers the whole of the area except for a ring of bone around the outside, and firmly attach the discs to the bone. The bone on the vertebral side of the endplates is often called the subchondral (”under the cartilage”) bone.
In the lumbar spine the discs are of an average height of 10 mm each. During the day, as we perform activities with the weight on our spines, the discs lose a certain amount of water as the pressure squeezes it out. Lying down will allow water to be reabsorbed into the disc and the height to be restored.
You may notice you are taller in the mornings! I notice I have to adjust the car mirror at different times of day, down in evenings and up in the mornings. I don’t think I sit any different!
Many of us think that disc heights decline with age, one of the reasons why we may get shorter as we get older. In fact disc do not narrow naturally with age, discs narrow due to a disruption of the discs capacity to function, in other words something has to go wrong.