THE LUMBAR VERTEBRAE
The vertebrae are the individual building blocks of the spine. Each has a front and back compartment. The front compartment consists of the circular vertebral body, shaped like a cotton reel, which is specifically designed to stack easily and bear weight.
The back compartment protects the spinal segments together so they stay in place.Five lumbar vertebrae make up the low back. At the base of the spine, the bottom vertebrae (L5) sits on the sacrum and the junction between the two is called the lumbosacral or L5-S1 joint. As the most compressed level in the spine, it is the most problematic. Nearly all back pain comes from dysfunction of the front or back compartment (sometimes both) at this level.The back compartment is a ring of bone extending backwards from the vertebral body which barely takes the weight. In standing it bears takes the weight.
In standing it bears approximately 16 percent of body weight, but less if the spine is more humped in the sitting position where the facets are less engaged. With severe disc narrowing, the primary form of breakdown of the spine-the facets may be forced to take much more weight (up to 70 percent of the weight through the spine), which is tremendously destructive.
Each ring of the back compartment has small projections of bone sprigging from the outside corners: two wings out either side, called the transverse processes and a fin projecting out the back called the spinous process (these are the knobs of the spine running down the back you can see through the skin). All these bony bars serve as levers for the attachment of muscle which make the vertebrae move.All the muscles working the segments exert a downward pull as they make them move.
This is noteworthy because lumbar compression is the main cause of low back pain. If you bear in mind how much time we spend upright, fighting the weighing-down effects of gravity, you can see there are two factors at the start: our weight and the muscular strings moving the vertebrae-contributing to compression caused by sitting.The cotton reels superimposed on one another at the disc-vertebrae union make up the beautifully bendable neurocentral core, and the junctions between are often called the interbody joints.
The bony inter-notching either side at the back makes a chain of mobile juicy apophyseal or facet joints, running down the entire length of the spine. Together, the two different types of joints of the front and back compartment make up the total ‘motion segment’ at each spinal level.
The vertebrae are prevented from grinding on one other by the intervertebral discs. These are high-pressure fibrous sacks containing a central unsquashable sphere of mucoid fluid called a nucleus. The long column of bones with its high-tensile fibro-elastic pillows makes the neurocentral core thrusting and resilient, able to take nearly all of the weight passing down through the body.
The actual shape of the bodies of the vertebral helps spread the load. They have a narrow waist which flares out to a broad weight-bearing upper and lower surface. Unlike the other lumbar vertebrae, L5 is thinner at the back which helps to form the lumbar lordosis. Its disc is also slightly wedge-shaped although it is still the fattest disc in the spine, helping it to bear the load of the rest of the spine on top.
Each ‘cotton reel’ is made up of a layer of hard cortical bone on the outside and honeycomb bone (called cancellous bone) on the inside.This is sometimes called the ‘spongiosa’ because it resembles a sponge and stores a rich supply of blood. The presence of the blood inside the bones is ingenious when it comes to dispersing forces through the bone.Apart from being a handy reservoir, the fluid inside helps absorb the impact of shock passing through the vertebrae. These box-like bodies, literally bursting with blood, transmit the forces of compression in all directions throughout the fluid, thereby dissipating the direct downward pressure. As well as reducing strain, this functions as a useful engine for shunting nutrients into the disc, which does not have its own blood supply.
The line of demarcation between the vertebrae and the upper and lower surfaces of the disc is called the vertebral end-plate. It is thin cartilaginous interface about 1mm thick and although each one is cushioned by the disc in between, it is still the weakest part of the spine. With the rigorous impact, each end-plate can seem like a semi-destructible membrane caught between two thundering fluid-transmitted systems: the vertebral body on one side and the disc on the other. Sometimes, impact through the spine can blow a tiny vent in an end-plate, like blowing a hole through hiding stretched over a drum.The honeycomb bone inside the vertebrae is actually a gridwork of tiny struts and spars, like internal scaffolding. Its three-dimensional structure prevents the roof of the vertebrae caving in and the walls collapsing inwards like a cardboard box being flattened.
It is a brilliant way of making the bones strong yet light. If the vertebrae were solid it would be much harder for our spines to operate. Not only would the bone tend to cleave off in chunks when subjected to compression and torsional strains but we would hardly be able to move our own weight.
When the vertebrae are superimposed on one another, the consecutive bony rings at the back make a hollow tube inside the spine called the spinal canal. The canal houses the fragile spinal cord of the central nervous system which hangs down from the base of the brain like a long plait of hair.
Filaments of nervous tissue branch off either side all the way down and become the spinal nerve roots. The cord itself actually ends at the level of the second lumbar vertebrae. The roots then continue on inside the spine, hanging down like strands of a horse’s tail (hence the name cauda equina) until they make their exit either side through their designated inter-segmental level.
Whereas the role of the front compartment is fairly straight-forward as a weight-bearing strut, the workings of the back compartment are more complex. Apart from acting as the casing to protect the spinal cord, it has two other important functions: to guide the movement of the vertebrae-favouring some and keeping other more troublesome ones to a minimum-and helping to lock the vertebrae together to stop them slipping off one another.