How a normal spine works

This chapter describes the nuts and bolts of how the spine works. Parts of it are quite technical, particularly the mechanics of bending and the function of the various muscles, but I'm afraid there is no way around this. It is simply not possible to understand how things go wrong without first seeing how they should work. More to the point, the information lets you know what you are doing when it comes to fixing your back.

WHAT IS A SPINE?

Figure 1.1 The human spine is a slender segmented column made up of 24 segments sitting atop the narrow sacral base.

The human spine is an upright bendy column. It consists of 24 separate segments called vertebrae which sit atop each other in a vertical stack. There are seven in the neck (cervical), twelve in the middle back (thoracic) and five in the low back (lumbar). The base of the spine sits on the sacrum, which is a solid triangular block of bone at the back of the pelvis. The sacrum tilts down at the front to an angle of approximately 50 degrees below the horizontal, making a concavity in the low back as the spine arches to compensate.

The spine rises out of the pelvis in three gentle curves like a cobra from a basket. Its 'S' shape helps hold it upright, and by arching back and forth over a central line of gravity it balances the top-heavy torso over its narrow base. With perfect spinal alignment (posture) a straight line can be drawn through the ear, the tip of the shoulder, the spine at waist level, the front of the knee joint and the back of the ankle.

The hollow in the low back is called a lumbar lordosis. This is followed by a gentle hump the other way in the chest region, called the thoracic kyphosis, and another arch in the neck called the cervical lordosis. The lumbar lordosis lessens with sitting when the pelvis tips backwards on the sitting bones (the ischial tuberosities) and increases with standing.

Figure 1.2 A lumbar hollow (lordosis) is critical for spinal health; it allows the spine to sink and spring to absorb impact while walking and also stacks the spine in equilibrium during sitting.

Perfect lumbar alignment achieves two important ends: it ensures the correct distribution of bodyweight through the front and back compartment of each lumbar segment, and allows your lower back to bow forward slightly to absorb impact during walking. As you might imagine, the right amount of lumbar lordosis is an important factor in avoiding back pain.

The following discussion highlights the anatomy which allows the spine to move in its free-flowing way — guiding it and controlling it so it doesn't go too far.

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 cord and hooks 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 vertebra (L5) sits on the sacrum and the junction between the two is called the lumbo-sacral or L5-S1 joint. As the most compressed level in the spine it is the most problematic. A high percentage of back trouble is caused by dysfunction of the front or back compartment (sometimes both) at this level.

The back compartment is a ring of bone, which barely takes weight, extending backwards from the vertebral body. In standing it bears approximately 16 per cent of bodyweight, 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 per cent), which is tremendously destructive.

Each 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 spinal knobs you can see through the skin running down the back). All these bony bars serve as levers for the attachment of muscles which make the vertebrae move.

Figure 1.3 The vertebral bodies of the front compartment are designed to stack easily and bear weight, while the bony ring of the back compartment protects the cord and notches the segments together at the facet joints.

The 'cotton reels' superimposed on one another at the disc– vertebra 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.

All the muscles working the segments exert a downward pull as they bring about movement. 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 plus the muscular strings working the vertebrae — contributing to spinal compression. But there is also a third: sitting. Long periods of heavy sitting slumped in a 'C'-shaped posture loads up the discs and is particularly compressive; even more so as the facet joints at the back disengage and the belly lets go at the front. This is noteworthy, because I believe lumbar compression is the background cause of most low-back problems.

The vertebral bodies are prevented from grinding on one other by the intervertebral discs. These are high-pressure fibrous sacks containing an unsquashable sphere of fluid in the centre, called the nucleus. The 24 bony segments interspersed with discs makes the spine into a dancing resilient column, readily able to carry load and absorb extraneous forces from all directions.

The actual shape of the vertebral bodies 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 body towering above.

Each 'cotton reel' is made up of a layer of hard cortical bone on the outside and honeycomb bone (or 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 an ingenious way of dispersing forces through the bone.

Figure 1.4 The honeycomb cancellous bone (spongiosa) is really a three-dimensional internal scaffolding which stops the vertebral body crumbling under load. The blood reservoirs in the vertebrae also help absorb shock.

Apart from being a handy reservoir, the fluid inside helps absorb the...