The Anatomy of the Lower Spine

Our spine performs several functions which to some extent seem quite contradictory.

The spine supports us and keeps us upright, and must transmit large forces through our body, for example when we run or jump. In consequence the spine has to be strong and relatively rigid. But the spine is also a mobile structure. We need to be able to bend, lean and twist. These movements must be controlled to protect the delicate spinal cord, which runs within the spinal column in the spinal canal, and the nerves which emerge at each level. Meeting all these different requirements demands some complex structures and joints.

The building blocks of the spine are the individual bones called vertebrae. There are five in the lower back or lumbar spine, numbered from L1 at the top to L5 at the bottom. These are located above the pelvis. The central part of the pelvis, called the sacrum, is in fact part of the spine, formed from several vertebrae which have become joined together, or fused, into one solid bone. Below the sacrum are a few rudimentary vertebrae (the evolutionary hangover of our tail) which form the coccyx.

The main bulk of each vertebra is the cylindrical vertebral body. The intervertebral discs are found between the vertebral bodies. These discs act as a firm cushion or shock absorber but also allow some movement between the vertebrae. Contrary to popular myth, the discs cannot slip in and out of position since they are very firmly bonded to the vertebral bodies. The discs are named according to the vertebrae they lie between, so the disc between the 4th and 5th lumbar vertebrae is called the L4/5 disc, while that between the 5th vertebra and the sacrum is called the L5/S1 disc.

The normal, healthy disc has a thick and very tough outer ring, called the annulus, which is made up of many layers of strong collagen fibres. Contained within this is the softer centre, or nucleus, made from a gelatinous material which strongly soaks up water, keeping the disc pumped up and firm. The annulus can be thought of as the tough outer wall of a tyre, while the nucleus is like the inner tube, keeping the disc firmly inflated yet moderately flexible.

Pairs of small joints towards the back of the spinal column at each level help to control and limit movement within the spine. These are called facet joints and are similar in basic structure to most other joints in the body, with smooth articular cartilage allowing movement.

Movement of the spine is achieved and controlled by muscles which surround the spine and are attached to the vertebrae through various processes or levers. The most prominent of these are the spinous processes, which point backwards. The tips of these are the bony prominences you can feel in the middle of your back. There are also similar processes pointing to each side which are called transverse processes. Excessive movement is restricted by various ligaments which run between adjacent vertebrae. One of these, the ligamentum flavum, runs within the spinal canal and can cause problems if it thickens or buckles since it can then affect the nerve structures within the spinal canal.

The spinal cord, which runs in the spinal canal down much of the length of the spine (normally ending at the first lumbar vertebra), gives off a pair of spinal nerves for each level of the spine. These nerves emerge from the spinal canal, one on each side at each level, through an opening between adjacent vertebrae called a neural foramen. The nerves from the lumbar spine pass into the legs, each one sending impulses to particular leg muscles and transmitting sensation from specific areas of the legs.

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