The Spine - Basic Anatomy and Mechanics

The bones in the spine are called vertebrae, and in between them are the inter-vertebral discs. The human spine consists (usually!) of 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the chest which along with the rib cage and the sternum (breast bone) house the lungs, heart and protect a large amount of the abdominal contents as well, 5 lumbar vertebrae, a sacrum and a coccyx. These are the 5 segments of the spine – the cervical spine, the thoracic spine, the lumbar spine, the sacrum and the coccyx. In each segment of the spine apart from the sacrum and coccyx an inter-vertebral disc separates the vertebral bones. There are of course variations in this. If an intervertebral disc exists in the sacrum it is usually fully enclosed by bone and rarely if ever becomes painful.

Each inter-vertebral disc is associated with 2 facet joints behind it. A disc and its associated facet joints make up an individual motion segment. In between the disc and the facet joints lies the spinal cord or nerves and to the sides are bony struts called pedicles (see image below). Sprouting off from the spinal cord and shooting out from underneath the pedicles are the nerves, which in the lumbar spine mostly form the femoral, and sciatic nerves and in the cervical spine form the brachial plexus (the nerves which travel down the arms). In the thoracic spine the nerves supply sensation to bands of skin radiating around to the front of the chest wall and abdomen, and move the intercostal muscles to that we can inflate our lungs.

The vertebrae are labelled according to which segment in the spine they lie in and whereabouts in that segment they lie. Hence the first lumbar vertebra is called L1 and last cervical vertebra is called C7. Each disc is labelled according to which vertebrae it lies between – hence the disc between the last lumbar vertebra and the first sacral vertebra is called the L5-S1 disc.

In most people the distinction between each segment of the spine is very clear – in other words it is very clear where the lumbar finishes and the sacrum begins – but in some the distinction is less so and the segments kind of blend into each other. This leads to the very confusing scenario where people are told they have 6 lumbar vertebrae or a 'lumbarized S1'. This is just anatomical variation between individual human beings and whilst it certainly has an impact on spine surgery in terms of planning and getting things right it rarely means anything as far as the patient is concerned.

In basic terms, and as far as the patient is concerned, the spine has two main functions. The first function is a purely mechanical one – to bear load. The second function is a purely structural one – to protect the spinal cord and the associated nerves. We will deal with each in turn.

Bearing Load

The spine bears load which basically means it enables us to stand upright, absorb shocks or impacts from the ground, and generally perform basic movements like bending down to tie our shoelaces or lean forwards over a sink to brush our teeth in an efficient manner.

The way in which the spine bears load is highly specialised and if not for the load bearing properties of the spine life would be a very jarring and inefficient experience. Two major mechanisms are at play here:

The first involves looking at the whole spine in its entirety – i.e. all 5 segments. In the diagram labelled "The Spine" you are looking at the spine 'side-on'. In medical jargon this view is called the 'sagittal view'. The opposite of the sagittal view is the 'coronal view’, which is 'front-on'. In the sagittal view the spine is curved. Curves in the spine are described as lordosis and kyphosis. The cervical spine is naturally lordotic – it curves backwards, the thoracic spine is naturally kyphotic – it curves forwards, the lumbar spine is naturally lordotic and the sacrum and coccyx are kyphotic. This is normal and in this normal, natural state the spine is balanced with the base of the neck lying just over the centre or front of the sacrum. Inside this balanced spine are the natural curves of each segment. The curves are extremely important because they make the spine, as a whole, act like a big spring or shock absorber. You will recall Newton's third law of physics, which describes each action or force having an equal and opposite reaction? Every step you take and certainly every jump you make involves a ground reaction force being transmitted though the body as per Newton's third law, and this force is absorbed in a large part by the spine. The spine, being able to coil and recoil because of this natural, built in spring can absorb these shocks. A more extreme example would be examining the reason a parachutist turns his/her whole body into a spring on landing. If he/she did not do so many fractures would result. In the spine the same principle applies but in a less extreme manner. If the spine was one straight column, each and every step would jar and cause trauma. The curves in the spine also allow for very natural, efficient movement. For example if you carefully think about how your whole body subtly moves during the simple act of leaning slightly forward to read a book when you are sitting upright in a chair you will realise that it involves the whole of your spine from the base of your skull to your lumbar-sacral junction. The curves inherent in your spine make this a very small and efficient movement. Try to imagine how inefficient and difficult this simple movement would be if your spine was one straight column. Not only would this be an inefficient movement but maintaining this position with a straight spine would quickly and easily fatigue the muscles down the sides of your spine (the para-spinal muscles) and this is painful. Patients whose spines have straightened out as a result of degeneration or disease live like this on a day-to-day basis.

The second way in which the spine bears load involves looking at each individual motion segment. At leach level of the spine the inter-vertebral disc, by virtue of its composition and anatomy has the ability to absorb shock acting like a mini-shock absorber in its own right.

In the schematic, the disc is seen to have a nucleus (green) in the centre, which can be visualised as a tense cushion of water under quite a lot of hydrostatic pressure. It is surrounded by a very fibrous capsule called the annulus fibrosus (coloured purple) The annulus fibrosus is made up of many strong connective tissue bands called lamellae which are oriented at right-angles to each other throughout its radius to provide maximum support for the nucleus. The end-plate (yellow) attaches the disc to the vertebral body – it is made of cartilage and supplies nutrients to the inner 2/3 of the annulus and the nucleus. ALL and PLL (red) stand for the Anterior Longitudinal Ligament and the Posterior Longitudinal Ligament respectively. These are two very important and extremely strong ligaments. The ALL in fact is one of the strongest ligaments in the whole body and certainly is the strongest in the spine – its integrity is very important in maintaining spinal stability. The water content of the nucleus pulposus what gives the disc most of its properties. Once the water goes it can never come back and the disc loses its function. Unfortunately, this starts to happen in everyone around the age of 20 as part of the normal aging process. Once the hydrostatic pressure is lost the disc essentially starts to deflate and this can be seen on an MRI scan.

Most people visualise the disc's ability to absorb shock by the compression and recoil of the nucleus. In fact, in a healthy motion segment, the nucleus is so strong and the hydrostatic pressure inside it is so high that in the process of absorbing load, the nucleus actually indents the end-plate and bulges into the soft(-ish) honeycomb-like bone in the middle of the vertebral body rather than compress itself. This honeycomb-like bone is called cancellous bone unlike the stronger cortical bone which surrounds the periphery of the vertebral body. It is the recoil of this cancellous bone which returns the motion segment to normal when the load is removed. Normal movement in the spine i.e. flexion and extension in both the sagittal and coronal plane occurs over this nucleus – the vertebral bodies rock backwards, forwards, side-to-side etc. over a normal nucleus like a primitive ball and socket joint. In fact, the movement is very far from primitive – it is a very well-developed motion that involves not just flexion and extension, but also complex translation with very efficient movement of the centre of rotation to allow for interactive and efficient movement.

Protecting the Spinal Cord

In the lumbar spine the spinal cord – i.e. the neural component – has in fact already finished. It ends at the level of about T12/L1. What continues, in terms of neural tissue, behind each motion segment in the lumbar spine is the cauda equina – a collection of nerves. For the purposes of this description we will just talk about the 'spinal cord' referring to both just because it is easier for descriptive means.

So, behind each vertebral body is the spinal cord, and surrounding the cord are the pedicles, facet joints and laminae, which form a very tight and strong protective housing for this very important structure. The spinal cord / cauda equina travels in the vertebral canal. The spinal nerves sprout from the cord and exit the spine to travel into the arms and legs through the neuroforamen (see diagram labelled 'Two Columns of the Spine'). Each vertebral body (not disc) has a spinal nerve associated with it so that the nerve labelled L1exits the spine at the neuroforamen under the L1 pedicle.

These then are the two main functions of the spine – bearing load and protecting the spinal cord. The spine has two columns and not surprisingly each column sub-serves one function.

To the right is a red line roughly dividing the spine into the Anterior Column (anterior simply means at the front) and the Posterior Column (posterior simply means at the back).

About 80% – 85% of the weight of the body (in the lumbar spine) and the head (in the cervical spine) is borne through the anterior column. This is the column primarily concerned with bearing load, shock absorption, movement etc. The posterior column bears little load but serves the protective function of the spine. Understanding this division of function is of prime importance because it leads to the realisation that in order for surgery of the spine to restore function, the anterior column needs to be addressed. Surgery that doesn't address the anterior column has a much higher failure rate overall.

So, the spine has two functions and two columns. However, it is the Inter-Vertebral Disc that allows both columns and therefore both functions to work – this is the 'take-home' message of this page. Without normal, healthy discs, both functions are lost. Why is this? Well firstly the discs are responsible for the natural curves of the spine. They are not uniform rectangular fillers as most people visualise them. They are trapezoidal in shape and in the lumbar spine taller at the front than at the back (Figure 6 is not a good representation of this). The vertebral bodies are by comparison, rectangular in the sagittal view. If you place these discs – trapezoidal wedges – between the rectangular, vertebral bodies you get the natural, healthy curves of the spine. Therefore it is the discs which give the spine its overall 'spring' mentioned above – when a disc in a lordotic segment of the spine deflates (so we're talking here about the neck and lumbar regions) the forces dictate that it loses its trapezoidal shape and actually becomes more rectangular in shape resulting in the spine flattening out – giving rise to the so-called flat-back deformity.

As mentioned above, it is the individual discs which act as mini-shock absorbers at each motion segment via the structure and integrity of the nucleus.

The protective function of the posterior column is dependent on the intervertebral disc keeping the vertebral bodies a normal, healthy distance apart from each other. The nucleus, again by virtue of the height it gives to the disc, keeps the vertebral bodies a certain distance apart, which keeps the neuroforamen (see figure "Two Columns of the Spine") open and wide so that the nerves can escape unimpeded. Once the neuroforamen closes up as a result of disc deflation the nerves get pinched in a scissoring type of mechanism. Similarly, when a disc deflates the surrounding ALL and PLL lose their tension and buckle. Not only does this buckling cause infolding of ligaments which bulge into the vertebral canal where the cord is, but this also gives rise to segmental instability at the affected level – instead of normal physiological flexion and extension around the nucleus, the vertebral bodies are free to wobble and shake over each other – this is called instability and is discussed further in the Causes of Back and Neck Pain page. The other structures that suffer when a disc loses its height and collapses down are the facet joints which sublux (a mini version of, or precursor to, dislocation if you like) and can become arthritic and painful.

Both the lumbar and cervical spine are lordotic in their sagittal plane, bear load in a similar fashion and thus degenerate in the same predictable way. The lumbar spine has most of its lordosis from L4/5 to L5/S1 and these are the discs which degenerate and cause problems most frequently. In the cervical spine most of the lordosis arises from C4/5, C5/6 and C6/7 and not surprisingly it is these three discs which degenerate and cause problems most frequently. We now know that it is imperative to restore lordosis to these segments of the spine when reconstructive surgery is undertaken to maintain spinal balance, and reverse the pathological effects of losing this important attribute.