The vertebrae of the spine are held apart by a specialised structure called an ‘intervertebral disc’. This disc is made up of two parts; an inner ‘nucleus pulposis’ which is a jelly-like structure, and an outer a ring of fibrous tissue called the ‘annulus fibrosis’ and which looks somewhat like the tyre of a car.
The function of the intervertebral disc is to allow a certain amount of movement between the adjacent vertebrae, which gives the spine its mobility. To do this effectively the disc is normally pressurised, to keep the vertebral bodies apart. The structure of the nucleus pulposis is such as to bind water to the molecules within it and it is the pressure of this water which holds the vertebral bodies apart. For a number of reasons the molecules within the nucleus pulposis can break down. If this occurs the nucleus pulposis loses its water with a consequent loss of pressure and the disc collapses to a greater or lesser extent. In this event the disc is said to be ‘degenerate’.
As a result of degeneration the nucleus pulposis it penetrate (or prolapse) through the surrounding annulus fibrosis and bulge out into the spinal canal adjacent to the disc. Because this prolapse commonly occurs in the postero-lateral aspect of the disc the discharged disc material frequently comes to lie against the nerve root which is leaving the spinal canal at this site. This event is often sudden and, if it occurs, can give rise to an excruciating pain in the lower back. In addition the pressure directly on the nerve root irritates it, causing a pain down the leg called ‘sciatica’. Along with the sciatic pain, the affected individual may also experience ‘pins and needles’ down the leg as well as numbness and muscle weakness.
The disc material which has prolapsed into the spinal canal sets up an inflammation in the surrounding tissues which adds to the pain. If the amount of prolapsed material is small it can shrink and the inflammatory process will settle down, particularly if aided by anti-inflammatory tablets and perhaps physiotherapy. In this event the pressure on the nerve is reduced and the pain relieved.
Injections of a steroid solution into the epidural space within the spinal canal can be effective in reducing the inflammation around a disc prolapse and these are often administered by way of an ‘epidural injection’. While this procedure cannot be guaranteed to work, it does reduce or eliminate the pain of a prolapsed disc in many affected individuals.
As spinal degeneration giving rise to a disc prolapse is often accompanied by arthritic degeneration of the small facet joints at the back of the spine, injection of these joints, if affected, often contributes to pain reduction when given with the ‘epidural injection.
Unfortunately, in some instances, the pressure and inflammation around the nerve persists and the affected person continues to experience pain in the back and/or down the leg. In this event the only way in which to obtain relief may be by fthe surgical removal of the prolapsed disc.
The prolapsed disc in the lumbar spine is approached through an incision over the back of the spine. The operation is usually performed as an ‘open’ procedure but more recently ‘microdiscectomy’ techniques have been devised, using a microscope or small video camera, in order to minimize the size of the incision.
Whatever method is used, the operation is designed to take the surgeon down to a soft membrane, about the size of a thumbnail, called the ‘ligamentum flavum which lies between the adjacent vertebrae. The prolapsed disc lies just underneath this ligament.
Once the ligamentum flavum has been opened the nerve root, which is often thickened and swollen, is retracted to expose the prolapsed disc lying underneath. An incision is made into the disc and the prolapsed material is removed. The disc may be explored to remove any further loose fragments of disc material. Finally the nerve root is replaced and covered to protect it before closing the wound.
This operation is relatively minor although not entirely without risk As a rule, minimal structural damage is done to the spine. After removing the pressure on the prolapsed disc the pain, in most cases, settles fairly rapidly. Long-standing pressure on the nerve causes damage within it and some numbness may persist for a long time. As the nerve ‘recovers’ there may be an increase in the sensation of ‘pins and needles’ for a while. Mobilisation after surgery is generally rapid and discharge from hospital is usually within a few days. After surgery early movement of the spine is encouraged although it is not recommended to return to work for about a month to six weeks. A long-term, continuing program of back care and exercises is also recommended in order to maintain the back.
As the result of its degeneration, the affected disc bulges out like a soft automobile tyre. In some individuals it is the bulging of the disc, rather than an extrusion of its contained disc material, which presses on the nerve root within the spinal canal and gives rise to the sciatic pain which radiates down the leg. In this situation Nucleoplasty may be helpful in alleviating the symptoms.
This is a new technique in which a thin electrode is inserted into the disc and a portion of its substance is ‘coblated’. As a result the disc shrinks in size which takes the pressure off the nerve and relieves the pain. Some presently view this as experimental surgery. It must be emphasised that not all ‘prolapsed’ discs are amenable to this form of treatment. It is only helpful where the disc is ‘contained’ and the pain is present mainly in the leg. While it is a relatively safe procedure it must also be noted that there is a small risk of nerve root and dural sac damage from the cannula.
CAUDA EQUINA SYNDROME
Although, fortunately, relatively uncommon, pressure on the nerve roots in the lower lumbar spine can impair bladder and sometimes bowel function. Typically this is often accompanied by an exacerbation of pain in the legs and the onset of numbness about the anus and perineal region. At the same time there is the onset of difficulty passing urine and urinary retention and, occasionally faecal incontinence.
A number of conditions can cause this problem, including spinal fractures, subluxations and dislocations, infection in the spine, bleeding within the spinal canal, tumours in the spine and prolapses of an intervertebral disc.
Only a few individuals suffering from a lumbar disc prolapse (slipped disc) may develop the cauda equina syndrome but, as it s effects can be permanent if not treated rapidly, it requires very urgent attention. Any individual suffering from this condition who suddenly experiences any of the above symptoms should seek immediate medical attention.
SEVERE DEGENERATION OF THE LUMBAR SPINE
Continuing degeneration of the intervertebral discs can take place at a single, or at several levels, and bring about a series of untoward changes within the spine itself:
During the process of degeneration the nucleus pulposis shrinks down as it loses water. This shrinkage causes a slackening of the tissues which may be felt as a slight instability in the spine as it moves. The degeneration of the nucleus pulposis itself can cause an inflammatory reaction in the area around the disc. This, in turn, may give rise to a continuous, dull pain in the back. In addition its altered mechanics puts an abnormal load on the small ‘facet’ joints at the back of the spine. These, in turn, may become arthritic leading to an increase of pain and further instability of the spine. This series of events occurs most commonly in the lower back, but similar degeneration also occurs in the neck and occasionally in the thoracic spine.
As the degenerate intervertebral disc collapses, the tissues surrounding the spinal cord at this level settle in on themselves, causing a compression around the nerve structures; a condition called ‘spinal stenosis’. This can have two effects. The compression of the nerves cam cause pain in the lower back which radiates down through the buttock and into the legs. More commonly, the stenosis gives rise in a sensation of heaviness or weakness of the legs while walking (spinal claudication). This causes the affected person to stop after only a relatively short distance, usually a matter of only 50 to 100 yards, and rest for a while before continuing another short distance.
Found in association with arthritic degeneration of a posterior facet joint. The arthritis causes an effusion within the joint which, in this case, causes a fluid filled bulge of the synovial lining into the spinal canal. The effect of this is to cause a narrowing (stenosis) of the spinal canal. As above, this can lead to the symptoms of spinal claudication.
TREATMENT OF SEVERE DEGENERATION
A spinal support, combined with physiotherapy and analgesic tablets may be of assistance.
Epidural injections of steroid solution into the spinal canal may give some relief of the discomfort arising from spinal stenosis. Similarly, where the pain is shown to be due to arthrosis of the facet joints injection of these joints may bring relief of discomfort for a greater or lesser period.
The problem of spinal degeneration is a mechanical one. If the above methods of treatment are not effective, or are not acceptable, surgery may give a satisfactory result.
If the problem is primarily one of pain in the legs due to degenerate stenosis of the spinal canal at one or more levels, which does not respond to physiotherapy, surgical decompression often helps. This is done by surgically exploring the areas where the nerves are being compressed and removing the tight tissues around them.
The spine is approached in a similar fashion to that of a simple discectomy. In this case, however, the opening into the spine is usually bigger to allow access to all the constricting tissues about the spine. The ligamentum flavum is thickened and, like the disc, is usually degenerate. This membrane is cleared away where it is constricting the nerve tissue.
The small ‘facet’ joints at the back of the spine often respond to the degeneration which has taken place by developing small bony outgrowths, called ‘osteophytes’. By virtue of their position, these osteophytes may impinge on the nerve roots and, if present, need to be removed.
A third factor, which frequently contributes to the squeezing of the nerve tissue, is the degenerate intervertebral disc itself. It is this structure which has initiated the whole process and may be found to be protruding into the spinal canal. If this is the case it needs to be removed to complete the operation.
INTERSPINOUS SPREADING DEVICES
The main problem giving rise to spinal stenosis is collapse of the intervertebral space as a result of the degeneration of its intervertebral disc. Recently it has been shown that opening up of the collapsed space by means of a spreading device, inserted between the spinous processes, can relieve the compression on the nerves sufficiently to relieve much of the discomfort. The magnitude of the operation is of less than that for complete decompression of the intervertebral space and is perhaps better suited for those who are considered to be at risk for a major procedure. It is, nevertheless, only suitable to use this procedure where one or, at the most, two levels are affected and the stenosis is not too severe and where the bones are not osteoporotic.
If the problem is primarily one of pain in the back it is the movement of the damaged tissues which gives rise to the pain. The aim of surgery therefore is to stabilize the affected regions of the spine. Spinal stabilisation, in severely affected individuals, is usually obtained by spinal fusion
If there is significant degeneration and arthrosis of the spine it is stabilised in the areas where this problem exists. If decompression of the nerve roots is necessary this is performed at the same time through foraminotomies, or small openings into the spine, at the levels where there is stenosis.
The spine is usually approached from the back, although with some techniques the stabilising implants are inserted from the front, or from the front and back. The spine is stabilised by inserting screws into the vertebrae through their pedicles and joining them with rods, or what is termed a ‘cage’ which is inserted between the vertebral bodies themselves, or by a combination of these implants. An important part of the procedure is the insertion of bone graft between the vertebrae in order to aid in their support.
After the initial discomfort of the operation has settled the back and legs usually feel very much more comfortable and mobilisation, preferably with some form of support, is possible within a few days. Discharge from hospital is usually about a week after surgery. Activity is encouraged but care should be taken when bending the spine until the bone graft has consolidated, about six weeks.
While spinal fusion, with or without instrumentation, usually produces a satisfactory result, it solidifies the affected region of the spine and is a final and irreversible event. It has been shown, in appropriately selected individuals, that good results can be obtained by a dynamic stabilising the spine using Dynesis.
In this system screws are inserted into the pedicles, as with the standard type of posterior instrumentation, but they are joined by combination flexible rods rather than rigid rods. This arrangement offloads the damaged disc while, at the same time, allowing slight movement of the adjacent vertebrae.
It seems that this support and slight movement is beneficial to the intervertebral disc as it allows it to recover to some extent. While immediate relief of pain is often achieved, it is sometimes several weeks or months before the full benefit of this procedure are evident. As the supporting rods are radiolucent, the post-operative x-rays seem strange as there does not appear to be anything joining the screws which have been inserted into the vertebrae.
Lumbar disc replacement
As with arthritis in other parts of the body, artificial joints in the spine have become a recognized form of treatment for degenerative spondylosis. Disc arthroplasy is used to treat some of the same conditions which are treated by spinal fusion. Where indicated, the use of these implants has several theoretical advantages over those used for spinal fusion. An artificial disc maintains the motion of the spine. It thus overcomes the problem of obtaining a bony fusion between the vertebrae. By not holding the vertebrae rigidly it maintains spinal balance, allowing it to vary its position slightly in order to adapt to varying physical demands and in this respect is more ‘physiological. It is felt that because of the increased motion the operation is more effective in reducing the spinal pain. It is also felt that disc arthroplasty reduces the risk of ‘adjacent level degeneration’ which is though to be promoted by spinal fusion.
On the converse side it must be pointed out that disc arthroplasty is not indicated in all forms of spondylotic degeneration of the spine. It should only be performed where there is movement at the affected intervertebral level and there is no arthritis of the small facet joints at this level. It is necessary to perform the operation from the front and, consequently, there is a greater risk of damage to the vessels. Because of this approach, it may not be possible to deal with problems within the spinal canal. Finally it must be said that, although this operation has been used for several years, it is still regarded by some as being an ‘experimental’ procedure. The National Institute for Clinical Excellence (NICE) states that current evidence appears adequate to support the use of this procedure but cautions that there is little long term evidence of its efficacy.
An unusual cause for back pain with sciatic radiation down the leg is due to an abnormal, localised enlargement of the veins which surround the dural sac.
The surprising feature of this problem is that the nature and distribution of the pain, and the radiological findings, are exactly the same as those of a prolapsed intervertebral disc.
When this condition arises, the enlarged veins irritate the adjacent nerve root. The pain which this causes can be intense and is usually prolonged. Although there are reports in the literature of the pain settling, in our experience the only satisfactory way of treating this problem is by surgical decompression.
Although the MRI scans typically are identical with those of a prolapsed intervertebral disc, there are instances of vein induced pain where the radiological signs are subtle. In these cases the significance of the radiological changes can be overlooked and the affected individual may experience varying degrees of pain in the back and legs for many years.
COMPLICATIONS OF SPINAL SURGERY
You must note that surgical procedures on the spine are not without their problems. While every effort is made to undertake the procedure carefully and safely, things sometimes (inadvertently) do go wrong. Although the risks are very low some of the following mishaps rarely can occur during the operation, some of which may require further surgery:
- Continuing pain in the lower back.
- Continuing pain, numbness and/or weakness in the leg
- Dural tears with meningocoele and/or cerebrospinal fluid fistulas.
- Excessive blood loss.
- Epidural haematoma.
- Injury to blood vessels or other structures in the abdominal cavity.
- Infection of the vertebrae, discs or meninges.
- Epidural scarring.
- Damage to the nerve roots or part of the spinal cord, resulting in postoperative neurological defects, including permanent sciatica, bowel and bladder dysfunction.