Snugly tucked between the 24 vertebrae of the spine are spongy discs which act as shock-absorbers and impart suppleness to the spine. Each disc has a gelatine-like soft centre (the nucleus palposus) contained within a fibrous outer layer (annulus fibrosis). A PLID occurs when the disc ruptures, releasing some of its gelatinous contents. Such a rupture may have little or no effect on back function. However, the patient becomes aware of a 'slipped disc' when the leaked nucleus puts pressure on a spinal nerve, causing localised pain that can be severe. If pressure is put on the sciatic nerve, the pain can also be in the leg as far down as to the ankle.
The precise pathophysiology of the condition is not completely understood, but it's believed that pain results not only from a mechanical effect on the nerves, but also from 'biochemical irritation' (Ann Intern Med, 1990; 112: 598-603).
PLID is primarily a degenerative condition but, paradoxically, it does not worsen with age. If you've got to age 55 without getting a slipped disc, you'll probably never have one. That's because the annulus gradually becomes more fibrous over time, thus preventing ruptures in later life. The danger zone is the 20-year window over age 35, when the nucleus of the disc begins to dry out and lose elasticity. Adolescents can also suffer PLIDs, but these are mainly caused by specific injury.
Three main factors can bring on PLID: heavy, awkward pressure on the spine; repetitive movements of the back; and being overweight.
A slipped disc usually happens when someone bends forward while lifting a heavy weight, which puts increased pressure at the front of the spine while releasing pressure at the back. Twisting motions while lifting are particularly hazardous. Heavy manual workers are most at risk but, less obviously, so are people who do a lot of driving - thought to be due to engine vibrations compacting the spinal discs.
However, fewer than one in 20 cases of acute back pain are due to PLID. Most are the result of sprains, injuries to the ligaments and muscles or a locked facet joint between two vertebrae. As a result, PLID is frequently overdiagnosed by GPs.
What doctors tell you
Doctors often recommend surgery to excise the offending tissue. There are two main operations, one more drastic than the other.
The simpler operation is a 'standard' or 'open discectomy'. This involves removing the damaged or bulging part of the ruptured disc to relieve pressure on the nerves. This is increasingly being done by 'keyhole' surgery and is the most common operation for PLID.
The alternative is to remove the entire disc - called a 'laminectomy'. It's a much bigger procedure, partly because it leaves a gap between vertebrae that must be closed. This is achieved by first removing the bony arches of the vertebrae (the laminae), then either filling the gap with bone chippings (usually taken from the patient's leg), or screwing the vertebrae together, a procedure known as 'fusion'.
There are a number of other, less invasive treatments such as cortisone injection, which has been a standard procedure for 30 years, and chemonucleolysis, in which the soft nucleus is dissolved away by a powerful enzyme such as chymopapain, derived from papaya. Newer procedures include electrical cauterisation of the annulus nerve endings to block pain signals ('intradiscal electrothermal annuloplasty'), the insertion of hydrogel cushions as artificial disc substitutes, and Vax-D, a sophisticated traction device.
What doctors don't tell you
Taken as a whole, PLID surgery has a poor track record. The most recent review of the clinical data has shown that about half the standard operations need to repeated, as are a staggering 80 per cent of keyhole surgeries (J Gen Intern Med, 1993; 8: 487-96).
When discussing your surgical options, orthopaedic surgeons rarely tell you that:
* your back is likely to heal all by itself. Surgical intervention should not be a knee-jerk response, as spontaneous remissions are seen in more than 60 per cent of cases (Schmerz, 2001; 15: 484-91).
* with an open discectomy:
- it won't make you feel any better and, indeed, could make you feel even worse (as experienced by at least 4 per cent of patients) (Eur Spine J, 2005; 14: 49-54)
- your back pain is likely to persist. About 75 per cent of patients have residual lower-back pain, 12 per cent of which is described as 'severe' (Spine, 2001; 26: 652-7)
- you risk inadvertent damage to the surrounding nerves.
* with fusion:
- you're likely to suffer surgical complications. 'Iatrogenic soft-tissue morbidity', as it is known, is an almost routine hazard (Orthopedics, 2002; 25: 767-71)
- common complications include excessive bleeding, nerve root lesions and recurrent disc herniation - the problem you had in the first place (Eur Spine J, 2003; 12: 239-46)
- the surgeon could damage your internal organs. Perforations of intra-abdominal structures have been reported (Neurocirug [Astur], 2004; 15: 279-84) as well as rupture of large blood vessels (Eur J Vasc Endovasc Surg, 2002; 24: 189-95)
- it may actually make the pain of PLID worse because of scarring or damage to the meninges. Henry La Rocca, clinical professor of orthopaedic surgery at Tulane University in New Orleans, points to "substantial" evidence that iatrogenic scarring of the nerve roots causes long-term pain, and that iatrogenic damage to the membranes covering the spinal cord can be "catastrophic" (Weinstein J, Wiesel S, eds. The Lumbar Spine. Philadelphia: W.B. Saunders, 1990).
In addition, spinal fusion has the highest failure rates. One reason may be that the operation puts an extra load on other parts of the spine. By locking the movement in one vertebral joint, fusion forces the adjacent joints to do 50 per cent more work than they were designed to do. This can lead to later PLIDs - as one osteopath puts it, "You simply chase the problem up the spine".
Fusion brings other long-term medical problems such as osteoarthritis, probably due to friction between the unbuffered vertebrae. The patient is also left with a weaker spine and so a reduced quality of life.
Nevertheless, there are occasions when surgery is necessary to prevent paralysis. The tell-tale symptoms are specific - either faecal incontinence or an inability to pass urine. Both are caused by the prolapsed disc pressing on the nerves that control the bladder and/or the bowel, and indicate a potentially hazardous condition requiring emergency surgery. Secondary symptoms may be back pain or numbness in the pelvic area.
Medical alternatives to surgery
Chemonucleolysis using papaya enzymes is more effective than surgery in the long term, but may also have occasionally severe adverse consequences - in particular, a fatal allergic reaction. It can also cause severe back pain for up to three months after the procedure (Spine, 1996; 21: 1102-5). As a result, it's now falling out of favour, especially in the US.
However, chemonucleolysis has been tried with substances other than papain such as alcohol and oxygen/ozone, which appear not to cause allergic reactions nor many side-effects. Ozone therapy claims to benefit 80 per cent of cases, although subsequent tests have shown that it fails to reduce the size of the disc in about a third of patients (J Neuroradiol, 2004; 31: 183-9). Alcohol chemonucleolysis has a better record, claiming a success rate of nearly 100 per cent (J Neuroradiol, 2001; 28: 219-29). But neither technique has yet undergone any clinical trials.
The lack of objective trial data is also the problem with electrothermal therapy and artificial hydrogel discs, both of which can only be described as 'promising' (Orthopedics, 2002; 25: 767-71).
Trials have been done on cortisone injection - and subsequently led to its being largely abandoned. In a controlled, blinded study of more than 150 patients, cortisone initially showed a slight benefit but, after three months, there was no difference between it and the saltwater placebo. In fact, regardless of treatment, the patients were 17 per cent worse (N Engl J Med, 1997; 336: 1634-40).
Clinical trials of Vax-D traction have been generally positive, with a claimed "success rate" of nearly 70 per cent. It is also alleged to be safe (Neurol Res, 2001; 23: 780-4). However, there have been - albeit rare - reports of a "sudden, severe exacerbation" of pain due to "marked enlargement of the disc protrusion", requiring emergency surgery, with the device (Mayo Clin Proc, 2003; 78: 1554-6).
More conventional traction, despite being a long-standing technique, has not withstood the scrutiny of clinical trials, which have shown it to be of equivocal effectiveness (J Spinal Disord, 2000; 13: 463-9). In fact, many osteopaths believe traction to be positively harmful, as it may exacerbate any tears in the annulus.