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.