This is a soft , gelatinous, mucoid material at birth. It lies almost in the centre of the intervertebral joint. But as age advances, the anterior part of the body of the vertebra grows much faster than the posterior part. Hence it ultimately lies strictly behind the centre. It forms a cushion between the vertebrae. There is a resultant compression which exerts evenly distributed hydrostatic pressure. The pressure within the nucleus is considerable.
The disc can be damaged by direct or indirect trauma. If the disc is healthy, it would need to be hit by a considerable force to be damaged. Even an impact enough to damage the body of a vertebra is not sufficient to damage a healthy disc. It has been calculated that a normal adult disc can withstand a compression force of 545 kg per square inch before rupturing , while less than 450 kg of pressure is enough to damage the vertebral body. In normal weight bearing, when a person is standing or sitting, the compression force is 45 kg.
But it is estimated that it increases considerably, reaching upto 225 kg when a person is bending forward. In this position when a load of 30 kg is lifted by using only the spine while bending, the force increases upto about 450 kg, which is dangerously near the breaking limit. A weightlifter trained to lift with proper techniques can lift as much as 272 kg without any apparent damage to the disc. This is why it is very important to learn the correct method of weightlifting.
When the correct method is employed the weight is lifted with the help of the arm and leg leverage, and the weight is supported by the spine only when the person is erect. When a person lifts a weight, a small role is also played by the abdominal muscles, and part of the force is absorbed by the intra-abdominal structures. It is estimated that, while lifting, upto 30 percent of the force is absorbed by these structures. This is why it is very important to have strong abdominal muscles in order to protect the lumbo-sacral spine. Exercising the abdominal muscles to make them strong is also important in case of lumbo-sacral pain.
When the disc has undergone degenerative changes, smaller weights may be sufficient to cause damage. So it is important to understand the degenerative changes of the disc to understand pain. It is also important to check or minimise degenerative changes as preventive measures.
The gelatinous properties of the nucleus depend upon its mucosaccarides which tend to break down with age. Imbibation of fluid diminishes, making the nucleus more rigid. The distance between the vertebral bodies diminishes; the annulus fibrosus bulges, growing weak at certain points. Under some circumstances and especially with trauma, the internal hydrostatic pressure rises, and the weakened annulus gives way. The disc herniates or prolapses through the weakened annulus, which may, in turn, pass through the end plate. Symptomatology depends upon the location of the prolapse. The most susceptible area of the spine is the lower lumbar spine.
It is here that spondylosis is most common. The intensity of the pain depends upon the sensitivity of the site of protrusion. Sometimes a fragment of the cartilage, generally in a degenerated disc, can break off and move inside the intervertebral joint, lodging against a sensitive area and causing pain. This pain may have a sudden onset. The broken fragments may consist of fibro-cartilage or nuclear tissue. The lumbar spine, however, is not the only part subjected to such pressures. For example, take the cervical spine. Here in spite of the smallness of the vertebrae, their bodies and facet joins support a large ball weighing approximately 5 kg – that is the man’s head. The head is balanced over two small facets, small as nails, yet mobile in all directions. Some people can even carry a weight of upto 54 kg on their heads, entirely supported by the joints of the two upper cervical vertebrae.
After 20 years of age, degenerative changes start occurring, which may result in necrosis or breakage of the nucleus pulposus, and the softening and weakening of the nucleus fibrosus. Under these circumstances even a minor strain can cause internal derangement in the joint as the nucleus is displaced and the nucleus fibrosus is weakened, making the nucleus pulposus bulge out. Unequal tension within the joint causes disc-prolapse, resulting in a sudden onset of acute lumbago. Prolapsed nucleus material can cause irritation in the adjacent nerve root. This may cause pain known as sciatica in the leg. This generally occurs in the lower lumbar or lumbo-sacral joint or lower cervical joints.
During middle age, when degenerative changes start, bulging of the disc material may take place in any direction, producing a pull on the ligaments during weightbearing. A ligamentous pull lifts the periosteum from the margin of the body of the vertebrae. New bone formation takes place under this periosteal lift and in due course, osteophytes appear. The osteophytes, when viewed through an X-ray, called osteoarthritic changes. They limit mobility. Ligaments also become hard by this age. Generally the osteophytes are thought to be responsible for pain, but in most cases they are not. This can be proved just by looking at a number of X-rays taken to reveal kidney stones in patients belonging to a middle or higher age group. Most of the time, the patients insist that they have never had a backache in their life.