Intervertebral disc

Intervertebral disc - Structure and Functions

The intervertebral disc is a real natural shock absorber, interposed between one vertebra and the other in order to attenuate the pressures developed during the movements, for example while jumping, running or suffering jolts on the car seat. Nevertheless, the functions of the intervertebral disc extend well beyond its, however, very important, antishock action. This bearing, in fact, gives the overlapping vertebrae a certain motility so that the column can, within certain limits, bend in all directions and perform modest rotational movements; if the intervertebral discs did not exist, the vertebrae would, due to their anatomical conformation, an even more limited articular excursion.

The intervertebral disc is a flexible fibrocartilaginous structure; it has the shape of a biconvex lens that adapts well to that of the vertebral bodies to which it is interposed. In each disk two parts can be recognized:

• the POLPOSE CORE: a central, gelatinous, yellowish mass formed by highly hygroscopic mucopolysaccharides (retaining water); it aims to respond to the stresses of the forces acting on the column and to distribute them uniformly to the annulus.
• the ANULUS FIBROSO: solid and concentric peripheral scaffolding, whose fibers are arranged in regular concentric layers that cross each other. Its purpose is to contain and protect the central core and gives the disk great resistance to compression.

The function of the disks is particularly important in the lumbar tract, where the vertebrae are more stressed by the overlying load. For this reason, between L1 and L5, the intervertebral discs reach a higher thickness and proportionally greater than the vertebral bodies. This ratio, equal to 1/3, drops to 1/4 in the cervical vertebrae and to 1/7 in the dorsal ones, also for this reason with less mobility.

In addition to slightly varying the shape depending on the location of the column, the intervertebral discs are normally thicker in the front (directed towards the abdomen); they are also absent between the sacral and coccygeal vertebrae, as well as between the first two cervical ones.

The intervertebral discs are connected, anteriorly and posteriorly along the entire column, by fibrous ligaments which constitute a powerful reinforcement structure.

Adult intervertebral discs do not have blood supply; thin blood vessels enter and leave the disc in the first years of life, but then tend to disappear around 20-30 years. Consequently, the intervertebral disc draws its nourishment essentially by osmosis from the capillary beds that surround it; in the same way it eliminates the waste substances. This mechanism is activated by pressure changes within the disc, generated during column movements.

Degeneration of the intervertebral discs

When a pressure is applied on the intervertebral disc, a liquid spillage is obtained and a reduction in its thickness. Conversely, when pressure is removed (for example during sleep or using an inversion bench) a liquid is recalled inwards and its structure is restored. It is known, in fact, that the height on awakening is about two centimeters higher than that measured at the end of a working day, since each intervertebral disc undergoes daily variations equal to 10% of its thickness.

In young people, the various discs make up 25% of the height of the spine, but this percentage is destined to fall with aging. In fact, advancing age brings with it a progressive and irreversible loss of water and intervertebral disc functionality, which turns into a "drain cushion".

While the water content in the disks of young people is around 80-85%, in elderly subjects this percentage falls below 70%.

According to Nachesom the pressure on the third lumbar disc varies considerably depending on the position assumed. Once the load exerted in the natural upright posture has been made 100%, the pressure is reduced to 25% in horizontal decubitus position and increases to 150% in seated position and to 180% in anterior trunk flexion.

If the stresses facing the intervertebral disc are particularly intense, the resistance of the annular container can be overcome and produce a displacement of the nucleus from its central position. The same result can be the consequence of a chronic exposure to vibrations and wearing stresses, which considerably lower the tolerance threshold of the annulus. In these cases there is talk of herniation of the disc, which can occur in different degrees and types, depending on how the nucleus moves.

In the most serious cases the nucleus pulposus separates completely from the intervertebral disk, like a "crushed denitrific". Depending on its position, prolapse can cause pain or symptoms of paralysis in the back, which sometimes also extend to legs and feet and / or arms and hands. These symptoms are the result of direct compression of the disc on adjacent nerve roots and their irritation due to the release of inflammatory agents resulting from the degradation of disc proteins.

The weakest point of the disc is located at the back of the fibrous nucleus, close to the intervertebral foramen, and it is for this reason that most hernias occur at this level.

Among the many treatment options, but in a few and selected cases (given the delicacy and invasiveness of the intervention), there is the possibility of replacing the damaged intervertebral disc with an artificial prosthesis.