The spine is not straight. If it is viewed from a side, four curves can be seen:
The cervical curve which is convex forward.
The thoracic curve which is convex forward. The upper part may have a slight lateral curvature directed towards the right side in a right-handed person and the left side in a left-handed person.
The lumbar curve is convex forward. It is more pronounced in females than males. It extends from the lower thoracic vertebra to the lumbo-sacral angle.It is larger than the upper two.
The pelvic curve extends from the lumbo-scaral joint to the apex of the coccyx. Its concavity faces downwards and forwards.
The vertebrae are held together and perform their functions of protection, movement and support. The spine with the help of its inter-vertebral joints tries to perform the functions of movement and support in the best possible way.
Intervertebral Joints
Intervertebral joints are the joints between two adjacent vertebrae. They comprise the anterior joint-containing discs, the posterior joints constituted by facet or surface joints, A connecting ligament system, muscles, intervertebral foramen, and nerves.
Intervertebral Discs
The discs are interposed between the adjacent surfaces of the bodies of the vertebrae and form the chief bond of connection between them. Their shape correspond with those of the bodies of the vertebrae between which they are lodged. Their thickness varies in different regions of the column and in different parts of the same disc. They are thicker in front on the cervical and lumbar region, thus constituting the anterior convexity of these curves. They are uniform in size in the thoracic region, and the anterior concavity of this column is due to the shape of the vertebral bodies, which are thinnest in the upper thoracic area and thickest in the lumbar region.
The disc absorbs the pressure transmitted to it by the central core, and, at the same time, keeps the vertebrae together. It buffers the action of compression upon bones. It is the chief shock absorber of the body. It constitutes one-fourth (quarter) of the entire length of the spine. The shock absorption is based more on the hydraulic system, also akin to the elastic properties of rubber. The disc consists of 3 parts: the end plate, the peripheral portion called the annulus fibrosus, and the central portion called the nucleus pulposus.
The End Plate. This consists of a narrow zone of hyaline cartilage covering each surface of the vertebral body. The end plate, along with the annulus, is perforated by thousands of small holes through which the tissue fluid diffuses. The fluid diffuses both into and out of the disc.
The Annulus Fibrosus. The annulus fibrosus comprises the narrow outer zone of collagenous fibres and a wider inner zone of fibro-cartilage. It is attached to the end plates. Fibres of this layer run obliquely, thus giving great strength to the rotational movements. The annulus fibrosus surrounds the nucleus pulposus in the form of a number of layers which can be compared to the layers of an onion. Around its periphery the annulus inserts itself on the vertebral body. The marginal fibres are particularly tough. The weakest point is located at the posterior near the intervertebral foramen.
The disc is nourished by synovial fluid and if a piece flakes off, it remains alive inside the joint cavity. The cartilage has no nerve or blood supply. It is nourished by the bone of the body of the vertebrae. It is therefore slow to react to a trauma, and also slow and often incapable of complete repair. That is why there is no immediate pain if the cartilage is damaged. The pain is felt only when adjacent sensitive structures are also affected.
Following the trauma to any tissue, there is swelling owing to the liberation of histamine and other substances. The cartilage swells after the forced activity, but due to the lack of nerve supply, it does not cause pain, and due to the absence of blood supply, it swells up slowly. Two or more days following the injury may pass before the cartilage swells up. However, ligaments, if injured, swell up in 2-3 hours.
The swelling may stretch to adjacent ligaments or the periosteum (outer layer of a bone), causing pain, or the swelling may block the full range of movements. Adequate rest required for the repair of the joint is usually not given to it. Repair is therefore often incomplete and consequently, degenerative changes in the annuals are induced much earlier than desired. All the damage to the annulus is permanent: a union and regeneration never take place here.
A disc has the quality of a sponge and is able to absorb fluid as well as diffuse its own fluid content. This is why the consistency of the disc keeps on changing. This can be demonstrated by measuring the height of a person at the end of the day and early in the morning when he gets up. The height of the person increases by ? to ? of an inch after a night’s sleep.This height difference is not due to the straightening of the curves of the spine but due to an increase in the thickness of the discs.