It is very important to know the structure and functions of the spine before we begin to understand the cause of pain and what actually happens during manipulation. We must be familiar with the preventive methods and precautions we should take after the pain has gone so that we may not suffer from it again.

Understanding and becoming familiar with the anatomy or the structure of the human body is imperative to understand pathology or the disease process. Then only can we think of a remedy or treatment. Let us examine what our aim in manipulation is, and how these measures help to keep us healthy. Somebody has compared the human spine to a sitar and an osteopath to the maestro who plays the sitar. To learn the sitar, to master it, to produce new ragas, calls for a deep understanding and years of devoted practice. Appreciating the light and almost imperceptible touch and, at other times, the deep pressure applied on the strings, is what differentiates a maestro from an ordinary player. So it is with the skill of an osteopath. To manipulate the spine requires an equal amount of devotion and understanding added to years of an uninterrupted practice. The osteopath becomes a master of his job only after a devoted practice of at least five years after his graduation, during which he learns the basic knowledge only. It is also true that he never stops learning.

Man is a vertebrate. This means that he has a backbone called the spine, extending from the neck to the tail bone. The spine consists of a series of small irregular bones called vertebrae placed in such a way that they carry on different movements and support the weight of the trunk, thus making weight bearing easier for the lower limb. These small bones called vertebrae are thirty-three in number. There are 7 vertebrae in the neck which comprise the cervical spine, 12 in the upper back comprising the dorsal spine, 5 in the loins called the lumbar spine. Five sacral bones fuse together in the tail bone region to form the sacrum, and below that is the coccyx formed by four rudimentary coccygeal bones.

The part of the vertebra situated in the front mainly helps to support the body weight. The posterior part called the neural arch,, encloses the neural canal through which passes the spinal chord. The neural arch consists of:

• The pedicle or foot of the vertebra
  
  
  
• A pair of transverse process or projections
  
  
  
• Superior articular process or surfaces.
  
  
  
• Inferior articular process or surfaces.
  
  
  
• Spinous process
  
  
  
• Laminae or lining covering the body of the vertebrae

The Cervical spine has three peculiarities:

• First, the transverse process of the cervical vertebrae is perforated by an opening through which pass the vertebral artery and the vein which supplies blood to the brain.
  
  
  
• Second, the first cervical vertebra named Atlas supports the globe of the head and has no body.
  
  
  
• Third, the second cervical vertebra called Axis provides the pivot upon which the atlas, the first vertebra, and the skull rotate.

Curves of the Spine

The spine is not straight. If it is viewed from a side, four curves can be seen:

1. The cervical curve which is convex forward.
   
   
   
2. 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.
   
   
   
3. 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.
   
   
   
4. 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.

The Nucleus Pulposus.

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.

The Posterior Joints

From a strictly anatomical point of view the posterior joints of the spine are the true joints of the spine. The extent and the variety of movements depends on the shape and direction of the facet joints. They determine the extent of movement and direction of a particular segment. These facet joints are covered by a dense articular capsule which is quite elastic. thin and loose. They are attached just beyond the margins of the articular facets, and are larger and looser in the cervical than in the thoracic and lumbar spine.

The facet joints are supplied by a nerve which runs to the two adjacent joints. Each joint therefore derives its nerve supply from two segments. Each vertebra has a pair of superior and a pair of inferior facet joints. These play an extremely important role in the minor traumatic pathology of the spine, as they have a central area of rich blood and nerve supply. They are the most richly innervated structures of the entire spinal column. This innervation helps the spine to adapt to the variation of tensions to which the capsule of facet joints is exposed. Derangement of these joints can be extremely painful.

In the cervical region the superior facet joints are inclined upwards at forty-five degrees. This helps in free flexion and extension of the neck. At this level usually, extension can be done more easily than flexion. Lateral bending and rotation are always combined. In the thoracic region, the facet joints are more oblique, say, at an approximately sixty degree inclination. In the upper part of the thoracic spine, movements are greatly restricted due to the direction of the facet joints and attachment of the ribs to the body of vertebrae. Rotation at this level is of a greater range than flexion and extension. Lateral bending is restricted due to the ribs and the sternum. Extension (backward bending) is freer in the lumbar spine. A considerable amount of side bending and a small amount of rotation can occur at this level.

Ligaments

The spine consists of a series of joints which are united from the second cervical to the first sacral by a number of ligaments. The vertebral bodies are united by anterior and posterior ligaments, and the posterior series of facet joints and neural arches are united by the ligamentum flavum. The function of these and other ligaments is to hold the bone together and yet allow some calculated movements. The ligaments are elastic structures with an elastic limit. They remain healthy with intermittent stretching. Ligaments can be torn in two ways: sudden force and uninterrupted prolonged moderate stretching. This is why intermitted traction is more physiological than continuous sustained traction.

Anterior Longitudinal Ligament

A long and strong fibrous ligament runs on the anterior surface of the vertebrae. It is attached firmly to the disc and margins of the vertebral body, and loosely attached to the middle part of the body. It is wider at the level of the disc and narrower at the level of the body. A tear in any of the ligament fibres leads to haemorrhage, oedema and fibrin formation. If given sufficient time, the ligament becomes as strong as before, but if stretched too soon, it remains weak and repair is incomplete. The ligament may get elongated, making the joint hypermobile. When stretched continuously for prolonged periods, the ligament starts aching and its elasticity gets diminished.

Posterior Longitudinal Ligament

The posterior longitudinal ligament is attached to the posterior margin of the vertebral body within the vertebral canal. It forms a bridge over the body of the vertebrae and is attached firmly to the intervertebral disc and to the margins of the vertebral body. Thus it reinforces the disc posteriorly. This ligament plays an important role in disc protrusion. The resistance of the ligament helps to push the protrusion back again. The constant occurrance of lumbago in some persons may be caused because the protrusion is enlarged and pushed to the area of lesser resistance, thus producing unilateral sciatica pain. It is due to the posterior longitudinal ligament rupturing so completely, backwards and thus the cauda equina (lowermost roots of the spinal chord) is subjected to great pressure which may result in bilateral sciatica.

Muscles

There are quite a few muscles which act on the spine and help in its different movements. There are short muscles which act directly and long muscles which act indirectly and aid in the movements of the spine. They help to steady the spine. They produce extension, lateral bending and rotation.

Short muscles help to maintain the posture. They contract intermittently, and during an upright posture, there are slow spontaneous swaying movements. They help to initiate flexion and assist the short muscles in further flexion and control of it. It is surprising to know that in incomplete flexion, the short muscles are inactive and controlled by the spinal ligaments. Any imbalance and weakness of these muscles produces a deformity of the spine known as scoliosis.

Side bending is also helped by short muscles. They play a great role in mechanical vertebral pathology. A sudden, enexpected movement can produce a harmful distribution of the forces on the intervertebral joints. If certain parts are compressed or put into a traction beyond their capacity., they can damage the joint to a varying degree, depending on the force causing the painful muscle spasm. Synchronisation of muscular activity is more important than just increasing the strength of these muscles by various spinal exercises. This is why manipulation is an important therepeutic measure.

When the muscles are weak, the ligaments and joints are more strained, thus becoming more vulnerable. Generalised muscle weakness is also the cause of bad posture. Excessive powerful muscle contraction can also damage the bone: for instance, there can be a fracture of the kneecap due to contraction of the high muscles.

Intervertebral Foramen

The intervertebral foramen is a short canal lodged between contiguous. It is ellipsoid in form. Its form changes with the mobility of the intervertebral joint. In the dorsal and lumbar spine it is directed laterally to the right and left. In the cervical spine it is directed slightly to the front, say by fifteen degrees, as compared to the dorsal or lumbar foramen. The canal is covered by a fibrous structure which is connected to the intervertebral disc and capsule of the posterior joints. Through it passes the spinal nerve which consists of a ventral root and a dorsal root. These appear to be united with each other in the canal, but when seen microscopically they are found to be seperated. The dorsal root contains the spinal ganglion. Each spinal nerve, after coming out of the intervertebral foramen, has an offshoot of a small branach called a meningeal branch which re-enters the vertebral canal through the intervertebral foramen and ennervates the vertebral ligaments and blood vessels of the spinal chord.

The compression or irritation of elements contained in the intervertebral foramen may occur due to the degeneration of the disc, osteoarthritis ofn the intervertebral joints, posterior profusion of the disc, or rupture of the disc with herniation of the nucleus pulposus. This may cause pain, some muscular weakness, lumbago, sciatica and a diminished feeling all over the skin.